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Gary, ASE Certified Technician
Category: GMC
Satisfied Customers: 16773
Experience:  GCM master tech over 25 years
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GMC astro: 2000 astro van. Having intermittent problems with

Customer Question

2000 astro van. Having intermittent problems with alternator not charging. Have replaced alt 3 times and brand new battery. I believe problem is the signal wire but I need more info to how it works
Submitted: 1 year ago.
Category: GMC
Expert:  Gary replied 1 year ago.
I am sorry for the long waitif the big lug stud at the alternator shows 12 volts and you have been through 3 alternators al ready the issue does boil down to the brown wire ( PCM control and monitoring wire ) showing high resistance or shorted to power or the PCM connections at the PCM or a bad PCMthe PCM control the charging through pulse width control which can not be tested with a multi meterthe best bet is to ohm out that brown wire from the PCM to the alternator and if that wire ohms good and is not shorted to power or ground or has high resistance replacing the PCM if the PCM connections and terminals are good should correct this issueyou really have cut this problem down to that wire being bad or a bad PCMhere is the technical on how the system worksThe generator provides DC voltage to operate the vehicle's electrical system and to charge the Battery. A magnetic field is created when current flows through the rotor windings. The rotor is belt driven by the engine creating an AC voltage as this field rotates within the stator windings. The AC voltage is converted to DC voltage by the rectifier bridge and is then supplied to the electrical system at the generator battery terminal.When the engine is running, the generator turn-on signal is sent to the generator from the PCM, turning on the regulator. The generator's voltage regulator controls the current flow to the rotor, thereby controlling the generator output voltage. The rotor current is proportional to an electrical pulse width modulated voltage supplied by the regulator. When the engine is started, the regulator senses generator rotation by detecting AC voltage at the stator through an internal wire. Once the engine is running, the regulator varies the field current by controlling the pulse width. This regulates the generator's output voltage for proper battery charging and electrical system operation. The generator L terminal is connected internally to the voltage regulator and externally to the PCM. When the voltage regulator detects a charging system problem, it grounds this circuit to signal the PCM that a problem exists.When the ignition switch is turned to RUN, the charge indicator turns on for a few seconds (bulb check), then turns off. The Powertrain Control Module (PCM) commands the bulb of the charge indicator on by sending a Class 2 serial data line message to the instrument panel cluster when the PCM detects a charging system problem.
Customer: replied 1 year ago.
What do you mean by high resistance on that wire? What should I read? I have found 12v at that wire but honestly if it's a damaged wire I can't rule out that when I plug it back in the signal gets lost. Also... i believe that wire goes to the instrument cluster before the alternator. Could the problem lie in the instrument cluster? The battery light used to come on while the alternator wasn't working. Now is does not come on except for when I start the engine for a few seconds as it normally does.
Expert:  Gary replied 1 year ago.
the one brown wire goes from the alternator to the PCM then the PCM tells the charge light what to doend to end on that wire should read close to 0 ohms and when ohming to ground should read open
Customer: replied 1 year ago.
Pcm didn't fix the problem at all. Now I'm completely lost. Is there a certain voltage I can put through that brown wire to force the alt to charge?
Expert:  Gary replied 1 year ago.
no that wire the computer sends a signal through pulse width modulationif there is a local Pep Boys or Auto Zone they should have to tool to plug in to see if it shows green lights of the computer telling it to charge for free for you
Customer: replied 1 year ago.
It plugs into the van diagnostic port?
Expert:  Gary replied 1 year ago.
no they unplug the alternator connector and plug that connector into a tester