Does anyone know for sure which MAP sensor is used for boost reference?

 

Neither one directly. The ECM has some sort of AlGoreithm to compute it

 

the pressure is measured at one or both of the map sensors. Perhaps it is an average of the two. Although I would like to know for sure.

 

Nothing definitive, but I just disconnected the upper and went for a drive.

Gauge still worked, vacuum down to 20", but would only boost to about 10 PSI. So I think it was still accurate, but boost was limited.

And no CEL.

 

The ECU uses input from the MAF, MAPs, engine speed, ambient temp, intake temp and probably more sensors to calculate the boost and torque. Then it sends a PWM signal to the boost gauge.

You could do what RJ did and disconnect them 1 at a time to see what happens.

 

thanks for doing that. It was very helpful to eliminating t one possible cause of my issue.

 

You could tell us what issues you're having.