Alternator problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

wdrthird · 11-20-2008, 06:04 PM

The single wire alternator on my '71 didnt seem to be charging like it should so I tested it and found it was only putting out 11.5 volts. I had it rebuilt and the guy said it had a bad rectifier. I put it back on, drove the truck once then tested it, again it was only putting out 11.5 volts. Took it back to the guy who rebuilt it and he said it had the same problem again. He told me that usually when it burns up diodes like that it means theres a short somewhere. So I unhooked the positive battery cable and attached one lead of my digital multimeter to it and the other to the positive post of my battery and it showed there was a draw with the truck turned off and all the accessories off. I had the wife watch the meter to tell me when it went to zero while I went to start pulling fuses to find which circuit was creating the draw. When I opened the driver side door, the meter went to zero, so Im guessing the door jamb switch was shorting out against the door and when I opened the door it broke the connection and killed the draw on the battery. I opened and closed the door a bunch of times but the meter would never show a draw again after that first time. Would something like that kill my alternator or do I need to keep looking?

JimKshortstep4x4 · 11-20-2008, 08:17 PM

Although a slight draw like the one you have will run the battery down over time it is very unlikely that the alternator would be affected. I would take the alternator back and have him check it out.

Jim

wdrthird · 11-21-2008, 07:35 PM

Quote:

Originally Posted by JimKshortstep4x4

Although a slight draw like the one you have will run the battery down over time it is very unlikely that the alternator would be affected. I would take the alternator back and have him check it out.

Jim

I took it back and he said it was the same diode as before that was bad. He fixed it again, but I havent driven the truck yet because Im not sure where to start looking. I replaced the door jamb switch, but was wondering if that really could be the problem or if theres something else I should look for before I fire the truck up again. Anyone have any other ideas? I hate to fry the alternator again and have to have it repaired a third time.

Security guy · 11-21-2008, 07:47 PM

I'm not that good with the wiring part of cars, but does yours have an external voltage regulator? I've got a '71 that had been sitting for a while. It wasn't charging either and I took the alternator to the parts store and had it checked. They tested it and said it was bad. Bought a new one and it wasn't charging either. Tried tapping (also hitting lightly with a hammer) the voltage regulator thinking it was stuck. That didn't work. Found an old factory manual from the '70's and figured out how to test the regulator. If I remember right, we shorted 12 volt battery to one of the terminals of the regulator, and that 'woke it up' and it charges great now. Worth a try?

Richard8971 · 11-21-2008, 08:01 PM

I was also getting a low charge reading. What I had to do on mine was wire the alt directly to the battery and not go through the wiring harness. Run the charge side of the alt to the poss side of the batt and the neg side of the batt to the alt bracket for a good ground. (I hope the pic helps)

I am assuming that you are using the wiring that went to the "old" three wire alt.

That kicked up my charge by almost 2 volts. Also, I have heard that you cannot hook up the alt when the batt is connected. There is a very real chance of frying the diodes inside.

I don't know for sure that is what is causing it, but just a heads-up.

Good Luck.

Don

Longhorn Man · 11-21-2008, 08:50 PM

First... when doing the draw test between the battery terminal and cable, you should use the neg cable. The reading is the same, but the neg cable is much safer to work with. EVERYTHING metal under the hood is a ground, flopping the cable around is asking for grounding out a hot positive cable
Check your tester now and see if it is working at all. I wonder if you haven't blown a fuse inside of it. Opening the door should have made it read HIGHER... not zero. When you open the door, the dome light turns on, which would require more amps through the cable, and the meter. I suspect you popped a fuse in the meter.
Regardless of that, I question your rebuilder. If there was a short somewhere, then there'd either be a fuse blown from said short, or a wire melting and making smoke.
To a person who works on automotive electronics, short means short to ground. Open circuit means a wire or switch not completing a circuit, which, I can't see either causing a one wire unit to burn up.
How ever, I also don't have any answers for you on the issues either.
Honestly, i would recomend just running a GM 3 wire internally regulated alt. It's not like it is hard to wire, and they are better than a one wire unit.

Richard8971 · 11-21-2008, 09:02 PM

Yeah, what Andy said.

Don

wdrthird · 11-22-2008, 12:01 AM

Thanks guys for the advice and the tip on the proper way to test the draw. It has an internal regulator and Im not using the original wiring, its just a single wire running from the alternator to the battery. The guy who rebuilt it has a pretty good reputation for doing quality work, but maybe this is just a fluke or something. Im not much on electrical sytems so Im kind of stumped as to where to start looking. As far as I can tell, theres no blown fuses, no burning wires and everythings working like it should. My meter seems to be working ok, but just to double check I used a simple test light and it showed no draw today. The first time I had the alt. rebuilt I checked as soon as I fired the truck up and the output was good, so I know the alt. was good when I first installed it, but after driving the truck some it was back to 11.5 volts and only putting out about 30 amps. I have not fired the truck since having it fixed the second time.

Longhorn Man · 11-22-2008, 12:15 AM

the 11.5/30 amps you tested was most likely the battery power, the alt wasn't putting out anything.
I would say make sure the positive wire from the alt to the battery is solid, no splices/bitt connectors, and has tight clean connections, then also clean the point between the alt brackets and the block, removing all paint and corrosion... maybe even a ground strap/wire from the alt to the battery like mentioned before, and fire it up... see what happens.
The more I think about it, the diode probably blew from an on again/off again condition... a sporadic connection on either the positive or negitive connection.

11-20-2008, 06:04 PM	#1
wdrthird Registered User Join Date: Dec 2007 Location: Ponca City, Oklahoma Posts: 229	Alternator problem The single wire alternator on my '71 didnt seem to be charging like it should so I tested it and found it was only putting out 11.5 volts. I had it rebuilt and the guy said it had a bad rectifier. I put it back on, drove the truck once then tested it, again it was only putting out 11.5 volts. Took it back to the guy who rebuilt it and he said it had the same problem again. He told me that usually when it burns up diodes like that it means theres a short somewhere. So I unhooked the positive battery cable and attached one lead of my digital multimeter to it and the other to the positive post of my battery and it showed there was a draw with the truck turned off and all the accessories off. I had the wife watch the meter to tell me when it went to zero while I went to start pulling fuses to find which circuit was creating the draw. When I opened the driver side door, the meter went to zero, so Im guessing the door jamb switch was shorting out against the door and when I opened the door it broke the connection and killed the draw on the battery. I opened and closed the door a bunch of times but the meter would never show a draw again after that first time. Would something like that kill my alternator or do I need to keep looking?

11-20-2008, 08:17 PM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: Alternator problem Although a slight draw like the one you have will run the battery down over time it is very unlikely that the alternator would be affected. I would take the alternator back and have him check it out. Jim __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

11-21-2008, 07:47 PM	#4
Security guy Registered User Join Date: Mar 2008 Location: Sarasota, Fl. Posts: 441	Re: Alternator problem I'm not that good with the wiring part of cars, but does yours have an external voltage regulator? I've got a '71 that had been sitting for a while. It wasn't charging either and I took the alternator to the parts store and had it checked. They tested it and said it was bad. Bought a new one and it wasn't charging either. Tried tapping (also hitting lightly with a hammer) the voltage regulator thinking it was stuck. That didn't work. Found an old factory manual from the '70's and figured out how to test the regulator. If I remember right, we shorted 12 volt battery to one of the terminals of the regulator, and that 'woke it up' and it charges great now. Worth a try?

11-21-2008, 08:01 PM	#5
Richard8971 Still livin the "dream" in '54 Join Date: Nov 2006 Location: Tucson, AZ Posts: 2,656	Re: Alternator problem I was also getting a low charge reading. What I had to do on mine was wire the alt directly to the battery and not go through the wiring harness. Run the charge side of the alt to the poss side of the batt and the neg side of the batt to the alt bracket for a good ground. (I hope the pic helps) I am assuming that you are using the wiring that went to the "old" three wire alt. That kicked up my charge by almost 2 volts. Also, I have heard that you cannot hook up the alt when the batt is connected. There is a very real chance of frying the diodes inside. I don't know for sure that is what is causing it, but just a heads-up. Good Luck. Don Attached Images __________________ Current build. 1954 Chevrolet 3800 "Ella" SOLD! My '67 stepper "Tangerine Dream" "Anything worth doing is worth doing right" James Dean “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.” Sherlock Holmes "There is no spoon." Neo Last edited by Richard8971; 11-21-2008 at 09:03 PM.

11-21-2008, 08:50 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Alternator problem First... when doing the draw test between the battery terminal and cable, you should use the neg cable. The reading is the same, but the neg cable is much safer to work with. EVERYTHING metal under the hood is a ground, flopping the cable around is asking for grounding out a hot positive cable Check your tester now and see if it is working at all. I wonder if you haven't blown a fuse inside of it. Opening the door should have made it read HIGHER... not zero. When you open the door, the dome light turns on, which would require more amps through the cable, and the meter. I suspect you popped a fuse in the meter. Regardless of that, I question your rebuilder. If there was a short somewhere, then there'd either be a fuse blown from said short, or a wire melting and making smoke. To a person who works on automotive electronics, short means short to ground. Open circuit means a wire or switch not completing a circuit, which, I can't see either causing a one wire unit to burn up. How ever, I also don't have any answers for you on the issues either. Honestly, i would recomend just running a GM 3 wire internally regulated alt. It's not like it is hard to wire, and they are better than a one wire unit.

11-21-2008, 09:02 PM	#7
Richard8971 Still livin the "dream" in '54 Join Date: Nov 2006 Location: Tucson, AZ Posts: 2,656	Re: Alternator problem Yeah, what Andy said. Don __________________ Current build. 1954 Chevrolet 3800 "Ella" SOLD! My '67 stepper "Tangerine Dream" "Anything worth doing is worth doing right" James Dean “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.” Sherlock Holmes "There is no spoon." Neo

11-22-2008, 12:01 AM	#8
wdrthird Registered User Join Date: Dec 2007 Location: Ponca City, Oklahoma Posts: 229	Re: Alternator problem Thanks guys for the advice and the tip on the proper way to test the draw. It has an internal regulator and Im not using the original wiring, its just a single wire running from the alternator to the battery. The guy who rebuilt it has a pretty good reputation for doing quality work, but maybe this is just a fluke or something. Im not much on electrical sytems so Im kind of stumped as to where to start looking. As far as I can tell, theres no blown fuses, no burning wires and everythings working like it should. My meter seems to be working ok, but just to double check I used a simple test light and it showed no draw today. The first time I had the alt. rebuilt I checked as soon as I fired the truck up and the output was good, so I know the alt. was good when I first installed it, but after driving the truck some it was back to 11.5 volts and only putting out about 30 amps. I have not fired the truck since having it fixed the second time. Last edited by wdrthird; 11-22-2008 at 12:04 AM. Reason: rewording

11-22-2008, 12:15 AM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Alternator problem the 11.5/30 amps you tested was most likely the battery power, the alt wasn't putting out anything. I would say make sure the positive wire from the alt to the battery is solid, no splices/bitt connectors, and has tight clean connections, then also clean the point between the alt brackets and the block, removing all paint and corrosion... maybe even a ground strap/wire from the alt to the battery like mentioned before, and fire it up... see what happens. The more I think about it, the diode probably blew from an on again/off again condition... a sporadic connection on either the positive or negitive connection.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.