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?


_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[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!
_________________
Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

[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:

[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.

_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[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.
_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[RonD2]

[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?
_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[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!
_________________
Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

[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.
_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[RonD2]

[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.
_________________
keep 'em rollin'
RICKG MC 51986 DOD 01-52, '50 CJ3a

[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!!”
_________________
Don Alvarez
Retired HS Teacher
Central Florida
M38 Project

[Pierre]

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

https://www.steinair.com/product/mil-spec-circuit-breaker-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/product/circuit-breaker/
and
https://www.armyjeepparts.com/p-3725-circuit-breaker-30-amp-nos-klixon-m38-fsn-5925-740-9996.aspx

And here's the spec sheet on the Klixon circuit breakers: http://www.webmail.sensata.jobs/download/klixon-circuit-breaker-thermal-catalog.pdf

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

Good luck!
_________________
Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

[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!
_________________
Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari