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Thread: M-37 Electrical draw with everything off

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    Default M-37 Electrical draw with everything off

    Hello. I'm new to the forum but it sure seems like this is the place to go for all things to do with military vehicles.

    I'm working on a 1951 M-37 for my Uncle and I have a current draw of about 8 A even with everything off. I'm new to military 24 V systems but I'm fairly experienced with antique trucks otherwise.

    Checked so far all with no change:

    1. Gen to regulator harness disconnected. Regulator housing opened -- all three relays look OK and do not heat up.
    2. Light switch disconnected
    3. Turn signal unit disconnected
    4. Harness from Bat. box to bed (for bed mounted radio?) disconnected.
    5. Harness to radio plug near glove box disconnected.
    6. Ignition switch disconnected.
    7. Harness from firewall to distributor disconnected
    8. Brake switch disconnected at master cylinder
    9. Horn disconnected at horn

    Perhaps it was foolish but I also left the batteries hooked up and fire extinguisher at the ready (with the 8 A or so flowing to who knows where) for an hour and just constantly inspected and felt every harness and connector I could get my hands on to see if something heated up -- nothing did. The wiring isn't perfect but it's not horrible either. Of course, most of it is in conduit for waterproofing purposes so there could very well be some bad bits that I can't see but with only an 8 A load and nothing burning out I was thinking something is 'on' vs. shorted out.

    Any ideas or advice would be greatly appreciated.

    Thanks in advance.

    Pete P.
    Harborcreek, Penna.

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    4 Star General NDT's Avatar
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    Welcome. Seems like the only remaining item you have connected is the voltage regulator. There is a large noise suppression capacitor inside, sounds like it may have degraded and is passing electricity.

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    The M37 starter switches can carbonize up after years of use and cause current draw. Disconnect the battery lead from the switch and see if it goes away.

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    Ditto on the starter switch. I had to take mine apart and scrape all the carbon off. After that the contacts looked good. Starts better now too. I have seen the contacts get pitted. The contact switches are available so you might could just replace it.

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    Don't know if your uncle is a purist or not, but I wound up dealing with such a situation on my '51 M37 by installing a master battery disconnect switch. And, ultimately, by going with a 318 and all new electronics under the hood. Heh. One way or another that bug was squashed.

    If your uncle hasn't heard of them before, uglytruckling.com makes some interesting M37 mods. I have their disc brakes (almost a requirement when you repower it) and their 318 engine mounts. Can't say enough good about their stuff from back in the day.

    Good luck!
    SJ

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    Pulled all four wires off the ignition switch and no change so I'm not sure if the switch is actually good but it's not the culprit at the moment. Pulled the whole regulator ass'y out and lo and behold the draw went away. Pulled the relays out and found the capacitor that NDT mentioned. Since there was still a draw with the gen. disconnected from the regulator but no draw with the regulator itself disconnected from the truck I'm liking the capacitor for the fault.

    There are (unfortunately) no radios currently on the M37 and at least for the foreseeable future there won't be. Can I just eliminate the capacitor at least for testing purposes?

    Thanks.

    Pete P.
    Harborcreek, Penna.

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    4 Star General NDT's Avatar
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    Quote Originally Posted by Pete P View Post
    Pulled all four wires off the ignition switch and no change so I'm not sure if the switch is actually good but it's not the culprit at the moment. Pulled the whole regulator ass'y out and lo and behold the draw went away. Pulled the relays out and found the capacitor that NDT mentioned. Since there was still a draw with the gen. disconnected from the regulator but no draw with the regulator itself disconnected from the truck I'm liking the capacitor for the fault.

    There are (unfortunately) no radios currently on the M37 and at least for the foreseeable future there won't be. Can I just eliminate the capacitor at least for testing purposes?

    Thanks.

    Pete P.
    Harborcreek, Penna.
    Yes, just make a jumper. Tube radios may have a whine in the speaker, otherwise no issue.

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    Ahh, the old glove box light staying on when the door is closed.... LOL

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