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Gary
Gary, ASE Certified Technician
Category: GMC
Satisfied Customers: 16520
Experience:  GCM master tech over 25 years
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My 2007 GMC Sierra 1500 Classic keeps burning up alternator

Customer Question

My 2007 GMC Sierra 1500 Classic keeps burning up alternator diodes, what could cause that?
Submitted: 8 months ago.
Category: GMC
Expert:  Gary replied 8 months ago.

how long to they last ?

what brands are they ?

are they reman or brand new alternators ?

Customer: replied 8 months ago.
Ill give you the back story.....for about a year now, I have been receiving the "Service Charging System" light on the dash, but everything was working as advertised. This morning I went to start it up and heard the dreaded, "click, click click", so I had my wife jump the battery. Truck started up and I drove it an hour to a cheerleading event. When I stopped and tried to start the engine again, same thing, "click, click, click". I got another jump and took it to Autozone. They tested the alternator and it said that it was putting out the correct voltage, but that the diode check failed. I bought a new alternator, and while installing it, he put the battery on the charging terminal at the store. It took a full charge and I started up the truck with no problem. I drove the truck to a few stops and everything was working fine. When I went back to the football stadium and let the truck sit for two hours, I came back and it was dead again. I took it BACK to AutoZone and the said that the diodes were bad again. I kept the truck running and drove the hour back home. When I got home, I shut the truck off and tried starting it again, no joy. The new alternator was a Duralast
Expert:  Gary replied 8 months ago.

even thought he diode failed what was it putting out for volts and amps when tested ?

Customer: replied 8 months ago.
14.3vdc i believe
Expert:  Gary replied 8 months ago.

and the amps where good ?

no check charging or batter lights on in the dash ?

are they 100 % sure the batterys volts and cold cranking amps and it was tested for bad cells ? and 100 % sure the battery is 100 % good for amps and for bad cells ?

Customer: replied 8 months ago.
Ive had the battery light on the dash along with the message "Service Charging System" like I mentioned before. They gave me a printout from the machine that said that the battery was "good". I cant tell you what they checked on it.
Customer: replied 8 months ago.
I saw the machine and it said everything else was good except the diode check
Expert:  Gary replied 8 months ago.

if that battery is confirmed good with cold cranking amps and volts and it has NO bad cells in it pick up a GM new alternator and install it the Autozone will refund your money and see if this still happens

the odds are there is only 2 wires from the computer to the alternator and they are good and even if bad or the computer was bad it should not cause an internal alternator issue

also keep in mind a battery that had been knocked down to dead over 2 times can never fully bounce back to full cold cranking amps

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Customer: replied 8 months ago.
could a bad cable cause this issue, the ground one is a little stripped out.
Customer: replied 8 months ago.
I am an electrician, is there any test that I can do to help me narrow it down?
Expert:  Gary replied 8 months ago.

bad cables would not cause this

the only test to be sure the computer and wiring from the computer to the alternator is good is most shops have a plug in tester that you unplug the alternator and plug the alternator connector into the tester and look for green lights'

if all green lights its good if any red the wires are bad or the computer is bad

the problem is the signal is sent pulse width modulation which no meters can read

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Customer: replied 8 months ago.
The battery is about 4 years old, how long are they normally good for?
Expert:  Gary replied 8 months ago.

4 to 6 years BUT every time a battery is deep cycled so to say it never will come back they can loose @ 20 % off the top each deep cycle

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Customer: replied 8 months ago.
Im sorry that I have a lot of questions, but talk to me about the Regulated Voltage Control System. I have that on my truck.
Expert:  Gary replied 8 months ago.

the computer tells the alternator how much to charge and when to charge v/s the voltage readings the computer is reading off of its input powers so it knows when to charge and how too

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Customer: replied 8 months ago.
Do you think that could be an issue? Ultimately, do you recommend that I take it to the dealer?
Expert:  Gary replied 8 months ago.

i would highly recommend the local GM dealer and just have them test it for you

it will not cost much to be 100 % sure what it is then either have them fix it or fix what ever they say the problem is

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Expert:  Gary replied 8 months ago.

how are you making out ?

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