Posted: Wed Jul 04, 2018 5:19 am Post subject: Circuit breaker?
Hi,
I am the proud owner of a 1952 M38 bought in Crete, Greece last year.
Itīs a good rebuild by the former owner, but Iīm having problems with the gauges. The ammeter and the oil pressure were functioning when I bought it, but ceased operation after that. The fuel and the temperature never worked. I tried to change gauges, and I have ordered a new oil pressure sender and the temperature sender, but I am wondering if one of the circuit breakers could "shut" all the gauges down?
As I am living in Sweden itīs a small challenge to aquire parts (and it takes time...)
Thankful for your expertise!
Tom
The M38 instrument system is a robust yet sensitive system.
There are several important basic requirements that must be met when dealing with them.
1 - Grounding. These M38's were originally produced with ground straps where needed and bare metal contact areas that insured good grounding between metal surfaces that rely on mechanical fastening to provide a ground. Over the years corrosion, too much paint and missing ground straps have created many problems with these gauges. The gauge housings must have a good ground contact with the instrument cluster mounting panel. The instrument cluster mounting panel must have a good ground with the dash. The engine must have that ground strap at the right forward engine mount to insure the two senders have a good path to ground. The fuel tank must have a good path to ground for it's sender to function properly. In many cases removing paint for better contact is more work then just adding a special ground strap for that item.
2 - The M38 instrument system complexity is further aggrevated by the need to match early gauges with correct resistors and both early and late gauges with the correct matching sender. Early M38's thru serial MC17855 had external resistors on the power lead connections to reduce 24V to 6 Volt. After that serial the M38 used gauges with internal resistors. You cannot mix and match these! The Army contributed to this matching issue when they converted most of the M38 fleet from 100 PSI oil pressure gauges to 60 PSI oil pressure gauges. Each of these use a different sender. Be patient and study your gauge and sender part numbers and compare them to the m38's ORD 9 SNL G-740 parts manual.
3 - The oil pressure, water temp and fuel quantity gauges all get their power from wire #27 which is called the spider harness. Early M38's thru MC65042 had a Circuit Breaker (CB) mounted on the instrument cluster and the #27 spider harness connected to it. Power was supllied via wire's #10 & #11 thru the ignition switch to the 27 CB and harness. To further complicate this the early M38's thru MC65927 had three circuit breakers (CB) on the inboard side of the cowl battery box. One for the auxilliary power receptacle, one for the Bendix type light switch and one for the horn.
Do not rely on the dash plate serial number to decide how your M38 is equipped. A visual inspection is quite necessary beacuse you should always remember that the two serial plates on an M38 were attached only with screws and were easily switched from one M38 to another!
4 - Finally you must carefully examine each of the Douglass connectors in your wiring harness. These have a strong tendency to become short circuits to ground when the wires in them and the rubber grommets in them deteriorate with age and their metal shells contact a metal surface or another Douglass connector's shell.
5 - The Army did help a bit with a TB that covers most of the troubleshooting guidelines you need to follow with the gauges.
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.
http://willysmjeeps.com/downloads/instruments.pdf
6 - Ammeter uses wires #8 & #9 from the voltage regulator where the ammeter's shunt is located. If you have a jeep converted to a transistorized voltage regulator then the ammeter is removed and replaced with a volt meter since there is no shunt in the transistorized regulator and wires #8 & #9 are not used. Instead the voltmeter is connected to the #27 wire spider harness.
I am overwhelmed of your kind and fast fact-filled reply!
Chrystal clear pics compered to my blurred ones I have in my TM reprint...
Iīll have too roll up my sleeves and get going!
The serial # at my jeep is 101921. It has been used in the Greek army and was saved/rebuilt by a retired greek MP captain at Crete.
He didīnt speak english, and my greek is limited to a few words, so it was exciting to negotiate the prize...
Glad to help. You serial number is an odd one. The US M38 production at Willys ran from Sep 1950 thru June 1952. The first serial was MC10001 and the last was MC72329. Perhaps you can post photos of the data plates on the dash and on the front end of the right rear wheel house?
I am trying to find out how to attach a photo to my reply....
The wheelhouse data plate is missing but the rest of the plates on the dash is there.
Will post a couple of pics when I can.
You are aware that the photo you wish to post cannot be placed directly in your post from your computer? It must be currently on a web site IE a photo storage web site like photo bucket or our member's albums on our web site. Also they must be open to the general public. If access to your stored photos require a pass word to access them they can't be posted because our website is blocked when it tries to access the program to pick up the photo. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Good looking jeep. That plate is clearly an overstamp. The serial stamping should have matched the standard Willys stampings of the period.
Your stampings are way too large. I would guess it was an added Greek serial. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Do anyone know if the Greeks bought M38s directly from Willys, or did they buy surplus jeeps from the US later? Or some kind of Military Aid Programme?
I am wondering why they would use the original data plates in english, instead of putting greek data plates on them.....
Maybee, we will never know.
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