Home » Public Forums » GMCnet » Alternator follies (I think it's a bad alternator, but...)
| Alternator follies [message #212208] |
Tue, 25 June 2013 14:58  |
habbyguy
Messages: 896
Registered: May 2012
Location: Mesa, AZ
Karma:
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Senior Member |
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I just picked up my GMC from storage, getting it ready for a trip with the grandkids to a lake near Williams, AZ (read "30 degrees cooler than Mesa, AZ). The engine battery was stone cold dead when I got there, but it fired off the charge that was left on the house batteries with the boost switch on.
The alternator light glowed dull red all the way home, so to be safe I ran the generator. Got it washed, inspected (necessary to renew the plates) and home without incident.
Here's what I'm seeing with the engine running, and dash A/C on max, after some time charging the engine (chassis) battery with the PD converter.
Engine battery: drops to around 12 volts (keeps dropping if I don't plug the charger in)
Engine battery sense lead at alternator: 12 volts (mimics the battery voltage)
Brown (resistor) sense wire at alternator: 10.4 volts
Output (big red wire) at alternator: 10.4 volts.
Thing is, with little or no load, the alternator will put out 14 volts, give or take. And I noticed that when I cycle the A/C off, the alternator output jumps up over 15 volts for a split second. Also, turning on just the headlights will drop the alternator output by a volt or so.
I'm thinking it's about 98% likely that the alternator (rebuilt Delco 80A unit installed in 2009 by the PO) is cooked, but I'm very willing to hear dissenting opinions at this point (while I let the engine compartment cool down so I can even get to the alternator without spontaneously combusting). Thanks, all!
Mark Hickey
Mesa, AZ
1978 Royale Center Kitchen
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