I have a 1988 GTU 13b n/a that has no spark and fuel pump does not turn on when ignition is turned on. Checked fuses all fuses ok, it gets 12v to the coils, has good compression, and replaced PCM but neither come on. Is there anything else I can check that might be the problem to take both out at the same time?

 

There's a check connector near the lead coil which has four wires. Three are Yellow based and a Black/White wire. W/key to on the B/W wire should measure 12 volts. Does it? This wire powers the ECU. If it does read 12 volts w/key to on then proceed to measure the voltage w/key to on at the Brown/White wire found at the TPS or Pressure Sensor and it should read 5 volts. If the reading is close to 0 then the AFM is likely shorting out which could cause your problem.

And did you mean to say start as opposed to on?

 

Looked at the voltage to both the afm and the pressure sensor and both read .5v when both connectors are unplugged with the key on and does not change when either are plugged back in

 

And just looked the tps has .5v too could that mean a bad PCM again or high resistance in the reference circuit

 

You would first want to check the B/W wire as suggested because if the ECU is not powered up then it would help explain the low voltage reading you are getting. If the B/W wire checks out then there are other sensors that have the Vref going to it. Besides the AFM and TPS you also have the variable resistor and the atmospheric sensor (it's located behind the kick panel near the ECU). And if you have access to the ECU already then pin 3I (B/W wire) should have 12 volts w/key to on (this is how the ECU is powered)..

 

Checked the b/w wire and it has 12v

 

The B/W wire at the check connector is before connector FEM-02. After it goes through this connector it ends up at the ECU so where you measured it is important. If it was measured at the ECU then you need to check the other two engine sensors mentioned as in unplugging them and rechecking the Vref wire. The resistor is easier to access probably so focus on that sensor first. And w/respect to the Vref wire make sure you are measuring the proper wire as in some S4's the Vref wire and ground wire are both Brown.

 

The fuel pump will works when the ignition is in Start position when cranking,as the AFM sees air,hits a "switch" and turns it on so the car gets fuel.(if you move the flapper on the AFM the pump will turn on).
Once the car is seeing fuel and starts the AFM then continues to send info that the pump needs to be on and keep on pumping fuel.(the Afm is open now and gets air,flapper is moved due to vacuum of engine needing air)
The Pump does not receive signal to it when you just turn the key on.

 

Thanks for the info

 

Thanks for the information I went back to the garage today and found my afm was rubbed the sheathing off the wires to the variable resistor and was grounding against the body of the afm