Ammeter problems

citabra · Post by **citabra** » Sat Sep 13, 2008 2:52 am

Got my 1953 M38A1 back together and running, but cannot get the new ammeter's needle to move in either direction. I have tested the ammeter and it checks out on the bench. Both wires 8 and 9 individually reflect the battery voltage. The generator is working and will run the jeep without the battery. The wires are not shorted out anywhere so I think something is wrong in the voltage regulator. I have a VBC 400 2UT. Would this be something I could fix myself or should I take it to the electrical automotive shop?

Post by **Ryan_Miller** » Sat Sep 13, 2008 9:26 am

I would take it to an automotive shop. Many of the larger trucks are running 24 volt, so they should be able to test it. Go to the downloads section under M38 electrical. I put the most recent update to the voltage regulator diagnositic manual with the expected test values.

If you want to do testing yourself, look into getting one of the surplus test adapter kits and the testing unit.

Voltage regulator parts are available, but can be hard to find.

OKCM38CDN · Post by **OKCM38CDN** » Sun Sep 14, 2008 6:23 am

Are you running a breadbox style regulator or the newer solid state version?

Newer solid state version requires a voltmeter instead of the ammeter...

Just asking to be sure ...

citabra · Post by **citabra** » Sun Sep 14, 2008 11:21 am

This is the old breadbox mechanical VR VBC-4002UT. I have downloaded the Autolite maint. information (TM9-1825) for comparison. I have continuity between the B and C pins on the VR. My unit acts as if the shunt has been modified so that the #8 and #9 circuits would connect to a voltmeter instead of a Ammeter. Has anyone every heard of the Military retrofitting a mechanical VR to use a Voltmeter?

Post by **Ryan_Miller** » Sun Sep 14, 2008 1:41 pm

If it was modified, the most likely would be after military service.

The military had thousands of the new solid state versions. It is a simple bolt in unit.

Taking the 4002UT apart and modifiying it would be a lot more work.

Most likely the springs/contacts are worn out on the 4002UT unit.