GMC 1500 alternator problem

W

WA1DH

Joined
Dec 16, 2009
Messages
131
Location
Rhode Island
My 1995 GMC K1500 seems to have a serious issue with the alternator. I started it up this morning and it seemed to be running rough. I pulled out my my driveway and noticed I needed to keep on the gas to keep from stalling. It has done this in the past when sitting a few days but usually clears up quickly. Today I noticed the gauges were all acting strange and then it completely stalled out and wouldn't restart - battery completely dead. I replaced the battery (which was 2 years old) and soon after connecting the cables I smelled something hot. I felt the small alternator to positive battery wire which was very hot, and the alternator itself was very hot. I quickly disconnected the positive cable. The key was removed and off the entire time, so there should have been no draw.

Am I looking at a failed diode with a dead short in the alternator? Alternator is over 10 years old and unknown brand. I'm going to order a new AC Delco off Rockauto, but is there anything I can try to bench test the old one just to verify?
 
The bridge rectifiers connect the stator to the battery always through the output terminal. Diodes in them can fail as a short circuit, and it sounds like that's what happened. If you take an ohmmeter and put the positive lead on the disconnected output terminal and the negative lead to the case, you should measure an open circuit, but if you measure a low value, then yes, at least one diode has failed.
 
I will test that and report back. I just went back out and connected the battery again briefly. This alternator has 1 output stud with a wire connecting it to the positive battery terminal, and 1 small wire (16 ga?) that clips on, which I imagine is just for the charging monitor circuit. With the battery connected, the alternator to battery wire heats up and the back of the alternator gets very hot.
 
I've had a rectifier diode short inside the original CS130 series alternator (1996 car). It was red hot and squeeling, no indicator light issue. They are not particularly good series of alternators.
 
WA1DH said:
Am I looking at a failed diode with a dead short in the alternator?
Click to expand...

Most likely.


WA1DH said:
is there anything I can try to bench test the old one just to verify?
Click to expand...

Various resistance readings on the unit and some of it's internal parts can verify this. If the diodes are bolted on, I'd unbolt them and measure those out of the alternator. YouTube should show you how to do this.

For fun you can take it to an auto parts store for testing.
 
That will also happen should the battery be connected backwards, though a side terminal battery makes that very difficult to do.
 
Check to see how much alternating current (ac) is being sent out by the alternator. Use a digital volt meter. A friend's old Nissan Z was doing weird electrical stuff. First thing I did was check the power at the battery. Yup, about 30vac on the 12VDC. He had to replace the alternator.
 
CorvairGeek said:
I've had a rectifier diode short inside the original CS130 series alternator (1996 car). It was red hot and squeeling, no indicator light issue. They are not particularly good series of alternators.
Click to expand...

Yeah, the CS alternators were really terrible.
 
I have continuity between the output post and case with the alternator disconnected and removed. I switched the leads and still had continuity. I tested a different alternator and only had continuity in one direction but not if I switched the leads.

The local auto parts place wanted almost $200 for their rebuilts. I have a new AC Delco unit on the way from Rock auto for half that and it should be in tomorrow.
 
WA1DH said:
I have continuity between the output post and case with the alternator disconnected and removed. I switched the leads and still had continuity. I tested a different alternator and only had continuity in one direction but not if I switched the leads.

The local auto parts place wanted almost $200 for their rebuilts. I have a new AC Delco unit on the way from Rock auto for half that and it should be in tomorrow.
Click to expand...
Yep, one of the 3 diodes is shorted from the stator is shorted (if remember the schematic correctly). Exactly what mine did. I took a look at it and was not impressed, throw away junk IMO. I used to repair the 10SI/12SI units.
 
The new alternator is in and working. I recharged the completely flat old battery (2 yrs old Everstart Plus from Walmart) and tested with my Ancel tester:

Old battery: Resistance 3.15 mOhm, est. 946 CCA

New battery: Resistance 3.27 mOhm, est. 915 CCA

Should I eat the $20 core fee and keep the old battery as a back up? At least the new one has dual terminals so I can use jumper cables if I ever need to.
 
WA1DH said:
The new alternator is in and working. I recharged the completely flat old battery (2 yrs old Everstart Plus from Walmart) and tested with my Ancel tester:

Old battery: Resistance 3.15 mOhm, est. 946 CCA

New battery: Resistance 3.27 mOhm, est. 915 CCA

Should I eat the $20 core fee and keep the old battery as a back up? At least the new one has dual terminals so I can use jumper cables if I ever need to.
Click to expand...
a spare battery is always good, keep it on a trickle.
 
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