TPS voltage

[John Robinson]

Today it was 98 degrees out and I drove my car about 6 miles and then parked it for 3 hours. When I went to start it it ran very poorly and then it would quit. H ad a service engine soon light and so I went through several turn off engine restart cycles and it would not clear. Got the trusty paper clip out and read the codes. I had Code 15 Engine coolant sensor circuit. The FSM pointed toward a bad ground in circuit 410 or 808. Code 21 TPS high voltage Ground circuit 808 is the one for the TPS. Code 23 Intake air temp sensor circuit Low temp indicated. The FSM says if the air intake is warm the sensor will read low temp to the ECM. I suspect the fresh air tube was very hot from heat trapped under the hood.

Now hear is my question on the TPS I checked the voltage on it and it has .63 volts with a closed throttle and only 4.1 volts WOT. Per my FSM it should be .54 volts closed throttle and 5 volts WOT. Would this be a good indicator that the TPS is bad?

 

[jsup]

Today it was 98 degrees out and I drove my car about 6 miles and then parked it for 3 hours. When I went to start it it ran very poorly and then it would quit. H ad a service engine soon light and so I went through several turn off engine restart cycles and it would not clear. Got the trusty paper clip out and read the codes. I had Code 15 Engine coolant sensor circuit. The FSM pointed toward a bad ground in circuit 410 or 808. Code 21 TPS high voltage Ground circuit 808 is the one for the TPS. Code 23 Intake air temp sensor circuit Low temp indicated. The FSM says if the air intake is warm the sensor will read low temp to the ECM. I suspect the fresh air tube was very hot from heat trapped under the hood.

Now hear is my question on the TPS I checked the voltage on it and it has .63 volts with a closed throttle and only 4.1 volts WOT. Per my FSM it should be .54 volts closed throttle and 5 volts WOT. Would this be a good indicator that the TPS is bad?

Make sure it's adjusted correctly and opening all the way up. What ECU do you have? The newer ones, 90+ auto calibrate the TPS, so even if low should work.

 

[John Robinson]

The TPS is not adjustable. Unlike the L98 that had a TPS with slots for the mounting bolts my 93 LT1 is just two holes for the bolts. The voltage values are whatever that particular TPS will do. Yes you are right the ECM will adjust itself to zero out the low voltage reading but I am unsure if the low WOT voltage effects the fuel pulse to be lean. My thinking is that if the WOT voltage is 1 volt lower than what the ECM is programed for. Then the ECM is only providing the fuel of let's say a 3/4 throttle instead of an open throttle.

Well you have forced me to think about how all this works and in so doing that I believe I have the answer to my original question. The ECM can only do so much based on what it is programed to do. I see it now the ECM

 

[jsup]

The TPS is not adjustable. Unlike the L98 that had a TPS with slots for the mounting bolts my 93 LT1 is just two holes for the bolts. The voltage values are whatever that particular TPS will do. Yes you are right the ECM will adjust itself to zero out the low voltage reading but I am unsure if the low WOT voltage effects the fuel pulse to be lean. My thinking is that if the WOT voltage is 1 volt lower than what the ECM is programed for. Then the ECM is only providing the fuel of let's say a 3/4 throttle instead of an open throttle.

Well you have forced me to think about how all this works and in so doing that I believe I have the answer to my original question. The ECM can only do so much based on what it is programed to do. I see it now the ECM

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I've been doing aftermarket performance for too long, I have to remember some of this..

When I said "adjusted correctly" I really should have said "no interference". It was late, I was tired. I have seen cases where something interfered with the TPS restricting it from going all the way. I don't know if it was bench tested or tested on the car, just ensure something didn't change that interferes with the motion of the TPS, that it moves and returns freely.

IF you open the throttle blades all the way,and don't have enough fuel, you should be creating a lean condition that will not run right. You would hear "popping"However, like I said, it's been a while since I did worked on a stock car, I can't be sure when the blades come 100% open related to the TPS off the top of my head. They could be "all in" at 80%.

A TPS is $25. Cheap if you want to just change it, but I'm not a fan of parts swapping, I like to know WHY.. So to answer your initial question, I don't know if 4.1 volts is "too low" but I would change it because of that. I was mentioning to look for physical interference. Aftermarket ECUs will show you TPS%, I don't recall if the stock one does that. If the stock ECU does report ECU percentage, just look at that and as long as it goes full scale, it will work.

Here's what I remember on the stock ECU. Turn on the car, without starting, put the pedal to the floor, that will calibrate the TPS, it may throw a code if it's not a full 5V. Wish I could be more help on the stock stuff, just been a while.

 

[John Robinson]

I don't know about the TPS voltage yet but I just discovered that the throttle cable from the ASR to the throttle body was about 5/8 to 3/4" slack I released the cable adjuster and tightened it up and now I have a charging beast again at WOT in both closed and open loop. At least it does not feel like it is lean anymore and I have to think about why that is. I would think the ECM would be setting fuel pulse based on demand and if the throttle was not able to open all the way the ECM would simply adjust to what it knows. :confused

 

[LT4man]

Congratulations, John, on repairing your Vette. Amazing how things lose their adjustment just with use.

 

[John Robinson]

Congratulations, John, on repairing your Vette. Amazing how things lose their adjustment just with use.

This car keeps me thinking now if I could just remember the answer's to what I was thinking. Let's see the manual is open to adjusting the TV,Throttle Cruise cable. That is it I have to go do that. I sure hope the wind doesn't flip the page to removing the transmission.

 

[boomdriver]

John, to answer part of the question, the TPS adjustment range of .54 to 5v is not etched in stone, as to what the range MUST be on each unit at a given position. That is simply the working range as reference. The ONLY important number is the minimum. That is because that is one end of the scale so its the reference for the rest of the operating range. As long as the ECM reads increase in position of the TPS with throttle & rpm increase it is ok. The ECM can see these other things from other sources. The max value is 5 v and rarely ever seen because that is what the value is for a return reference signal. Throttle response is metered by the ECM calculation of several inputs...temps, TPS, o2 and MAF or MAP....there are endless possible results from an almost infinite range of conditions...such as
Ex: IF: tps is .59v and o2 is0.004v and MAF shows .87V then the result WILL be X.

X being the solution to a mathmatical formula that changes over 600 times per second (resulting in a single injector pulse)....
These ECMs on C4s are the 8 tracks of the computing world.. as much as this particular calculation involves, the new automotive computers have 1000 times more speed and computing power/ability. OBD-II was the newest hottest latest greatest thing in '93...now even thats antique.

So, its just a math problem with infinate variables. The operating range is just that, a range for reference, not a specific point that the mechanical throttle has to meet. The 5v is the normal reference signal that the ECM uses to bounce back & forth as a fixed or known value.

 

[John Robinson]

Thanks Boom. I was stuck with the FSM telling me that I had to have 5 volts at WOT. What you say makes perfect sense. My error was that since the FSM stated the voltage should be .5 to 5 Volts that was cast in stone. I removed the TPS from the TB and ran the idle to wot by using a screw driver to move the potentiometer. The TPS did go to 5 volts with the potentiometer turned all the way. I then checked to see if the butterfly's were wide open at WOT and they were. Since I can now open the butterfly's all the way after I adjusted the cable from the ASR/ABS unit to the Throttle body I won't replace the TPS.