PCM not turning alternator on?
03-18-2011, 03:01 PM
#1
PCM not turning alternator on? I have a good alternator and battery and my mechanic tells me the PCM isn't turning the alternator on. He's trying to find a way around me not having to get a whole to PCM. Any ideas what else this could be or someway around replacing the PCM?
03-18-2011, 04:17 PM
#2
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Two thoughts...
1) Exciter/field wire. This is the little red wire on top of the alternator. Are you very sure that this wire is making good contact at both points (PCM and alternator), and that it's not damaged anywhere inbetween? It would suck to get a new PCM only to find that the issue all along was a short in that wire.
2) Not all alternators require battery voltage to "excite"; there are one-wire/self-exciting alternators that are instead activated by rpms. Your factory alternator will not work this way, but you can buy ones that will (Powermaster is a good brand for that). I have one of these on my Nova that works great, but I don't know how well a one-wire system would work on an LS1 car, with it's advanced electronics and tons more amperage requirements.
1) Exciter/field wire. This is the little red wire on top of the alternator. Are you very sure that this wire is making good contact at both points (PCM and alternator), and that it's not damaged anywhere inbetween? It would suck to get a new PCM only to find that the issue all along was a short in that wire.
2) Not all alternators require battery voltage to "excite"; there are one-wire/self-exciting alternators that are instead activated by rpms. Your factory alternator will not work this way, but you can buy ones that will (Powermaster is a good brand for that). I have one of these on my Nova that works great, but I don't know how well a one-wire system would work on an LS1 car, with it's advanced electronics and tons more amperage requirements.
03-20-2011, 08:38 PM
#4
technically, there is no exciter wire on the ls1 alternator.
The voltage regulator is a microprocessor,
one of the wires coming from the pcm to the alternator is the turn on wire which is +5 volts, and not 12v battery voltage that was the traditional exciter wire for old style simple voltage regulators. If the alternator voltage regulator doesn't see this 5v then the regulator won't regulate field current and the alternator won't output. So you need a 5 volt source, not sure if you could tap off one the other wires coming out of the pcm that has 5v. Depending on how the mechanic is testing this 5v turn on wire, if the alternator isn't connected then the pcm probably sees this and doesn't put 5v on the turn on wire purposely, he would need to make sure the alternator is fully connected then probe the connector on the alternator and see if there's 5v.
And i believe the pcm does circuit tests on everything, one being the alternator, and for a lack of better description it tests the voltage regulator (microprocessor) on the alternator and if it doesn't check out the pcm thinks the alt. is bad and then doesn't put 5v on the alt. turn on wire, this is described in the service manual.
DTC P0620
the pcm uses the generator turn on signal circuit to control the generator. when generator operation is desired, the pcm sends a 5 volt signal to the voltage regulator causing it to control the alternator field circuit. Once the generator is enabled by the pcm, the voltage regulator controls alternator output independently of the pcm but the pcm can command the alternator to stop outputting by turning off the 5 volt signal. This is no different then unplugging the F connector going to the alternator. The voltage regulator has fault detection circuitry, and puts this F terminal to ground if there's a problem setting this DTC. And the PCM has fault detection on this circuit and if it senses a voltage other than expected it also sets this DTC.
The voltage regulator is a microprocessor,
one of the wires coming from the pcm to the alternator is the turn on wire which is +5 volts, and not 12v battery voltage that was the traditional exciter wire for old style simple voltage regulators. If the alternator voltage regulator doesn't see this 5v then the regulator won't regulate field current and the alternator won't output. So you need a 5 volt source, not sure if you could tap off one the other wires coming out of the pcm that has 5v. Depending on how the mechanic is testing this 5v turn on wire, if the alternator isn't connected then the pcm probably sees this and doesn't put 5v on the turn on wire purposely, he would need to make sure the alternator is fully connected then probe the connector on the alternator and see if there's 5v.
And i believe the pcm does circuit tests on everything, one being the alternator, and for a lack of better description it tests the voltage regulator (microprocessor) on the alternator and if it doesn't check out the pcm thinks the alt. is bad and then doesn't put 5v on the alt. turn on wire, this is described in the service manual.
DTC P0620
the pcm uses the generator turn on signal circuit to control the generator. when generator operation is desired, the pcm sends a 5 volt signal to the voltage regulator causing it to control the alternator field circuit. Once the generator is enabled by the pcm, the voltage regulator controls alternator output independently of the pcm but the pcm can command the alternator to stop outputting by turning off the 5 volt signal. This is no different then unplugging the F connector going to the alternator. The voltage regulator has fault detection circuitry, and puts this F terminal to ground if there's a problem setting this DTC. And the PCM has fault detection on this circuit and if it senses a voltage other than expected it also sets this DTC.
