My M38 Ammeter Episode

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
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dpcd67
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My M38 Ammeter Episode

Post by dpcd67 » Mon Jul 16, 2018 5:57 am

Hooked up my M38 electric system after making the wiring harness from an M35 one. (That takes patience). Everything seemed to work fine until I picked up the gauge panel that was laying on the cowl, and it sparked when I picked up. Not good. I began testing all the connections and I found that the Ammeter housing was 24.7 volts and so was the gauge panel. Disconnecting wires 8 and 9 fixed it. Engine not running.
Now, for you guys who have later vehicles, Ammeters are a recipe for disaster. If they get frozen up inside they will burn your vehicle up. And they do not provide much useful information anyway.
So, I am going to put a red, yellow and green Volt meter in it. I like them; OK, not original. Whatever.
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Re: My M38 Ammeter Episode

Post by Jeeps4Brains » Mon Jul 16, 2018 6:40 am

So you were doing this work without disconnecting the battery?

It sounds like the Amp gauge was shorted to the body of the gauge.

If I remember correctly one terminal gets the + of the battery and the other terminal gets the output of the generator.

After what you describe it makes me want to pull my NOS Amp meter I plan to use in my early M38 sometime and test it.
45' MB, ??' MBT, 47' CJ2A, 48' CJ2A,
51' M38 #1, 51' M38 #2, 51' M100,
52' M37, ??' M101A1 (1st Gulf War Vet),
53' M38A1, 53' M211, 65' M151A1, 67' M416,
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Re: My M38 Ammeter Episode

Post by dpcd67 » Mon Jul 16, 2018 2:26 pm

Of course the batteries were connected; I was testing my new wiring harness. At some point you have to connect them. The wires to the ammeter come from the voltage regulator; #8 and 9. Yes, the amp gauge was internally shorted, and grounded to the gauge panel too. This all is a new installation of parts that were on the jeep when I got it, after having sat in the water for 40 years; I am not surprised.
The needle is frozen; should have been a tip off; they usually will move when you shake them.
Old ammeters are not to be trusted. There is a reason we quit using them.
And my dimmer switch does not have a low beam either, #17. High and nothing. I will replace that as well. This M38 was a basket case; all new now.
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Re: My M38 Ammeter Episode

Post by Jeeps4Brains » Tue Jul 17, 2018 4:11 am

I will keep this in mind when I get to my M38. Not much military left on the old girl. I have collected an AMP gauge as wanted to go back original. I will have to test it the next time I dig it out. :roll:
45' MB, ??' MBT, 47' CJ2A, 48' CJ2A,
51' M38 #1, 51' M38 #2, 51' M100,
52' M37, ??' M101A1 (1st Gulf War Vet),
53' M38A1, 53' M211, 65' M151A1, 67' M416,
MVPA #31724

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Re: My M38 Ammeter Episode

Post by dpcd67 » Tue Jul 17, 2018 5:33 am

Yes, when I restored my Barracuda, and they have ammeters too, everyone said, splice the wires together and bypass it; recipe for burning up your car.
I have them on my GPW and WC63 though......
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Re: My M38 Ammeter Episode

Post by mdainsd » Tue Jul 17, 2018 7:48 am

I posted this response on willys M jeeps to the OPs post over there. The ammeters can be made safe.

"Yes, the ammeter in these vehicles is a recipe for disaster (electrical fire).

Both leads #8 and #9 are effectively connected to the vehicle batteries all the time. Any issue with insulation of these small #14 wires and the results can be BAD.

Myself, I prefer the originality of the ammeter, but have to lose some originality to make them safe. First, they are really voltmeters that are reading the small voltage drop across the shunt (A piece of solid core wire) in the mechanical style voltage regulators. I have interrupted both wires very close to the regulator and added an inline fuse to each. In reality one fuse "would work" but there is a risk to burning out the ammeter in the instance of the wires shorting to ground. Fuse rating is very low, 1A. This fuse modification could probably be done inside the regulator itself, but I didn't do it that way. The way I did it I can change out a stock regulator for another stock one and all works."
52 M38 2X, '52 M37, '44 WC51, '42 WC 56/57, '50 CJ-V35(U), '42 GPW, '44 M3A1 Scout Car, 2010 M-Gator

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Re: My M38 Ammeter Episode

Post by dpcd67 » Tue Jul 17, 2018 1:18 pm

That is a good safety measure you did. But mine is already toast and I have a volt meter, and I like the red, yellow, and green face. And I can install an ammeter any time want if I ever did.
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Re: My M38 Ammeter Episode

Post by Radtech » Tue Jul 17, 2018 3:44 pm

Did you test the gauge before you put it in?Put an ohm meter across the terminals and check the resistance.Use a AA battery across the terminals to check the movement one way then reverse the process to test in the opposite direction.Check continuity from + to the case then - to the case.If the meter movement locks up it will not burn your vehicle it just means the needle will not move.The only way it can short to ground is if you drop it and break the insulation around the terminal.There is nothing inside that can break and short to ground except the insulation around the terminals.They have ceramic insulators between the metal can and the meter movement and terminals.

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Re: My M38 Ammeter Episode

Post by dpcd67 » Tue Jul 17, 2018 7:28 pm

Did I test it before I put it in? No. The needle does not move at all, powered or not. And when I had it hooked up, it was hot, meaning the case has 24.7 volts on it. Something inside is making it do that. When I test the terminals with my Ohmmeter, I get 15. Like a solid connection. I know nothing about these, but, I ain't using it.
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Re: My M38 Ammeter Episode

Post by Radtech » Wed Jul 18, 2018 9:42 am

One of the two terminals is shorted to the case.This causes one or both of the needle rotor bearing surfaces to freeze the needle due to the heat.

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Re: My M38 Ammeter Episode

Post by dpcd67 » Wed Jul 18, 2018 2:54 pm

Yeah, and it looks perfect from the outside. No rust or corrosion at all. Inside must be nasty.
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