Alternator issues
 

I am looking for someone who knows how the vehicle control module and instrument cluster work together to control the alternator. I have a 97 Blazer that stops charging at 4000 rpm. As soon as the rpm drops below 4000 the alt starts to charge again.

The alternator checks out. The signal to the alternator actually disappears.

I have an electronics background but do not know what sort of logic is actually taking place in the instrument cluster and VCM. I am familiar with Bolean algebra so if some GM elctronics engineer has hijacked logic diagrams I can read them! LOL!

Thanks in advance!

On the '98_ trucks, the field is controlled by the PCM through a PWM (pulse width modulated) signal. Looking through the wiring diagrams for a '97, it appears that they are setup as a reference voltage. I would look at what is happening on the gauges fuse and check to make sure that the resistor in the cluster is intact.

Thanks! I am just coming off a mid-shift so I will probably tear into it tomorrow. Thank you for the wiring diagram! My diagram for the 97 didnt show the resister in parallel with the indicator light or give its value.

Have a good one!