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willysmjeeps.com :: View topic - M38 dash lights - dim & full
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M38 dash lights - dim & full

 
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Harleygaz
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Joined: Jul 18, 2019
Posts: 38
Location: Mississippi, near Memphis TN

PostPosted: Sun Oct 20, 2019 6:19 pm    Post subject: M38 dash lights - dim & full Reply with quote

Hi. How did the M38 light switch control dim & full brightness? I don't mean switch position but how did it limit voltage or current to give the dim setting? I checked voltage and on both settings I get the same voltage on the supply to the dash lights.
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Gary
M38 - June 1952
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wesk
Site Administrator
Site Administrator


Joined: Apr 04, 2005
Posts: 14461
Location: Wisconsin

PostPosted: Sun Oct 20, 2019 7:23 pm    Post subject: Reply with quote

I'll bet you have not looked in my photo gallery: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php

In my sub-album: M Series Light Switch

Then sub-album: Bendix-Scintilla Switches

Then finally: TM 9-1825E Ch 4 Sect 1 Light Switches

Fig 29 on page 54 note item "WW" Look at the index for Fig 29 on page 55. Now go to page 56 and read paragraph 22c (3).

or go to our downloads page and download the PDF copy of: TM 9-1825E Chap 4 Light Switch repair
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RonD2
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Joined: Oct 02, 2014
Posts: 579
Location: South Carolina, Dorchester County

PostPosted: Sun Oct 20, 2019 8:42 pm    Post subject: Reply with quote

Hi Gary,
Edited and deleted my original post. I beg your pardon, and my apologies to you and all. I totally mis-read your question.

Thanks Wes. Wise counsel.

Regards, Embarassed
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

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



Last edited by RonD2 on Tue Oct 22, 2019 10:02 pm; edited 2 times in total
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BBDE
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Joined: Mar 05, 2008
Posts: 50
Location: Berlin Germany

PostPosted: Mon Oct 21, 2019 12:25 am    Post subject: Reply with quote

Hi Gary,

as Wes has explained, there is a internal resistor inside the lightswitch.
Inside the "BENDIX-SCINTILLA" switch (following the description to the sub-album from Wes) you can see the resistor.

Inside the "newer" Lightswitches, the resistor is more like a cable but the function is the same.
Dim of only the panel lights, dependent the levers position.

Here you can see the "Internal Main Fuse, or better circuit breaker"
and the "Resistor"


If you have trouble with the "late" switch, look to this link:
http://pages.suddenlink.net/randygar/switch/switch.html

With voltage you running your system?


Sincerely yours
Frank
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Harleygaz
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Joined: Jul 18, 2019
Posts: 38
Location: Mississippi, near Memphis TN

PostPosted: Mon Oct 21, 2019 7:42 am    Post subject: Reply with quote

Thx Frank, thatís what I expected to find but wanted to be sure. Headlights, service tail & brake, BO marker, drive and brake all work fine. I took the dash lights out and cleaned everything and they both work except for one blown bulb. New bulbs have arrived so hopefully Iíll get it back together in a day or so. Iím running 24 volts.
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Gary
M38 - June 1952
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mdainsd
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Joined: Oct 20, 2017
Posts: 315

PostPosted: Mon Oct 21, 2019 8:01 am    Post subject: Reply with quote

With only one dash bulb working, the "dimming" effect will only be half of what it would be if both bulbs are working. It is a matter of having a parallel resistance (2 bulbs) in series with a single resistance (the resistor in the light switch).

Note: measuring the voltage at the bulb sockets with no bulbs will read 24V in either full or dim position. Again it has to do with resistances. This is also why the military light switch will not give full and dim when used on a 12V system, the resistance of the resistor in the light switch is calibrated to the resistance of two (sometimes 3) 24V dash light bulbs.
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'52 Dodge M37, '42 GPW, '48 FrankenJeep CJ2A/M38, '50 CJV-35(U), '51 M38, '42 WC-57 Command Car Undergoing factory original restoration, '44 WC-51 Weapons , (2) M1941 Sperry 60" Anti-Aircraft Searchlights, John Deere M-gator
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Harleygaz
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Joined: Jul 18, 2019
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Location: Mississippi, near Memphis TN

PostPosted: Mon Oct 21, 2019 9:16 am    Post subject: Reply with quote

Mdainsd, thatís what I was thinking, I just wanted to confirm the resistor in light switch itself. I should have it working tomorrow and Iíll post here to confirm itís all working correctly.
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Gary
M38 - June 1952
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jnissen
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Joined: May 12, 2018
Posts: 51
Location: Austin Texas

PostPosted: Mon Oct 21, 2019 10:42 am    Post subject: Reply with quote

For those of us who have gone to the dark side of 12V you can still get the dimming function if you change out the resistor. Just seal up the light switch after the modification.
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Jim Nissen
1955 M38A1
Patent Plate MD74784
Engine Plate MD85578
Contrived Hood Number 20974784
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Harleygaz
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Joined: Jul 18, 2019
Posts: 38
Location: Mississippi, near Memphis TN

PostPosted: Tue Oct 22, 2019 11:19 am    Post subject: Reply with quote

Good news is that my dash lights are working both sides on both dim and full settings and they light up the speedo OK. Bad news is that repro gauges with opaque plastic cases donít appear to let enough light through but Iíll try again later when it is properly dark.
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Gary
M38 - June 1952
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mdainsd
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Joined: Oct 20, 2017
Posts: 315

PostPosted: Tue Oct 22, 2019 11:34 am    Post subject: Reply with quote

Get rid of the repo gauges? Also, if you have the correct early dash lights (the large ones), you can pop the red glass out of them and clean them up real well inside and out, it really helps with the light coming out of them.
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'52 Dodge M37, '42 GPW, '48 FrankenJeep CJ2A/M38, '50 CJV-35(U), '51 M38, '42 WC-57 Command Car Undergoing factory original restoration, '44 WC-51 Weapons , (2) M1941 Sperry 60" Anti-Aircraft Searchlights, John Deere M-gator
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Harleygaz
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Joined: Jul 18, 2019
Posts: 38
Location: Mississippi, near Memphis TN

PostPosted: Tue Oct 22, 2019 6:33 pm    Post subject: Reply with quote

NOS gauges are too expensive. Budget is already blown and more to do. I cleaned the glass bowls and they look like new. Iíll live with it until I can afford NOS.
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Gary
M38 - June 1952
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wesk
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PostPosted: Wed Oct 23, 2019 9:53 pm    Post subject: Reply with quote

You can still light those non-original gauges up at night. If they have the provision for internal lamps then add a 24 volt small lamp to the socket that snaps in the back of the gauge and wire it into a junction splice to the #40 wire. If there is no provision for internal snap in lamps then cut a small slot in the side of the case closest to the stock panel lamps to allow their light into the gauge. If you wish you can glue a thin piece of clear plastic over the slot.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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