Delco charging system problem

I have a Gradall 534B which has a typical Delco Remy charging system (internally regulated) identical to a million old GM cars. I have been having a discharge problem with the machine for some time now and have been unable to get to the bottom of it. When I disconnect the sensing wire from the alternator, the discharge disappears, reconnect and there it is again. I tried disconnecting the sensing wire and running another around it to eliminate it as the problem, but no luck, the discharge was still there. I had the alternator tested and it's fine. I replaced the alternator with new just in case and that made no difference. What the heck am I missing? Incidentally, in getting to this point, I had disconnected virtually every wire in the machine with no change until I unplugged the alternator. I've reached the end of my capabilities and figure that one of you guys may be able to set me straight.

I though that wire was supposed to be switched? Is there a relay somewhere in the sense wire circuit?

A typical delco system can have the sensor wie go either directly to the alternator battery terminal or, as in this case, to the remote ford style relay, where it also joins the main battery wire coming from the starter and a wire that goes back to a separate terminal on the starter. If I disconnect the sensor wire from the relay then the draw stops. If I then hook it directly to the battery wire coming from the starter and touching nothing else, the draw resumes.

Ok, when you say sensor wire, you mean the larger, usually red wire on the plastic plug, right? The other wire on the plastic plug, (smaller, usually white) should be switched (usually through the alternator light). If you disconnect this wire does the drain stop?

Yes, the larger wire. The small one in the same plug is the exciter wire.

Ok, good, I just wanted to make sure we were using the same terminology.

What happens when you disconnect the exciter wire?

How did you test to determine that the parasitic drain was there and then went away? By putting a volt meter from the battery cable to the battery post? Did you try that with a ammeter?

Nothing. Only when the sensor wire is disconnected does the draw stop.

I'm sorry, I didn't answer the whole question. I have a voltmeter,between the negative terminal and the post and it registers 9.5 volts which I don't quite get why it's not a full 12v. I haven't tried an ammeter, but know that if the machine sits for about a week the two healthy batteries will go dead.

The way I tried to think of the problem was that in order for there to be a continuos draw, the culprit has to have access to power even when the key is off. I started by disconnecting the dash which is easy because it has two plugs that take care of the whole thing, so theoretically, there should be no power to any sensors, relays or solenoids. I then went to the primary wiring and followed the battery cable to the starter and from the starter to the starter relay, then the alternator. So I found it or maybe not? I can't quite see what I'm missing here, but I know it's staring me in the face!

Have you tried unplugging the switch only and testing?

Yes. The dash can be entirely disconnected with two plugs which I did without any change.

Sorry this reply is late but I have had this problem on other pieces of equipment with Delco internal regulated alternators. A diode in the rectifier bridge is breaking down. That would also explain the 9.2 volts instead of 12 drain. The alternator output will test normal on the bench or in a machine. But when it is stopped it will pass a small amount of current. Sometimes enough to kill a battery over night.

To test the rectifier bridge you'll need to disassemble the alternator and remove the rectifier bridge. The rectifier bridge has the three leads from the stator attached to it. There are actually 6 diodes in the rectifier bridge and they have to be checked individually. After removing the bridge locate the thin copper tabs attached to the three studs coming out of the center of the bridge. carefully bend the tabs away from the studs so that none of the tabs are touching each other. Each tab is attached to a single diode. Test the diodes with an analog meter or self powered test light by touch one lead to a tab and the other to the side of the heat sink the tab comes out of. Then swap the test leads. You should have meter movement or the light come on in one direction but not the other. I think you will find that one of those diodes will pass current in both directions.

Thanks, I pretty much had given up! I'll give it a try in a couple days. Right now I'm knee deep in a drive motor problem on my Bobcat t190. Seems like every piece of junk I own has decided this fall is the time to take a sh**!!

Replaced more than a few alternators for parasitic draws. But the alternator's already been replaced. New doesn't mean "good" but unless he got his rebuilt and the rebuilder missed the bad rectifier.

turn the 2 wires in that small plug that plugs into the alternator around,your problem will be solved ,you must have switched them at some time or another.

john hofer said:

turn the 2 wires in that small plug that plugs into the alternator around,your problem will be solved ,you must have switched them at some time or another.

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Thanks for your input. I replaced the alternator and the problem was solved, even though I had replaced in the past. I had thought of your solution earlier and traced the wires to be sure they were properly located, which they were.

Thats good.... thanks for reporting back to us and finishing this out.