Making a 24 volt wiring harness for an M38 jeep.

BullRun · Post by **BullRun** » Thu Mar 14, 2013 9:56 pm

My early M38 was converted to 12 volts but came with many of the original 24 volt parts. It even has a 24 volt fuel tank sender that was leaded into a CJ3A fuel tank. Very clever job! Tank is like new too!

After pricing new 24 volt M38 wiring harnesses (around$700) I have decided it would be better for my rebuild (not a restoration) to cobble together NOS, hand made and repaired parts for the harness saving me about $500.

I am mostly concerned about swapping in a 24 volt distributor. Anything not work out that the manual says?

Anyone do this with the early triangular faced light switch and have suggestions or input.

The idea is get it back on the road asap instead of 3 to 5 years from now.

Post by **wesk** » Thu Mar 14, 2013 11:26 pm

I've rebuilt and /or modified and adapted many GI harnesses over the years. If you have a good head for electrical wiring it's a piece of cake. If you don't its a long, painful experience during which you will make many mistakes and often smoke a lot of jeep. It's a tough choice but if you want something that looks real and is 24 volt and waterproof, you will need to buy that $700 harness or you will need to take several important steps:

1-Stop and read all the book's info. TM 9-8012, TM 9-1825B (1952), TM 9-1825E (1952), TM 9-8627 (1953), TB 9-2300-228-20 are the starters.
2-Learn how to read & use the GI wire numbering system and learn what a pinout is and how to use it.
3-Print many of the wiring diagrams both full system and individual systems and use them as your work sheets making indications on them of what you did, how you did it and where you did it.
4-Invest in a good quality VOM and basic electrical hand tools.
5-Acquire an adapter test lead set.
6-Decide first which M38 harness you are going to build (there were about 6 electrical wiring changes during production), and what upgrades if any like turn signals, or a 60 Amp GI alternator, or a 24 V 60 Amp civvy alternator and so on.

I am mostly concerned about swapping in a 24 volt distributor. Anything not work out that the manual says?

I am not 100% sure of your meaning here but a 24V distributor will replace the old civvy distributor and coil and you will run a single wire #12 from the ignition switch to the distributor.

Anyone do this with the early triangular faced light switch and have suggestions or input.

There were three types of light switches used on the M38. Two are the triangular type with the early one having a large and small receptacle pointing downwards and later having a large receptacle and a blank face where the small receptacle was pointing downwards. Then the last switch type was the simple oval shaped switch with one large receptacle pointing straight out the back. All three use the same large receptacle. That 1st earliest switch had the second smaller receptacle just to car a separate harness to the rear for the trailer plugin.

Here's the three stock choices for the M38.

The new style push button plug is in use today and fits the same hole in the dash and uses the same plug.

This is a pinout for the large plug on the light switch and includes a functional view of the switch innards.

The applicable portions of TM 9-1825E 1952 for the two early light switches are here:
http://www.willysmjeeps.com/v2/modules. ... _album.php

You should browse thru the several hundred technical photos in my album as a primer:
http://www.willysmjeeps.com/v2/modules. ... _album.php

TB 9-2300-228-20 is here:
http://www.cj3a.info/sibling/milgauge/milgage.htm

Don't forget all the free PDF copies of manuals on our downloads page:
http://www.willysmjeeps.com/v2/modules. ... =Downloads

evanso1975 · Post by **evanso1975** » Fri Mar 15, 2013 1:33 am

Do you have the early (2-plug) light switch? If so, Brent Mullins has some front harnesses on Fleabay at the moment:

http://www.ebay.com/itm/Jeep-Willys-M38 ... 1a&vxp=mtr

Although the wires are colored rather than the original black, for $25 they're certainly worth it. Will only work with the 2-plug light switch though.

Post by **wesk** » Fri Mar 15, 2013 8:42 am

The $25 is reasonable but the $19.55 shipping is a bit high. $10 would be more correct.

BullRun · Post by **BullRun** » Tue Mar 19, 2013 5:59 am

Thank you Wes and Owen for your very generous sharing of information!

I do have the early front harness from Brent. Also I have the early 2 plug light switch pulled off a parts M38A1. Hopefully it still works!

I was concerned about the distributer because I need to get a 24 volt one and I have found that some of the NOS ones going around are actually reject parts that were originally not assembled correctly.

Some of my 24 volt parts have been converted to 12 volts so lots of inspections are needed.

The jeep runs and drives great but without repair has great potential to be a rolling fireball so I really appreciate the help!

Post by **wesk** » Tue Mar 19, 2013 8:21 am

What is the serial number of your M38 and your name? I want to review my M38 survey form list. You can PM or email me if you like with the answer.

Post by **wesk** » Tue Mar 19, 2013 8:23 am

This is the wiring diagram to use with that early front harness.

I assume you do not have the harness from the small plug on the switch aft.

ECVJeeps · Post by **ECVJeeps** » Tue Mar 19, 2013 8:56 am

I made a copy of the wiring harness for the M38CDN that I found helpful, possibly you could use a bit of information from my drawing in your harness?
I realize that there is a difference between early and late versions; I just found that the originals were confusing and I used different colours
along with the numbers to clarify my drawing as much as possible.

I had my copy laminated for use in the garage.
Pat

BullRun · Post by **BullRun** » Thu Mar 28, 2013 8:39 pm

I really appreciate the interest and help!

I have been going over the jeep and discovering things that either make it easier or harder to repair based on your veiwpoint.

The jeep is/was I believe a Follow Me jeep when it left the military/government and has a rebuilt RMC engine. So the body is early but the drive train is late (I think). It has a screw on transmission knob. 40lbs oil pressure being driven and 20lbs at idle. First junked military jeep I have ever found without a worn out engine. Really got lucky with this one.

I fixed the fuel system so it runs without a jerry rigged fuel tank and lines. The fuel tank is a CJ 3A put in the jeep by the military as it had a 24 volt sender. I resoldered the tank fittings that leaked and repaired the tank fuel sender opening so that either a 24 volt or 12 volt sender can be used.

I have been going through my parts and have just about all the 24 volt parts but have to verify the year of the power train. There has been no maintenance on the jeep over the years as I replaced the original oil lines from the last rebuild from probably 50 years ago. After a new fuel pump, an oil change and hooking up an original air cleaner I was able to drive it around a field (no brakes)for about a half hour checking everything out. Who knew there was so much other work to making a wiring harness?

Pretty amazing seeing this old jeep come back to life!

I'll get that information to you Wes.