Possible Alternator short

[nakins]

I'm trying to determine if my alternator has a short. I took my truck to a parts store where they did a clamp on test while the engine was running. It showed the alternator to be OK with regard to voltage and diodes. However, prior to me taking it there, I had disconnected the red wire from the alternator, placed a volt meter between the red alternator wire that goes back to the battery and the stud on the alternator where that wire connects to and read 12.5 volts. Now I know the the wire will be hot normally when it is connected to the alternator, and there will be a 12 + volt reading if testing between the wire/stud and a ground. But there shouldn't be any reading when testing "inline" with the engine off, correct? What this seems to be telling me is that the alternator is shorted to ground and draing the battery when not running, correct?

[gmtech79]

No you should always have battery voltage at the alternator connected or not. You need around 14.7 volts there with the engine running (depending on the battery state of charge, you probably would have more like 14 volts.).
There is no need to remove the charge wire to check voltage, just check it on the alternator, if you didn't have B+ that would mean you have an open wire or open fusible link.

[MT-2500]

To test for a short you need to run a amp draw test with a good amp/ohm/volt meter.
Most good alternator on car testers have a diode short test.
What is the battery and alternator doing or not doing.
Is the battery going down sitting or running?
MT

[richtazz]

With the wire removed and a jumper between it and the stud, there should be no volt reading with the ignition off. If you do, there is an internal short in the diode/rectifier bridge assembly in the alternator. If you test the end of the wire to ground, you should have battery voltage at all times.

[gexter]

I have a 1990 GM suburban diesel
I noticed this old thread trying to look for issues that I have been having because I am not having much luck with other forums.
The answer I am looking for is why when my alternator is connected to the battery it drains it. when I pull the charge wire off the battery does not drain.
I also get a fluctuation of 15.11 to 15.55 volts when it charges and the lights flicker.
when I pull the charge wire from the battery that goes to the alternator and put a continuity tester between ground and the alternator charge wire I get continuity. In other words the charge wire that comes from the battery is a ground wire when the ignition is off, when the ignition is on I get 12 volts when off it becomes a ground.
If I take the alternator charge wire off and with the ignition key off and a test light between the charge wire that normally would go to the battery and positive on the battery I get a full brightness on the light.
Is this normal?
With the alternator charge wire reconnected the wire that is going to the ignition from the alternator that plugs into the alternator is a flat ground when the key is off but 12 volts when on.
Does this mean my diodes are shot and the ignition wire that is ground when off is going through the alternator and killing my battery.

Any takers on this forum?
I am right in thinking the alternator has a bad diode and draining the battery to ground though the ignition wire?

Thanks in advance!
Gexter

[j cAT]

welcome to the forum.

you should start a new post with your issue.

your problem is quite common . the alternator has full wave rectification. this causes the output voltage to be almost solid positive steady voltage . when a diode shorts out this then causes the output to be 1/2 wave rectification . this has voltage drop outs basicly only 1/2 the total power from the alternator. this causes lights to flicker, drains the battery thru the alternator shorted diode, and can cause with newer vehicles weird tranny shifting..

whenever the alternator draws current with the engine off replace the alternator.

reverse polarity or high current electrical equipment will do this

charge battery before operating the new alternator

[gexter]

Thanks j cat.
That pretty much answers all my questions and then some.
Thanks for the tip on starting a new thread and thanks for answering my question.

All the best.

Gexter.