Working on a 94 Z28 (6 speed) which is giving me some funky readings from the TPS.

When using a Tech 2 and looking at the TPS, I m only seeing a maximum opening of 24%. The TB blades are open 100%. When depressing the throttle and getting to 50% at the blades the Tech 2 is only signifying a 1% throttle opening.

I've got FSM on order, but am thinking that the ECM my be interpreting the data incorrectly.

Any suggestions?


Update below.

 

What is the closed throttle votage? What is the voltage reading at WOT when the Tech is indicating 24%. The PCM notes the closed throttle voltage at startup, and prorates throttle position percent over approx a 4 volt range. If your TPS voltage is off, your TPP will be off.

 

Voltage at closed throttle is .2x volts. WOT it gets to around 1V.

 

There's your problem.

Key on, engine off - check the voltage between the gray and black wires. Should be 5 Volt reference supplied by the PCM. If you have 5V between those wires, you have a problem with the TPS sensor or the blue wire.

Closed throttle voltage has to be between 0.20 - 0.90V. Anything below 0.20V sets a code. When you slowly open the throttle, the voltage should increase smoothly from the closed throttle voltage to about 4V above the closed throttle voltage at WOT.

 

Voltage gray-black is 1.65V. Does this indicate a defective PCM? I have not been able to verify the integrity of the wiring. Perhaps an extra load somewhere could cause the lack of voltage. It does not run particularly well, excess HC and O2 in the exhaust, seemingly compensating for a misfire that does not exist. Opti, plugs, wires, O2, MAF, TPS sensors have been replaced.

 

Here's the schematic (95, but your 97 - guessing that's what you have from your screen name - is similar for the TPS), courtesy of Shoebox.

http://shbox.com/1/1995_pcm1.jpg

Note that in the 95 the ground (black wire) for the TPS is shared with the coolant temp sensor. A problem on the two circuits would explain your problems. Check the voltage on the CTS circuit (harness on the sensor). If you can tell me the voltage and the correct temperature of the coolant I can tell you if you also have a problem with the CTS circuit. You should have 5V on the circuit with the connector off the sensor.

 

Getting 2.3 volts at 68 F.


Thanks for the help Injuneer. And this is on a 94.

 

2.3V corresponds to about 50*C or 120*F.

Where did you read the "68 F" from?

 

The car hasn't been driven in 2 days. But to check for sure, I opened the cap and used a thermometer in the radiator. It has been a cool summer in the NW. I measured the voltage by backpining the ECT with key on/engine off. Thanks again

 

swapped out the ECT: no change.

I guess I will now test the wiring for continuity: common sensor ground and v ref line to TPS

I think the PCM is next. How much to flash the eprom?

Thanks.

 

injuneer can correct me if this method is wrong, but i had huge issues with this, i had to set the throttle stop and make sure the blades on the tb were all the way closed and all the way open (basically that the throttle was working properly) then i had to stop the tps?(i think thats what it is) on the left side of the tb, i had to slot the bolt holes on it so i could rotate its position so that i could get the proper readings at idle and wot. took a little time but i finally got it right and it made a HUUUGE difference lmk if i need to clear this method up for u (but pm me it might not send me an email saying to check this thread and i might forget--sorry)

 

His problem is not adjusting the TPS. His problem is that the TPS is not getting the correct reference voltage from the PCM on the black and gray wires.

Question - when you got 1.65V across the black and gray wires, was the connector on or off the TPS? If it was with the connector still on the TPS, unplug it and check the voltage again.

 

1.65 volts with the connector unplugged.

 

Would be interesting if you could measure the voltage at the PCM connector pins without the wires attached. Would be hard to do though, unless you cut the wires.

 

Ok, took some time today after a hiatus for personal reasons.

Got the service manual and started the testing procedures. Got it narrowed down to the PCM. Ordered a replacement and programed, and installed. No change.

Got a new TPS pigtail and wired it directly to the PCM bypassing the factory harness. No change.

Started playing around with the two circuits that are the 5 Volt reference. I pulled the B28 pin for the TPS ref, and repinned it to the B29 (the 5 volt reference for the map, AC pressure and AC temp sensors). I know had 5 volts to the TPS like I should.

So lets daisy chain the TPS onto the B29 Circuit along with the original stuff. TPS drops back down to 1.7 Volts. No good. After racking my brain for a bit, repinned back to the factory settings. Hooked up, back to square one.

Now started doing some random checks for the 5 Volt reference lines.

MAP Sensor 5V
AC Temp 5V
AC press 5V
TPS 1.7

Unplug the map sensor, all the sudden I have 5 volts at the TPS!!!!!!

Plug MAP back in, try the same scenario with the AC press and Temp sensor, neither makes the TPS jump back up to 5V.

Tried isolating the MAP sensor, bypassing the factory wiring. No change at all. Then used a replacement MAP, and still no change.

I am about to torch the Camaro!

Any thoughts before the gasoline gets poured.