Dave, you say to set the TPS volatage to 1.0 volts with a digital meter. Somehow I missed where the two wires and color of the wires, and the test point location. I assume you adjust the voltage by adjusting the throttle screw on top of the manifold. Engine is in a 85 5.0L EFI.

 

Shoot for .8 to 1.0v. The screw is not really there to set the voltage. The screw is there to set the proper proportion of air through the throttle at idle, and the rest of the air goes through the idle air control valve.

The factory and everyone tells you not to mess with the screw, but these things are so old, that most have been messed with already. I have a third party book, that says when the idle air control valve is unplugged, the engine should slow way down, and barely idle if the screw is set right. If the engine keeps idling along fairly well, the screw is too far in. If it quickly stops when the IAC is unplugged, the screw is too far out.

When you get the screw correct, the TPS is supposed to be in it's correct voltage range. To check the voltage, you want to put your meter on the negative which is the black/white, and the signal wire to the computer which is the darkgreen/lightgreen wire. There is no real testpoint, you have to scrape a little bit of the insulation off the wires and then put tape back on them after you are finished. If the voltage is way off, you have to scratch your head and ask why, most of these things have very little adjustment, you may get some from the slop in the screw mounting holes.

I guess you could set the screw according to a certain voltage and then try the unplug test and see how it acts. It's a little bit different way of doing it, but doing things different is how we all learn.

 

Dave, on the 1985-86 EFI 302, the TPS voltage is critical. These computers work on absolute voltage, unlike the later ones that work on difference from start value. Archion learned the hard way on his 86 F-150. An aftermarket TPS was nominally in spec at 1.08v at closed throttle. His EGR vent solenoid sounded like a machine gun and the truck wouldn't idle worth a damn it also acted strange even driving. The spec to set the TPS voltage on these is measured between the dark green with light green strip wire and the black with white stripe wire. Orange with white stripe to black with white stripe should be 5.0v.

Set voltage is 0.95 to 1.05v, if you cannot get that and have it idle, or you end up backing the screw out until it doesn't touch and still can't get there, the TPS is probably bad. Unfortunately, the only ones that work correctly on these are Motorcraft ones.

Archion found an actual setting for the throttle plates I think. There was at one time an air bypass kit for these early systems from Ford. It allowed the throttle plates to be closed far enough that the TPS voltage was in spec, but had a bleed bypass to let more air in. Later models have a minimum air setting that is done with the engine warm and IAC unplugged.

 

Thanks Dave and others for posting the information on the voltage to set the TPS. Also, thanks for pointing out the color codes of the wires I need to be checking. Richard