tps voltage issues
 

My tps will not adjust enough to reach .54 volts (TB closed ). It will also only go up to 3 volts WOT. The connector is providing 5.05 volts and a solid ground. This is the third tps I have tried, so I don't believe it is the tps. I tested the connector and there no resistance from pin to wire (very little resistance since 0 is not possible) on each of the 3 pins. What should I be testing next?

What year?

Are you sure you are hooking the lever on the TPS to the TB right?

How do you know this exactly ? Are you looking at a scan tool ? or using a voltmeter.

Take the sensor off of the throttle body but keep it plugged in to connector and check to see if you get full range of voltage using meter. If you do, then you more then likely have min idle set wrong not letting tps to be set correctly. I had this problem once myself

Using a volt meter. There is only one way for it to go on the tb and function, I'm sure it is on the correct way. I have tested it on and off the tb. The highest voltage I could get was 3.00 volts with it off the tb, rotated to the stop. This is the third new one I have tried and all of them are reading the same, so I am thinking it is not the sensors. If they were putting out two different outputs i would suspect a faulty tps. It is getting a solid 5v in and ground in that is stable throughout the range, but the output is just not able to get over 3v. I thought it may be resistance in a pin or connection, but the resistance through the plug checks out to be negligible. Is there anything else in the circuit I should be looking for?

Oh, it's a 1985.

What happens to the 5 volts in once you have it the connector connected to the sensor ? Backprobe it.

It is holding steady at 5v unplugged, plugged, and throughout the wipe range of the sensor. The ground as well. That's why I'm on a third tps. All of them have yielded the same output though.

What your saying seems to defy the laws of electricity. I mean if you have 5 volts in but only 3 volts out then the other 2 is being consumed within the ground circuit after the sensor, but I know your going to say that it tests good. So, it is certainly a weird problem. If everything you said is true then I would use a jumper wire to create an additional ground and retest. AND I would install an additional ground for the ecm even if its just temporary with a jumper wire.

It is quite befuddling. It is either too much resistance between 5v and signal, or too little between signal and ground. There is no practical way to measure the current, which would have to sum to zero. The ground side is showing continuity with ground, but I will try a jumper(s).

Testing the tps, it has 1050 ohms between 5v and signal. It has 5830 ohms between signal and ground. That should yield 4.225 to 4.251 volts (1% tolerance). So it is good.

On my 84 I was getting about 3 volts when I tried to adjust it it would stay at 3.0 or go to zero. I decided there was a break internally at that point where the wiper could contact either side, and could only get zero below the break.

That's not quite right.

The resistance between the 5 volt reference (grey wire) and the signal wire (drk blue) will change with the throttle position. There is no connection between the signal and ground. The signal is isolated from the ground circuit.

Ok so we are clear right now on the ground - This is a sensor ground circuit through the ecm, which is not the same as the grounded side of the battery (negative terminal).

Ok this is what I would do next -
You should have 5 volts on the grey wire at all times. Dark blue is the signal wire back to the ecm. Black is the 5 volt return signal. Terminal D12 at the ecm connector is the 5 volt return.

So I would reassemble everything and turn on the key, then measure voltage at terminal D12 at the ecm. It should be basically zero. What do you get ???

If you get anymore than a few hundredths of a volt then I would jumper this wire to ground and retest the signal voltage at wide open throttle. If when you jumper d12 to ground and get a normal reading at the signal wire then verify all ecm grounds and if they are all good then I would replace the ecm and cross my fingers....

Do you have two voltmeters available ???

Check it out .....



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The resistance between the 5v Ref and the signal wire varies with the tps. The resistance between the signal and sensor ground ALSO varies with the tps. The signal ground is ground. It's ground at the plug, it's ground at the ecm. Otherwise the Ref voltage would have to be something other than 5v and the signal ground would have to be 5v different than the Ref voltage (say, Ref voltage =10v, signal ground =5v). The signal ground is continuous with ground at the plug and the ecu. Thanks for the ideas on what to test! Will eventually eliminate it down to the problem.

I've been struggling with the same issue on my '84 for several days. My TPS read .10V as it was adjusted when I began troubleshooting. Moving it to the fullest extent of adjustment would only give me .19V. WOT got me about 2.5V and my reference voltage was 5.03V. I was ready to go the with a new TPS when I read Amotoxracer's post about trying another voltmeter. I dug out my old RadioShack meter and found a whole different set of numbers. I was able to adjust the TPS to the lower range of acceptable voltage and read 3.9V at WOT. Now it's time to head out on a test drive to see if the CEL will stay off.

You know, I must have missed that part. It is a very valid point. This multimeter might very well be the problem. I'll bring a fluke home and see it it gives a different result. Thanks again!

Iirc, way back when being taught how to check for opens or variations in a TPS it was best to use a analog meter. That way you can watch the sweep and catch a problem. DVOMs have a delay and you may miss the open/variation. hope this helps.

Gary