Throttle position sensor
 

Hey everyone I own a 1999 xj with coil packs and a 2000 xj pcm. I'm testing my tps sensor because of a check engine light and obvious symptoms. Using my multimeter I did some tests. At closed throttle I have 12v ground, 5volts reference wire, and 4.6v return. When I open and close the throttle everything status the same nothing changes. With the sensor unhooked I tested just the harness and had the exact same results, I then tested at the pcm with the sensor connected and disconnected and got the same results. I'm not sure why but I think Bad pcm or short in the line?

Sounds like a short in the sensor to me. You are testing between the return and the ground correct? WOT should be about 4.6 volts with closed around .5

Yes I am testing that way, at closed throttle it is 4.6 and open is 4.6. It doesn't change at all

Have you driven it with this problem? If you can does it hold gears a long time before upshifting? Can you look at real-time data off the pcm to see throttle position?

Yes I have and yes that's excatly what is happening.

The tps sensor is brand new and was only 4 months old when it went out. If I get the same results with the sensor unhooked doesn't that rule out the sensor being bad? What's the voltage on the return wire supposed to be at the harness when unhooked from the sensor?

I don't believe there should be any voltage on the return when its unplugged though it wouldn't surprise me if there was as grounds tend to float in cherokees. Cruiser would know for sure exactly what's supposed to happen when its unplugged.

Here's a procedure for testing the TPS. It is not the sweep test.


I use a safety pin to back probe the wires in the connector.


Attachment 311677

Thanks for the advice ken, I tried this method and got the same results. With the sensor unplugged and multimeter connected to the harness what should the return wire voltage be at?

If its not pouring when I get home tonight I will check the voltage on that.

If by "return wire" you mean Ground, the voltage should be the same as in that chart with the TPS connector installed, or disconnected. Same goes for the 5 volt supply voltage. See chart.


The Ground circuit in the Jeep is what's called a live ground. All engine management sensors with this ground wire (Brown/Yellow tracer) are connected in a network, like a spider web. Each sensor ground wire should read pretty close to the same. If the voltage is too high in this circuit one of the sensors, or its connector, are shorted to the 5 volt supply or to battery voltage. To isolate the bad sensor each one should be disconnected to see if the voltage returns to the .015 - .020 range. The connector wires could be all ate up with corrosion or be butchered by mishandling. Look for it.

Here's what the '99 ground network looks like:


Attachment 311655

He is talking about the center wire that goes back to the pcm. He is getting voltage on it and worried there is a short in the wire.

If you think you have a short to an external voltage source look for chafing in the wire harness loom(s) that may have rubbed through to bare wires.


You say you have a coil rail and a 2000 XJ PCM. Do you install a 2000 XJ wire harness?

You may be seeing a 12 volt feed in the Signal wire from the TCM.


You can try removing power to the TCM by removing fuses in the Junction Block (JB) and the Power Distribution Center (PDC).


One at a time, remove these fuses and check the voltage on the TPS Signal wire after each fuse removal.


JB: Fuse F10


JB: Fuse F11


PDC: Fuse F16 (IOD)


Key to RUN/ON when checking the TPS.


PDC:


Attachment 311653


JB:


Attachment 311654