The Stovebolt Site Forums The Shop Area The Electrical Bay Charging problem - 40 GMC

Now that I have driven my new-to-me 40 GMC a few times now, it seems that the charging system is not working. I'm getting no postive charge at the ampmeter, but when I turn the lights on it shows discharge. Also a small discharge when its running.

I only have a 1939 GMC shop manual, not a 1940. I read the electrical chapter and it shows and discusses a regulator with 4 terminals - "Ignition", "batt", "Gen" and "F". I went and looked at mine and it only has 3 - "batt", "Gen" and "F". It doesn't look like any wires are cut off. I read somewhere that in 1940 they went to a larger generator, could that include a different 3 terminal regulator?

So I'm a bit baffled about how to proceede with troubleshooting. I think I can still ground the field output to see if the gen. is working - right? Is there a good clear wiring diagram online somewhere for a 3 terminal regulator?

I know, I need to get a '40 manual, its on my list.

This is a Canadian built 1 ton pickup with the stock 6v, negative ground system.

I'd appreciate any advice.

Steve

Steve - 1st things 1st - the amp gauge ain't the most accurate thing ever designed - run the batt down a bit, like lights on awhile, then start it and see if it shows some charge - both my trucks show what appears to be a slight discharge under normal conditions, and more discharge at idle when there's elec stuff on, but shows a charge when the batt is low and needs a charge, like if it doesn't start right up and takes a minute of cranking - gauge quickly goes back to neutral position

best to use a meter to check the gen rather than shorting anything out if you've started and driven it a number of times, I'd expect the battery to be gettin low if it's really not charging

the 3 connections on yer regulator is correct

Bill

The regulator has 3 terms. There should be a ground wire to the base. There possibly is 2 wires going to the bat. term. Remove the wire to the field term at the regulator & ground it. The generator should show full charge. Do not run it long or very fast this way. If it shows a charge the regulator is bad or defect in the wireing. If no charge the gen. needs rebuilt.

Well I just went out and made some checks.

I removed the field wire from he regulator and grounded it, then ran up the engine a bit, made no change on the ampmeter. Then I checked for voltage at the GEN terminal and had nothing. Finally, I checked for voltage at the BATT terminal and got 6.12 volts.

The ampmeter definatly shows a small discharge when the engine is idling, then goes back to the neutral position when shut off. I'm sure these ampmeters are not real precise, but it seems to be working well enough to show current being drawn from the battery to run the ignition system.

Sounds like its pointing to the generator, huh?
Thanks,
Steve