willysmjeeps.com

1951 M38 number 34347. Does anyone know what this is?

Location is under center of dash, against the firewall slightly to the right of center, behind circuit breakers. (you can see position in relation to starter switch)

Trying to chase down an issue with 2 gauges not working - Temp and Oil Pressure. replaced both sensors (as well as thermostat and water pump), got replacement Temp gauge as well which also doesn't work. I think it's electrical in nature.

Neither gauge has worked since I got the M38 fall of last year.

Hi Free,
It's the auxiliary power plug on the firewall. Pretty sure it's listed in the ORD9.

Most agree it's to plug in a trouble light like this one: https://www.ebay.com/itm/154793765919?h ... SwdOxhWMq2

Can't help with the gauges, not enough information.

Welcome to the race!

you should check continuity on your wire harness and spider harness at the gauges.

Thanks Ron and thanks Ken.

I'm at the continuity stage now, any good info on how I should do this? Is the spider harness something that goes bad?

Also, there is a circuit breaker on the back of the instrument panel, but PO had no wires going to it, bypasses it. fuel and battery gauge work, off the existing spider harness, temp and oil pressure don't. very odd.

much appreciated!

have you looked at the downloads section here ? page 2 has the trouble shooting section on gauges and senders...

Check to make sure the cases are grounded well. All the way to the tub.

FreeinFl,

1st let me mention most of us here are on a regular old fashion first name basis. You can join us by going to your account page and creating a signature element. Most folks include their first name, year make model Willys so their known more personably from then on and the year make model of your discussion topic is readily seen in all your posts.

2nd as mentioned above every manual you should have on your work bench is listed as a free PDF download on our web site's Downloads page. Just click on the word "Downloads" at the top center of the page.
http://www.willysmjeeps.com/v2/modules. ... =Downloads


3rd My photo album is the second album in "Member's Albums" which is in the link right next to the "Downloads" link called "Photo Gallery". In my album are over a thousand technical photos that will answer 90% of questions asked here.
http://www.willysmjeeps.com/v2/modules. ... _album.php

For example if you look in my album under the sub-album "M38 Electrical System" you will find these little gems:


That little dodad you posted a photo of is seen dead center in this early M38 wiring diagram as "Used up til MC47771"


Here it is ID'd as "Aux Power Plug"

The Military also published a general M series gauge troubleshooting guide: http://www.willysmjeeps.com/v2/modules. ... tit&lid=46

For a very long thorough list of Military and civilian manuals that can be used on the M38 just download this list:

http://www.willysmjeeps.com/v2/modules. ... tit&lid=40

A few hints on gauges and senders for your M38's.

1 - There are early gauges with external resistors attached to the back of the gauges. The later gauges had internal resistors.


Early cluster with external resistors.


2 - Gauges are matched to their senders. Make sure you review the ORD 9 carefully when ordering any replacement gauges or senders. Instrument panel parts list starts on page 149 section 0607. Senders are found on page 163 section 0610.

3 - Your biggest enemy with gauges and senders are poor grounds. Old jeeps have a ton of paint in many layers. This paint covers dozens of originally left bare spots for fasteners and gauge mountings to insure good grounding. Gauge housings must be bare where they contact the instrument cluster and the "U" clamps on the back must be bare where they touch the gauge housings and where they touch the cluster. The four 1/4 turn fasteners that secure the cluster to the dash must be paint free, they must have a paint free contact area on both the cluster panel and the receptacles in the dash. The engine mounted senders (oil press & water temp) must have paint free contact surfaces on the sender housing and the block. The gas tank sender must have paint free contact areas between sender housing and fuel tank. Fuel tank must be grounded to tub. Tub must be grounded to frame. Engine must be grounded to frame at right front mount. I personally always add a ground (bonding) strap to the instrument cluster and attach the other end to the firewall.

Over the years as supply stocks were reduced many jeeps were converted to different gauges and senders. You must investigate this carefully and choose wisely. For example the M38A1 came with a 60PSI oil pressure gauge & sender and eventually most earlier M38's were converted from 120 PSI to 60 PSI as well.

Does the auxiliary plug on the M38 have a female fitting (to accept a plug with a male bullet terminal) or does it have the male bullet terminal? I have seen both version as loose parts but never sure which was mounted on the M38.
The trouble light shown by Ron has a connector that simply plugs into a light bulb socket, not the style of plug that screws onto the M38 auxiliary power plug.
Early M37 also had this auxiliary plug. I guess there were so few applications than had the correct mating plug that the Army decided to discontinue this feature. ... Brian

Thank you so much Wes. seems I have alot of digging to do, not have a great history of this M38's upgrades over the years.

I tried creating a signature element, but keep getting an error message that I'm missing required fields. I'll keep working on it.

Thanks again
Phil P

There are a couple of things you can do.

1 - Download our M38 data base which shows many of the upgrades at their serial cutoffs.

2 - Download a copy of our M38 survey form and submit the completed form to my address.

About 80% of all upgrades and their serial cutoffs are listed right in your ORD 9 SNL G-740. The rest are usually only found on factory drawing/blueprint changes.