Secondary Vacuum Pressure = 0 kPa/0.56V

[dondon]

I'm trying to figure out if the secondary vacuum sensor by the battery on my 90 should be reading different values under normal operating conditions.

At the moment, with the engine idling it reads 0Kpa if I blip the throttle, it reads 0kPa... it basically reads 0 kPa all the time. I don't know if this is a symptom of why my secondaries don't appear to operate or not. My MAP pressure changes when throttle changes, should the secondary vacuum pressure not change aswell?

How do I troubleshoot why this is incorrect if it is?

[Franke]

The vacuum sensor monitors secondary port vacuum during full power operation only near WOT. Under normal operation there should be no vacuum applied to the vac sensor. If you are reading .56 volt under normal engine power then it is ok. FSM indicates .54 volt to .62.

[dondon]

The vacuum sensor monitors secondary port vacuum during full power operation only near WOT. Under normal operation there should be no vacuum applied to the vac sensor. If you are reading .56 volt under normal engine power then it is ok. FSM indicates .54 volt to .62.

Thanks for clarification. The sensor vacuum does not change any throttle position with full power on. I had a call with GK a short while back. He was of the opinion that the FP switch even though the light was on and the computer indicated full power, may be a problem. Guess I should try rebuild the switch prior to trying an ecm rebuild for this issue.

Thanks again for the help.

[Marc Haibeck]

Hi Don,

.56 volt is the normal output of the secondary diagnostic vacuum sensor when the secondary port throttles are closed. The voltage will go to about 1.5 volts when vacuum is applied to the secondary port actuators.

You really can't activate the secondaries with the throttle with the engine idling. To do so you would need to open the throttle 98% or more and hold it there long enough to read the meter. If you did that the engine would be on the rev limiter at 7100 rpm. That's not a good thing.

The voltage at the sensor will not change if the Full Power key is actuated on or off.

To test the sensor at idle, ground pin C17 at the ECM when the engine is idling. That will open the secondary port solenoid and the secondary actuator vacuum will be applied to the diagnostic sensor.

[dondon]

Hi Don,

.56 volt is the normal output of the secondary diagnostic vacuum sensor when the secondary port throttles are closed. The voltage will go to about 1.5 volts when vacuum is applied to the secondary port actuators.

You really can't activate the secondaries with the throttle with the engine idling. To do so you would need to open the throttle 98% or more and hold it there long enough to read the meter. If you did that the engine would be on the rev limiter at 7100 rpm. That's not a good thing.

The voltage at the sensor will not change if the Full Power key is actuated on or off.

To test the sensor at idle, ground pin C17 at the ECM when the engine is idling. That will open the secondary port solenoid and the secondary actuator vacuum will be applied to the diagnostic sensor.

Thanks Marc, was finally able to get things running well today. I must have a flimsy connection with the ECU and the third wire harness. Anyway, I was able to run your test and now have the pressure changing. Thanks very much for the help, it is much appreciated.