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 Post subject: GM Alternator Mystery
PostPosted: November 5, 2016, 10:15 am 
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Joined: February 28, 2009, 11:09 pm
Posts: 1286
Location: Connersville, Indiana
I installed a GM CS 130 alternator in my Alpine. So this may be viewed as an improper post, but the Alpine is homebuilt and the problem is something that could happen in any build.

The alternator worked fine for 50,000 miles, then the idiot light started to glow dimly. So dim it could only be seen at night, but otherwise things seemed okay. The verdict: Bad negative diode, so I replaced the diode module, light stayed on. So I replaced the alternator. Light went out for a couple of thousand miles, then back on. Changed the diode module, light stayed on. Lately I've noticed the battery becomes noticeably discharged overnight. Testing with a VOM tracked the drain to the alternator output lug. I'm pretty sure a diode on the output wire would solve the problem the discharge problem. However:

I have no idea the size diode that would be needed. The SC alternators put out about 80 amps.

I view the diode as a bandaid, not a solution. I'd like to solve the problem inside the alternator, but am at a loss just what the problem might be.

Any ideas?

Bill


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PostPosted: November 5, 2016, 1:37 pm 
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Joined: October 19, 2009, 9:36 pm
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Location: meadview arizona
where are you getting your alternators from?

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PostPosted: November 5, 2016, 3:48 pm 
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First one was a junk yard special as I had no idea of the configuration I needed. It was usable, but I was having ECM interference so I replaced the rectifier and regulator with BWD parts. I then discovered the interference was due to an inadequate alternator ground. The BWD parts were in it when things started going south. I had kept the "original" rectifier and regulator and reinstalled them. They were working perfectly when removed. When reinstalled, the light stilled glowed.

The second one was from Rock Auto, either Delco or Remy rebuild. Can't remember for sure. The replacement rectifier for it was the top line name (which I can't remember) from Advance Auto.

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PostPosted: November 7, 2016, 4:58 pm 
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Joined: July 4, 2006, 5:40 pm
Posts: 1927
Location: Novato, CA
I had a similar issue with my first alternator, very dim dash light, but it kept the battery charged for over a year until it finally broke for good, as indicated by a pegged voltmeter. Not knowing anything about diodes or rectifiers, I bought a new $80 MGB alternator from O'Reilly Auto Parts. It's worked fine ever since.

The GM alternator is a popular replacement for the MGB crowd, probably the Sunbeam crowd as well, but if there are only a couple of rebuilders out there, maybe it doesn't make much difference.


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PostPosted: November 8, 2016, 9:59 am 
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Joined: June 15, 2010, 8:29 am
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Location: Duxbury, MA USA
I had used a couple of these in my Europa. I had the exact same issues. Yes, the relay...first try to get rid of the discharge. ... and then a diode did make things work, but charging rates seemed to go high at times and I was eating a battery every couple of years. Finally, I said to myself "time for a change in strategy here". I bought a very tidy Denso alternator original fitment was Toyota. I did have to modify my GM mount which was a modification of the original Motorola mount. Took me all of an hour to weld the parts and do some light machining.
The Denso works perfectly, looks better, no relays or diodes. This battery is now 4 years old and starts the car without charging after the winter nap. One of my best investments in reliability!

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PostPosted: November 8, 2016, 11:47 am 
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Joined: January 10, 2008, 4:47 pm
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finally broke for good, as indicated by a pegged voltmeter.


Wow, most cars don't have a voltmeter. Car batteries can explode and this is how the process starts. My race mechanic was next to a car when the battery exploded in the pits once.

I don't have any other advice, sorry.

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PostPosted: November 8, 2016, 6:32 pm 
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Joined: February 28, 2009, 11:09 pm
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Location: Connersville, Indiana
I've considered a Denso alternator, but it seems there is no direct Denso replacement and altering the mount is out of the question at this time. It is integrated into the engine mount as well as the AC compressor mount and it would be necessary to remove the engine. So I'm stuck with what I've got.

I'm thinking that perhaps something is going on with the starter and it is throwing a voltage spike that fries the diode. The car is wired so the alternator output goes to the starter solenoid, so there is a direct connection. The fuse box is fed with a dedicated wire from the battery. If I wired the alternator output to this dedicated wire instead of the starter solenoid, would that give me some protection from starter spikes? Also, what if I installed a relay that disconnected the alternator output when the starter was engaged?

Bill


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PostPosted: November 17, 2016, 3:58 pm 
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Joined: March 30, 2011, 7:18 am
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A common cause is a problem with one of the alternator wires - bad plug or crimp connector, pinched spot, etc. causing resistance to go up.


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PostPosted: November 18, 2016, 3:30 am 
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Quote:
I'm pretty sure a diode on the output wire would solve the problem the discharge problem. However:


If you do this it probably messes with the voltage regulator. This is a modern alternator and it contains the regulator too or do you have a separate unit for that?

The diodes in the alternator should be designed to handle the voltage spikes. This is a really important, crucial job. If there are problems in your wiring you can have an "alternator load dump". That means it's charging and loses it's connection and will make a huge power spike. I think the spec for this is hundreds of amps and also hundreds of volts. It will act like an ignition coil. So voltage spikes should not be an issue...

The alternator warning bulb is part of the regulator circuit sometimes I think, it's involved somehow. Maybe somehow you are getting mixed parts? Diode packs for older alternators or something? This stuff has been pretty standard for a long time now, I think.

Sorry no real help here.... :(

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