91 2.9L TPS signal testing

[Skooby]

Truck will rev if throttle is opened slow, will pop and backfire if opened quickly. Trying to test the TPS and getting the 5V on the connector (disconnected) with the key on, not getting continuity to ground o. The other wire. With connector plugged in and key on seeing 5V on all three wires? Same result with 2 different TPS. Help? PCM ground?

 

[franklin2]

Orange wire should be 5v. Green wire should vary when the throttle is moved. The other wire should be ground. What voltage do you get from the engine metal to the ground wire on the tps? Should be close to zero.

 

[RonD]

+1 ^^^

But reads more like the SPOUT wire is unplugged
SPOUT is the "vacuum advance" for spark system, it comes from the computer
You disconnect the SPOUT wire when setting the base spark timing, then need to plug it back in when done

The Distributor does RPM spark advance on its own, so it works OK if you slowly accelerate, the SPOUT is needed if you want quick throttle response
But same for TPS, it replaced the "accelerator pump" on carb engines

 

[Skooby]

ok, finally got some time to play with the truck. Key on, connector unplugged- Top wire (red w/grey) +5v, bottom wire (brown w/white) +5v, middle wire (grey w/white) nothing. Key on connector plugged in +5v on all 3 wires regardless of throttle position. I should see continuity to ground on one if the wires and the middle should be varying from +1v to +5v.....

 

[franklin2]

I gave you the wrong wire colors didn't I? Brown/white is the 5v supply. Grey/white is the signal back to the computer. Grey/red is the signal return or "ground".

According to the diagram, you should also have a signal return or ground wire on the engine coolant temp sensor, the air charge temp sensor, and your MAP sensor. They all should have the grey/red or in some places it may be a black/white. All these different sensors having the same wire means there is a splice in the harness somewhere that splits this wire and leads it in all those different directions. Maybe you can take some readings on this wire at the other sensors, and see if you get a good ground anywhere in there and try to figure out where you are losing it.

 

[19Walt93]

You want as close to one volt on the signal return wire at idle with the TPS plugged in, 5 volts on the signal return indicated WOT.

 

[RonD]

The grey/red wire runs to ALL the sensors, so test the grey/red on say the IAT(air temp) sensor, should be on upper intake drivers side 2 wires
Its a reference Ground, the computer uses pull down circuit, so not a true ground but should show as Ground

That comes from pin 46 on the computer
Pin 47 is signal return from TPS, just FYI

You could have a short between the two wires at TPS which is why you want to test at another sensor

 

[Skooby]

Ended up with a new pcm. Pin 46 signal ground trace inside the pcm was cooked. Running good now!

 

[RonD]

Good stuff

Thanks for the update and the fix