So, my 90 F150 302 4x4 manual still hesitates now and then, and the idle starts out normal, but as the truck warms up it settles at 1500 rpm. I've cleaned the egr and IAC and deep creepe dthe intake. I'd like to check all my sensors (egr, tps, IAC, map) to see if they are functioning properly because I did get some codes. My haynes manual says where to pull the wiring harness' off, which electrodes to touch, and what the voltages should be with key on, engine off.

Problem: My volt meter reads zero when I make the proper contacts on all the sensors. I pulled back and touched the voltmeter electrodes to the battery to test if the multi-meter was reading properly, and it pops up as around 12, as it should be. The resistances for the sensors come out close to what they should in the book. How can all the votlages be zero? What am I doing wrong?

 

Make sure the ignition is on RUN.

 

it is. Key on, engine off.

I was thinking about going to the parts store, having them pull a tps off the shelf, and see if I can get a volt reading on it.

 

When you check voltage, its voltage going to the sensors. So, your supposed to be checking the harness plug, not the plug on the sensor. Its just to check reference voltage going to the sensor. Now, if your checking voltage on the TPS, you have to leave it plugged in. Either cut a small slit into the wire insulation on both wires or back probe the plug with sewing needles.

 

Don't slit or pierce the insulation in any way; it will do nothing but cause problems down the road. It is best to back-probe the connector with the test leads. For temperature sensors, resistance is also an acceptable way to test the accuracy of the sensor. This can be done with the harness unplugged and testing the pins on the sensor itself. The tps can also be tested on resistance, but I have found that voltage is more useful. The TPS, EVR, MAP have a 5V reference signal from the computer, with variable voltage output (except the MAP which outputs a frequency) that can be tested.

 

yeah, it helped when I checked the resistance of the sensor, and the voltage of the connector from the PCM. Brain fart.

I might have the frequency changing MAP sensor, because I'm not getting a change otherwise with vacuum applied. Is there a way to measure it with a regular multi-meter?

 

Unless there is a frequency or duty cycle mode on the multimeter, then no, you're not going to be able to test the MAP with the multimeter (the voltage will read around 2.5 volts and won't change). If you happen to have an old tachometer/dwell meter, you can use that set to the six cylinder scale on the tachometer to measure the frequency. I'm not sure what the exact procedure is, but I am pretty sure it is in a Haynes manual.