Early Cowl Battery Box Circuit Breakers

[Naugha]

I may go with the early M38 wiring pattern.

There is continuity across my three cowl battery box circuit breakers even using the rotten wires. (The test light is on.)

What does this tell me about the possibility of using these original CB's ?
Should I short them out to see if they work?

[RonD2]

Don,
Electrical terminology.

Your test light proves that all 3 are "closed" at the moment. It tells you nothing about their condition or current handling capacity. As I recall, these are thermally acting circuit breakers and should "trip" (or open) at or near the rated amp capacity under 28-volts, which without looking in the manual I recall is somewhere in the 25-amp range.

Making your "short to ground" test for each CB individually (and also measuring it), could be tricky and dangerous. And if the CB trips but you can't measure it, what's the point? I doubt you'll find any CB test procedure in an Army manual. Back in the day they weren't trying to re-use 50 year old electrical components. If there was any doubt about it, they replaced it.

In my military electrical-electronic experience, I was never taught to "test" a CB that operates solely on thermal action (i.e., that also has no manual on-off switch). If it wouldn't hold the load it was designed to hold, or was otherwise suspect (showing heat damage, cracked, warped, etc) then it got replaced. An old-school auto-electric or TV repair shop with a bench power supply might be able to test them for you.

It's not worth burning down the house (or melting your wiring harness) to save a nickel guessing about them. I'm pretty sure they're still made fresh today.... google "Klixon circuit breaker". I have the part numbers in my notes if you want. If in slightest doubt about them, I'd suggest replace them with new and not look back.

And before you think about shorting a 25-amp CB across your batteries to see if it'll trip and try to measure it, please remember that under most conditions, one amp is more than enough current to stop your heart.

Good luck!

[wesk]

Don,
Ron gave you some very good advice. Leave heavy duty service testing to folks with proper equipment.

A wee bit of history to get you on speed with 24V electrics. Since the advent of powered buggies and DC Electrics we have seen 6 volt then 12 volt and then in 1948 24 volt. Each new voltage out performed the earlier. Each increase in voltage resulted in lighter electrical equipment and increased power availability. It only takes 3 seconds to smoke a brand new 24V electrical harness.

How do you maintain these electrical systems? You use the applicable manuals, wiring diagrams and parts lists. How do you become proficient in maintaining these electrical systems? You gain experience thru learning and practice using the appropriate study guides, manuals, diagrams and parts list.

The 3 breakers (ORD# 7760416) on the early M38's MC10001 thru MC 65927 are rated 24V 15A. The instrument cluster breaker used on the early M38's thru MC 65042 was a different part number (ORD# 7762620) but was also rated 24V 15A. The early style main light switches also had an internal circuit breaker.



The manuals you should have on your bench before digging into your electrical system are:
TM 9-804 Early M38 Operator's Handbook.
TM 9-8012 Late M38 Operator's Handbook.
TM 9-1825B dated 1952 Autolite Electrical components
TM 9-8527 Dated 1953 Delco Electrical Components
TM 9-1825E Dated 1952 Bendix-Scintilla Electrical components & Connectors.
ORD 9 SNL G-740 Dated 1951 and ORD 9 SNL G-740 Dated 1955 supply manuals.
TM 9-2920-215-35 Auto-Lite Starter mdl MCZ-4001-UT Service Manual (Not covered in TM 9-1825B)
TM 9-2920-214-35 Delco Generator mdl: 1117495 Service Manual (Not covered in TM 9-8527)

Additionally the following manuals can be very useful as well:
TM 9-8000 Dated Jan 1956 US Army/USAF Principals of Automotive Vehicles.
ORD 8 SNL G-740 Date 1956 Supply manual
The Portrayal Press copy of Spence's Army Vehicle Parts Guide.

and finally our web site's basic list of manuals applicable to the M38:

M38 Manual and TB List
Based on data initially compiled by Howard Sabin for Army Motors
Updated and Edited By Wes Knettle
________________________________________________________________________

M38 Technical Manuals

TM 5-5213 17 FEB 53
Engine, Gasoline driven, L-head and F-head, Models MC38-MC38A1 basic engine: Conversion for stationary power, Willys Overland Motors.

TM 9-804 29 SEP 50
1/4-ton, 4x4, utility truck M38 (Willys-Overland) Operation and maintenance manual.

TM 9-804 Change 1 16 FEB 51

TM 9-804 Change 2 18 May 51

TM 9-804 Change 3 10 APR 53

TM 9-1804A 11 JUN 51
Ordnance maintenance: Engine (Willys-Overland Model MC) and clutch for 1/4-ton, 4x4, Utility truck M38

TM 9-1804B 16 JUL 52
Power Train, Body and Frame for 1/4-ton, 4x4, Utility truck M38

TM 9-8012 12 JAN 56
1/4-ton, 4x4, utility truck M38 (Willys-Overland) Operation and Organizational maintenance manual
Superseded TM 9-804.

TM 8-8012 Change 1 22 MAR 56

TM 9-8012 Change 2 13 JAN 58

TM 9-8012 Change 3 18 JUN 58

TM 9-1825A 12 JAN 44
Ordnance Maintenance: Electrical Equipment (Delco Remy)

TM 9-1825A Change 1 31 MAR 47
(supercede by TM 9-8627)

TM 9-1825B 20 JAN 44
Ordnance Maintenance: Electrical Equipment (Auto-Lite)

TM 9-1825B Change 1 30 APR 47

TM 9-1825B 3 NOV 52
Ordnance Maintenance: Electrical Equipment (Auto-Lite)
Supercedes 9-1825B dated JAN 44


TB 9-1826A 11 FEB 44
Ordnance Maintenance: Carburetors (Carter)

TB 9-1826A 10 DEC 52
Ordnance Maintenance: Carburetors (Carter)
Supercedes 9-1826A dated FEB 44

TM 9-1827C 21 JAN 44
Ordnance Maintenance: Hydraulic Brakes (Wagner Lockheed)

TM 9-1827C 2 JAN 53
Ordnance Maintenance: Hydraulic Brakes (Wagner Lockheed)
Supercedes 9-1827C dated JAN 44

TM 9-1828A 9 MAR 45
Ordnance Maintenance: Fuel Pumps

TM 9-1828A 29 DEC 52
Ordnance Maintenance: Fuel Pumps
Supercedes 9-1828A dated MAR 45.

TM 9-1828A Change 1 ? _______

TM 9-1828A Change 2 ? _______

TM 9-1829A 15 APR 44
Ordnance Maintenance: Speedometers, Tachometers, and Recorders.

TM 9-2853 7 JUL 45
Preparation of Ordnance Material for deep water fording.

TM 9-2853 Change 1 7 JUN 48

TM 9-2858 8 MAY 45
Cooling Systems: Vehicles and Powered Ground Equipment

TM 9-2858 Change 1 ? ________

TM 9-8627 Electrical Equipment – Delco Remy (supercedes TM 9-1825A)

TM 9-2858 Change 2 `? ________

TM 10-500-10 11 AUG 65
Airdrop of Supplies and Equipment: Rigging 1/4-ton Utility trucks.

TM 10-500-10 Change 1 ? _________

TM 10-500-10 29 JAN 71
Airdrop of Supplies and Equipment: Rigging 1/4-ton Utility trucks
Supercedes 10-500-10 dated AUG 65.

________________________________________________________________________

Technical Bulletins


TB ORD 247 14 May 51
Wll Transport Vehicles, Headlight adjustment

TB ORD 370 9 AUG 54
1/4-ton, 4x4, trucks, Ford Model GPW, Willys Model MB, M38, and M38A1: 3/4-ton Dodge trucks models WC51, WC52, WC53, WC54, WC56, WC57, WC58, WC59, WC61, WC64, M37, M42, M43, M56 and M201: 1 1/2-ton 6x6 trucks, Dodge Models WC62 and WC63: Use of front axle non driving flanges for temporarily converting vehicles from four-wheel drive to two-wheel drives.

TB ORD 434 14 Mar 52
Combat & Transport vehicles with 24-volt electrical systems: Instructions for checking 24-volt instruments that do not zero.

TB ORD 445 7 MAY 52

TB ORD 465 2 APR 54
1/4-ton, 4x4, utility trucks M38 and M38A1: Installation of pedestal truck mount M31A1 and M31C.

TB ORD 467 11 AUG 52
1/4-ton and 3/4-ton, 4x4 trucks and 2 1/2 and 5 ton, 6x6 trucks "M" models: Use of proper fuel tank filler caps.

TB ORD 470 9 SEP 52
1/4-ton and 3/4-ton, 4x4 trucks and 2 1/2 and 5 ton, 6x6 trucks "M" models: Instructions for proper cleaning of crankcase valve assembly.

TB ORD 479 21 NOV 52
Ordnance motor vehicles: Standardization of electric type instruments.

TB ORD 485 19 JAN 53
1/4-ton and 3/4-ton, 4x4 trucks and 2 1/2 and 5 ton, 6x6 trucks "M" models: Servicing data plate corrections.


TB ORD 487 3 FEB 53
Identification and maintenance of fuel filter.

TB ORD 499 4 MAR 53
1/4-ton and 3/4-ton, 4x4 trucks and 2 1/2 and 5 ton, 6x6 trucks "M" models: Identification and replacement of hydraulic stoplight switches.

TB ORD 524 8 JUN 53
Installed metal fuel tanks: Cleaning and preserving

TB ORD 547 18 DEC 53
1/4-ton, 4x4, utility trucks M38 and M38A1: Prevention of crankshaft pulley failures.

TB ORD 554 1 MAR 54 Rev 7 JAN 55
1/4-ton, 4x4, truck M38 series; 3/4-ton, 4x4, truck M37 series; 2 1/2-ton 6x6, truck M34 series; and 5-ton, 6x6, truck M40 series: Removal of flywheel housing drain plug.

TB ORD 560 13 MAY 54
1/4-ton, 4x4, utility trucks M38 and m38A1: Revision of tire pressures on servicing data plates.

TB ORD 570 9 AUG 54
1/4-ton, 4x4, trucks, Ford Model GPW, Willys Model MB, M38, and M38A1: Use of front axle non-driving flanges for temporarily converting vehicles from 4 WD to 2 WD.
.
TB ORD 589 7 JAN 55
1/4-ton. 4x4, truck M38, M38A1, M170: Installation of distributor point set.

TB ORD 610 13 JUN 55
1/4-ton, 4x4, utility truck M38, M38A1C, 1/4-ton, frontline ambulance truck M170: Operating of fording valve.

TB ORD 621 2 DEC 55
Tandem hitch 2540-735-8343 (8-H-1826): Installation on 1/4-ton, 4x4 trucks MB, GPW, M38 and M38A1.


TB ORD 625 5 Jan 56
All tactical wheeled vehicles: Cleaning of power train case ventilating valves

TB ORD 626 1 OCT 57
Tactical transport vehicles: Excessive fuel tank pressure.

TB ORD 1007 2 DEC 57
1/4-ton, 4x4, utility trucks, M38, M38A1, and M38A1C, and 1/4-ton, 4x4, frontline ambulance truck M170: Instructions for reinforcing clutch release fork at fulcrum seat or pivot point.

TB ORD 2300-20/4 18 AUG 58
1/4-ton, 4x4 trucks M38, M38A1, (etc): Stowage locations for pioneer tools and brackets

TB 9-804-1 8 JUL 52
1/4-ton. 4x4, Utility truck, M38: Prevention of damage to the oil filter flexible line.

TB 9-804-2 17 JUL 52
1/4-ton, 4x4, utility truck, M38: Eliminating ________ brake line entanglement.

TB 9-804-3 16 JAN 53
1/4-ton, 4x4, utility truck, M38: Replacement of the lifting shackle pin retaining clip.

TB 9-804-4 ?__________

TB 9-804-5 16 FEB 53
M38 Improper Thermostat retainer dimensions

TB 9-804-6 17 FEB 53
1/4-ton, 4x4, utility truck, M38 Correction of vapor lock in fuel pump to carburetor line.

TB 9-804-7 13 MAR 53
1/4-ton, 4x4, utility truck, M38: Mounting of vent line clip.

TB 9-804-8 13 MAR 53
1/4-ton, 4x4, utility truck, M38: Elimination of transfer case noise.

TB 9-804-9 20 MAY 53
1/4-ton, 4x4, utility truck, M38: Repositioning of trailer coupling receptacle to prevent damage to reflector.

TB 9-804-10 15 JUN 53
1/4-ton, 4x4, utility truck, M38: Removing engine hood hazard.

TB 9-804-11 22 JUL 54
1/4-ton, 4x4, utility truck, M38: Elimination of fuel pump diaphragm failure.

TB 9-804-12 3 DEC 53 Rev 9 MAR 53
1/4-ton, 4x4, utility truck, M38: To prevent contact of front wheels against chassis frame, fender and lifting hook.

TB 9-804-13 29 APR 54 REV 1 NOV 55
1/4-ton, 4x4, utility truck, M38: To prevent chaffing of fuel and vent lines at fuel tank.

TB 9-804-14 6 JUN 54 REV 19 OCT 55
1/4-ton, 4x4, utility truck, M38: Identification of speedometer flexible shaft assembly by vehicle serial number.

TB 9-1804B-1 24 FEB 54
1/4-ton, 4x4, utility truck M38: Kingpin bearing cap screw selection



TB 9-1825E-1 18 JUN 53
Electrical connector repair kit41-K-103-100: Proper use of tools.

TB 9-1859-8 9 MAR 53
1/4-ton, 4x4, Command Reconnaissance Truck: Time schedule guide.

TB 9-2300-206-15 28 NOV 58
1/4-ton, 4x4 trucks, M38, M38A1, M38A1C, M170 equipped with high capacity AC-DC 100 ampere 28v generating systems: Test and rebuild of generating system.

TB 9-2300-206-15 Change 1 ?_________

TB 9-2300-210-20 21 JAN 59
Truck, 1/4-ton, 4x4, M38, M38A1, M38A1C, M170: Interchangeability of air and oil pressure sending units and gauges and installation instructions.

TB 9-2300-225-20 10 DEC 59
Truck, 1/4-ton, 4x4, M38, M38A1, M38A1C, M170: Generator nut torque specifications.

TB 9-2300-228-20
Tactical Transport and Combat Vehicles: Troubleshooting for Instrument Cluster Gages, Switches, Circuit Breakers, Sending Units and Related Wiring dated 8 July 1960

TB 9-2300-233-20 31 JAN 61
Truck, 1/4-ton, 4x4, M38, M38A1, M38A1C, M170: Increasing length of generator belt tension brace.

TB 9-2830-55 23 May 55
Truck ¼-ton M38 Windshield glass conversion to 2 piece. It authorizes replacing a one piece windshield with a two piece windshield when replacement glass is in critically short supply. It states that if over half of the broken windshield is usable, then you cut the glass in half, ditch the broken half and cut a matching other half from a piece of scrap glass. The two halfs were joined together using a cut piece of aluminum flat stock and two lengths of weatherstripping and rubber strip filler.

TB 9-2855-2 23 DEC 52
Instructions for the installation of personnel heater kit for 1/4-ton, 4x4, utility truck M38 (Willys MC) (24V) (G-740).

TB 9-2855-3 16 DEC 52
Instructions for the installation of power plant heater kit for 1/4-ton, 4x4, utility truck M38 (Willys MC) (24V) (0 to -65 degrees F).

TB 9-2855-4 27 OCT 52
Instructions for installation of hardtop closure kit for 1/4 - ton, 4x4, utility truck M38 (Willys MC) (G-740).

TB 9-2855-37 10 DEC 56
1/4 - ton 4x4, utility truck M38: Instructions for installation of hot water personnel heater kit.

TB 9-8012-2 11 OCT 56
1/4-ton, 4x4, utility truck M38: Replacement of fuel pump.

TB 9-8012-1 20 JUN 56
1/4-ton, 4x4, utility truck M38: Elimination of interference between front spring rebound clips and tie rods.

________________________________________________________________________


Modification Work Orders

MWO ORD G740-W1 3 DEC 51
1/4-ton, 4x4, utility truck M38: Relocate Aux Pwr Recpt CB to firewall.

MWO ORD G740-W2 13 NOV 52
1/4-ton, 4x4, utility truck M38: Deactivate primer pump.

MWO ORD G740-W3 5 NOV 52
1/4-ton, 4x4, utility truck M38: Rear axle vent leakage.

MWO ORD G740-W4 24 NOV 52
1/4-ton, 4x4, utility truck M38: Bell housing front plate cover.

MWO ORD G740-W5 27 OCT 52
1/4-ton, 4x4, utility truck M38: ID On/Off position on ignition switch.

MWO ORD G740-W6 14 MAY 53
1/4-ton, 4x4, utility truck M38: Reinforcement of rear body corners.

MWO ORD G740-W7 15 JUL 53
1/4-ton, 4x4, utility truck M38: Addition of oil seals to front hubs.

MWO ORD G740-W8 20 OCT 55
1/4-ton, 4x4, utility truck M38: Addition of angles to battery hold dwon frames.

MWO ORD G740-W9 2 FEB 56 & Chg 1 5 JUN 56
1/4-ton, 4x4, utility truck M38: Waterproofing of steering gear and draining of steering jacket.

MWO ORD G740-W10 6 OCT 55
1/4-ton, 4x4, utility truck M38: Securing front passenger seat pivot pin to seat hinge pivot.

MWO ORD G740-W11 1 NOV 56 & Chg 1 18 SEP 57
1/4-ton, 4x4, utility truck M38: Installation ac-dc generator kit.

MWO ORD G1-W35 16 FEB 63
¼, ¾ 4x4 and 2 ½ and 5 ton 6x6 M models; provision for positive polarizing device for trailer receptacles.

MWO ORD G1-W43 12 JAN 54
¼ ton 4x4 utility truck M38 & M38A1, Removal of axle lube fitting.

MWO ORD G1-W44 12 JAN 54
¼ ton 4x4 utility truck M38 & M38A1, Welding rear shock absorber brackets

MWO ORD G1-W50 9 AUG 54
¼ ton 4x4 utility truck M38 & M38A1, Anchorage of front pass seat.

MWO ORD G1-W54 15 SEP 54
¼ ton 4x4 utility truck M38 & M38A1, Installation of new eng timing indicator

MWO ORD G1-W63 19 SEP 57
1/4-ton, 4x4, trucks M38, M38A1, M38A1C, and M170: 3/4-ton Replacement of air and oil pressure gauges, sending units, and installation of pipe coupling

MWO ORD G1-W76 26 AUG 57
1/4-ton, 4x4, utility truck M38, M38A1, Reinforcement of spare wheel support bracket.

MWO ORD G1-W85 22 MAR 57
1/4-ton, 4x4, trucks M170, M38, and M38A1: Installation of castellated nut and cotter pin on steering bellcrank pivot shaft.

MWO ORD G1-W88 3 OCT 56
1/4-ton, 4x4, utility truck M38, M38A1, M38A1C and front line ambulance truck M170: Modification of clutch release pedal rod.


MWO ORD G1-W94 29 OCT 56
1/4-ton, 4x4, utility truck M38: 1/4-ton, 4x4, utility trucks (Ford model GPW and Willys Model MB): Installation of 105-mm rifle M27A1 with 105-mm rifle mount M75A1 or 106-mm rifle M40 (M170E1) with 106-mm rifle mount M79.

MWO ORD G1-W104 23 APR 57
1/4-ton, 4x4, utility truck M38, M38A1, M38A1C, M170; Alteration of wiring harness used with switch 7729684 to make them applicable to switch 7368702.

MWO ORD G1-W105 8 MAY 57
1/4-ton, 4x4, utility truck M38, M38A1, M38A1C and front line ambulance truck M170: Installation of radio power feed kit.

MWO ORD G1-W108 26 SEP 57
1/4-ton, 4x4, utility truck M38, M38A1, M38A1C and front line ambulance truck M170 and ¾ ton……….: Removal of 330 ohm fixed resistor from generator regulator assy 8699744.




________________________________________________________________________

M38 Supply Manuals

SM ORD 8 SNL G-740, Vol. 9 25 JAN 55
Winterization Equipment for truck, utility, 1/4-ton, 4x4, M38 (SNL G-740)

SM ORD 7 SNL G-740 26 FEB 51
Organizational Maintenance Allowances for Truck, utility: 1/4-ton, 4x4, M38

SM ORD 7 SNL G-740 Change 1 29 APR 52

SM ORD 7 SNL G-740 26 FEB 53
Organizational Maintenance Allowances for Truck, utility,: 1/4-ton,4x4, M38

SM ORD 7 SNL G-740 Change 1 26 OCT 54

SM ORD 7 SNL G-740 15 FEB 55
Organizational Maintenance Allowances for Truck, utility,: 1/4-ton,4x4, M38

SM ORD 7 SNL G-740 25 JUN 56
$21 ebay feb 04 Organizational Maintenance Allowances for Truck, utility,: 1/4-ton,4x4, M38

SM ORD 7 SNL G-740 Change 1 25 APR 60

SM ORD 8 SNL G-740 1 DEC 50
Field and depot maintenance allowances for Truck, utility: 1/4-ton, 4x4, M38

SM ORD 8 SNL G-740 16 MAR 53
Field and depot maintenance allowances for Truck, utility: 1/4-ton, 4x4, M38

SM ORD 8 SNL G-740 16 FEB 55
Field and depot maintenance allowances for Truck, utility: 1/4-ton, 4x4, M38

SM ORD 8 SNL G-740 25 JUN 56
Field and depot maintenance allowances for Truck, utility: 1/4-ton, 4x4, M38

SM ORD 9 SNL G-740 ? 1950

SM ORD 9 SNL G-740 19 NOV 51
$212 Ebay Feb 04 List of all service parts of truck, utility: 1/4-ton, 4x4, M38

SM ORD 9 SNL G-740 1 SEP 55
List of all service parts of truck, utility: 1/4-ton, 4x4, M38

SM ORD 6 SNL J-16, SEC 38 OCT 52
Tool Sets, Field and Depot Maintenance for: Truck, ¼ ton 4x4, (Ford model GPW and Willys Model MB) (SNL G-503); Truck, ¼ ton 4x4x, (Willys Moedle M38) (SNL G-740); Truck, ¼ ton 4x4, Utility, M38A1 (SNL G-758).


Miscellaneous Pubs

DA Pam 108-1

DA Pam 310-

DA PAM 310-29 ORD 1 Index of supply manuals, ordnance

ORD 3 SNL K-1 Cleaners, preservatives, lubricants, recoil fluids, special oils and related maintenance materials.

ORD 3 SNL K-2 Items of soldering, metallizing, brazing,and welding materials, gases and related items.

[Naugha]

So, If I want to build an early M38 harness then put the original CB’s on display, buy some new ones and have a shop test them before installation . That sounds like a plan.

If I want to build a late harness then I still run into the reliability issue of that CB in the light switch..... or what to do about CB protection in general.

I just turned seventy and doubt if there is enough time left to master all the above info & skills. These ‘user ability roadblocks’ are usually solved with money ($$$), which introduces the ‘what do I buy/ where can I find it/ does anyone around here know how to do this’ issues.

Most of the parts are close to ready for reassembly except for electrical : the harness, gauges/senders, the CB issue, and possibly the light switch.

I am still thinking about the turn signal mod to use.

Situation Normal, all fouled up..... but progress continues.

The wife printed out a small poster size diagram version of the late harness for folks who need large print.

[Naugha]

Let me squeeze in some related questions. I am looking for a beginning point in making decisions about the wiring harness.

My M38 falls in the before MC65927 harness group but I used the late bell housing and late 24v starter in order to make any future starter repairs more ‘parts friendly’, which may have nothing to do with my questions.

* Is there a best choice for my basic harness. Late vs Early.

* Do the three CBs in the early harness create problems? Is that why they were dropped in the late harness?

* Is the one CB in a light switch enough protection and should I have it tested by a shop before installation?

The light switch that came with the jeep is a one cannon plug sealed unit.

[RonD2]

So, If I want to build an early M38 harness then put the original CB’s on display, buy some new ones and have a shop test them before installation.

Hi Don,
If you buy brand new Klixon CB's, there's no need to test them before using them.

[Naugha]

New Klixon CB’s can be trusted. Good to know. Thanks Ron.

Do you have any thoughts about the other questions I added to the above post?

[RonD2]

Hi Don,
Not sure there’s any such thing as a “basic” harness. Did you decide to build or buy your harness?

There's a lot of variables involved, and I'm not versed in them all for a 24V M38, partly because my jeep was 12V when I bought it. Because of some things the previous owner did, I ripped it out and built new to have no doubts (I’m using a Delco 10SI alternator with a 60-amp Klixon manual reset CB mounted on the cowl battery box, new Prestolite military wire, mix of Douglas and Packard connectors, main light switch with built-in CB, and old-school in-line automotive fuses on the horn and turn signal flasher circuits). As much as I’d like to have a 24V M38, I’m kind of glad I don’t, mostly because of the cost involved in buying and maintaining 24V stuff that gets more expensive and scarce every day. I’m confident of my electrical ability, so will offer some thoughts --- and I do wish you well in your endeavor. Grab a cup of coffee or your favorite single malt.

Digging into the September 1955 ORD9 on page 175 reveals 8 harnesses and partial harnesses used on an array of M38 serial numbers, not all corresponding to serial number 65927. My point is --- with no clear definition in the wiring diagrams or manuals about them or what changed and why, using the term “early” or “late” with a demarcation line at serial number 65927 may be confusing at best. A lot more research could uncover what changed and why, but I’ve had no joy. Still clear as mud to me.

The ORD9 lists 4 circuit breakers, one 15-amp on the Instrument Panel (page 149) used on serial numbers through 65042. And three more (also 15-amps) on page 155 under “miscellaneous” for the ignition switch, horn and generator regulator (redundant to the CB inside the VR), and the headlight switch (redundant to the one already inside that switch) --- mounted on the cowl battery box, for serial numbers through 65927.

In other words, the dizzying array of these harnesses and circuit breakers used on different serial numbers (with no other technical information describing them or the reason for the changes) adds additional confusion. No help so far.

Keeping in mind that your Voltage Regulator and the Light Switch also have built-in circuit breakers, I couldn't find anything definitive concerning why the Army changed from early (with CB's) to late (without CB's) and those harnesses. I’m pretty sure that the VR circuit breaker(s) are there to protect the output of the main electrical generating device (the Generator), and therefore also protect things downstream to it (to a certain extent). Perhaps that is the reason why the Army dropped the four 15-amp CB’s --- because if the VR operates properly those breakers are redundant and provide only limited additional protection. Or maybe it was because they were prone to “nuisance trips”. Aside from safety problems, nothing will get deleted faster than a nuisance. Who can know for sure? Naturally, should the VR circuit breaker(s) fail to do the job, really bad things can happen. Did you have your 65-year-old VR tested for overcurrent protection? If not, or you otherwise doubt your VR, that may be enough reason to build the early version and buy 4 new Klixon CBs for the redundancy (belt & suspenders).

With M38 production long over by the time the September 1955 ORD9 was published, and the absence of an Army-issued TB/MWO directing removal of the CB’s (and associated harness changes), it appears to me that the changes happened on the Willys assembly line. With the CB’s still listed in the ORD9, this raises the question: if an early M38 (with CB’s) had an electrical problem, would the mechanic order the appropriate parts and maintain the early configuration until the supply was exhausted, or would he “convert” the M38 to a later configuration by removing CB’s and changing (or altering) harnesses? Would some M38’s in service have a “hybrid” combination somewhere in between “early” and ”late”? Who knows? It probably depended on the nature of the problem as much as supply stock. Either way, more confusion 65 years later for somebody trying to put a stake in it and decide how to build and connect a harness.

I can’t find a copy to see details that might offer clues, but the list of MWO/TB’s shows TB 9-2300-228-20, Tactical Transport and Combat Vehicles: Troubleshooting for Instrument Cluster Gages, Switches, Circuit Breakers, Sending Units and Related Wiring dated 8 July 1960.

I know, not much help. Considering the complexity of building a vintage 24V system, the expense in time and cash, and the real danger involved, I’d advise you to only use brand new components (including every inch of wire and connectors). If you have to use vintage NOS electrical components, I’d make darn sure they’ve been competently tested, and I’d still watch them closely for a while. When the government sold off parts it was usually because they were no longer needed, but sometimes it was because they were defective (and should’ve been destroyed, but weren’t).

And if you aren’t confident of your automotive electrical ability, I’d recommend finding some local experienced help (your local MVPA, old school auto-electric shop, etc). This forum has lots of help, but it can’t replace eyeballs standing there looking at it.

If I were in your shoes, I believe I’d go with the belt and suspenders version. Good luck!

[Naugha]

I see NOS CB’s with Douglas connectors available from a top vendor. Not really ‘new’ but are there folks who have successfully built an ‘earlyish’ M38 harness with NOS CB’s.

Do folks usually test the CB’s found in the light switch & gauge cluster... can those CB’s be replaced?

Would using the three CB’s on the cowl BB compensate for uncertainty about the light switch & cluster CB.

I have some wiring harness subunits from other military vehicles which were advertised as being useful in building a M38 harness..... also have a Douglas connector crimping tool.... have several diagrams and a copy of the military wire numbers. May as well have a go at it.

Plan B is to buy a new M38 harness.

[RonD2]

Do folks usually test the CB’s found in the light switch & gauge cluster... can those CB’s be replaced?

Can't say what other folks do. Only the early triangular faced light switch can be opened for service and repair (good luck finding parts). The later oval face light switch is a sealed throw-away, no repair parts unless you want to crack the case and cannabalize (good luck with that). Testing the CB in either would be risky and dangerous unless done expertly using a bench power supply and measurement ammeter like already mentioned.
The CB on the back side of the gauge panel is also a 15-amp thermally acting CB just like the other three mounted nearby. Same comments apply.

Would using the three CB’s on the cowl BB compensate for uncertainty about the light switch & cluster CB.

No. LOL. "Compensate" and "uncertainty" aren't words used in electricity.

They might maybe afford some restricted (downstream) level of protection, if anything. It's not normal to see two forms of overload protection in a circuit, and perhaps that's why the Army shed the CB's (and fired the electrical engineer). The general application of protective devices like CB's, fuses, and the like is that protection is afforded to components located downstream from the device in the circuit. For the M38, the main protective device (also located furthest upstream) is the CBs in the VR. Keeping the 4 little CBs may result in the "nuisance trips" that I mentioned. If I had to bet, I suspect the Army got rid of them because because of the unnecessary design and nuisance trips. But that's just my opinion.

[RICKG]

My MC51986 has the 3 cb's on the batt box, the single plug lever type light switch. The wiring harness I used was an older unit I found in my brothers garage, in a box, after he passed. 10 years later I've yet to trip a cb, the cbs themselves were used takeouts I had, not nos purchases.

[Naugha]

Curiouser and Curiouser

Anyone else have M38 electrical build experiences that can can add data to the discussion ??

.... or let’s try a twist in the approach.... let’s look for failed electrical systems.

Anyone out there that used a wiring harness, light switch, and CB combination that SMOKED their M38 ??

What has worked GREAT? What has failed disastrously?

Not so much opinion but actual experience..... “It works fine!!”. vs. “It caught on fire!!”

[Pierre]

RonD2,
Will any 15A Klixon CB do or is there a particular model?
will this one do?

https://www.steinair.com/product/mil-sp ... er-15-amp/

Thanks

Pierre

[RonD2]

Hi Pierre,
Klixon part numbers CDA15 or 7854-18-15 are the closest thing to the original.
The only difference is they don't have Douglas connectors built on for wire connections. A short adapter wire could easily be fabricated.

To my knowledge, only NOS (and take-off) circuit breakers will have Douglas connectors, and I’ve seen NOS advertised in two places (others may also stock them).
They’re described as 30-amp rather than the 15-amp listed in the ORD9.

See https://shop.midwestmilitary.com/produc ... t-breaker/
and
https://www.armyjeepparts.com/p-3725-ci ... -9996.aspx

And here's the spec sheet on the Klixon circuit breakers: http://www.webmail.sensata.jobs/downloa ... atalog.pdf

The Klixon CDA15 can be found on fleabay for about $15 plus postage.

Good luck!

[RonD2]

There's something I should add about how these thermally acting circuit breakers work that some electricity-challenged folks may not realize.

Most folks are familiar with circuit breakers that when "tripped" (open) due to a circuit malfunction like a short --- they stay tripped (open) until someone manually re-sets it, because they have a built-in switch.

The M38 circuit breakers and the modern Klixon part numbers I gave are automatically acting circuit breakers. They don't have a built-in manual re-set switch.
Because of that, when the CB detects a short circuit, it heats up and trips open like it should, disconnecting the circuit. A few minutes later (after it cools down), it automatically closes again and reconnects the circuit. If the short-circuit condition is still present, the CB will heat up again and trip open again and keep repeating this open-close process until the short circuit condition is removed (repaired, disconnected, etc)....and provided nothing else in the circuit melts or starts a fire in the meantime while this is happening.

Be very careful before touching anything in a circuit you think is "dead" because the CB may close again when you least expect it and connect you into the circuit.
A very good safety rule is to never work around electricity alone. But that's just my opinion.

Be safe out there!