Where is the PID 'GM.VOLTS' measured from?

[samh_08]

I've been logging this to diagnose fuel pressure loss under load. Turns out that the voltage is all over the place so Im trying to find where I may have a poor connection, or alternator issues..

When I check voltage with my multimeter, I get .2VDC higher than the PCM reports through the scan tool.

[ferocity02]

I think it's at the OBDII port.

[joecar]

The PCM has several battery voltage feeds... the PCM uses one of them for writing the pid GM.VOLTS.

[joecar]

The power pins on the OBD port are fed directly from battery voltage (via fuses).

[samh_08]

Pins 20 & 57 both have system voltage (.2V more than scantool says.)

Anything labeled ground has a voltage drop of .010V.

[joecar]

GM.VOLTS reading 0.2V lower than DMM/voltmeter is ok, 0.2V is about the correct drop for the ADC chip/circuit in the PCM.


0.01V drop across your grounds is way too good (I would expect 0.1V)... 0.01V is the DMM/voltmeter open circuit noise (check this out).


Also put your DMM/voltmeter on the alternator and verify its voltage at load; then move the DMM/voltmeter to the next "node" in the wiring and repeat.

[gmh308]

GM Volt. That is hugely successfull Chevy electric car right?

[joecar]

Lol.