Your OBD-II vehicle is different from the OBD-I vehicle that most of us on this forum have. You can't get the codes without a scanner, the check engine light works differently, the implications of disconnecting the battery are somewhat different.

Pin numbers are too ambiguous, wire colors are different year to year. Unless we extensively research your paritcular truck, we can only answer in generalities that you must then apply at least some legwork to resolve to your particular situation.

So I use signal NAMES rather than wire colors. VREF is the source of 5 volts to the sensor. It is the only one (on the harness side) that should have voltage on it with the key on.

SIGRTN is the ground return for the sensor. It is the only one (on the harness side) that should show a low resistance with the key OFF.

TPS Signal is the other wire. Most likely it is the middle wire of the three wires on the connector. This is the signal that the computer is complaining about. The computer expects the voltage (with everything hooked up and the key on) to be between about 0.5 and 4.5 volts.

I had intended that you make all ohms readings with the key OFF. Any crappy ohmmeter will do for what we are looking for. Test that the meter is at least this functional by putting it into "ohms". Use "1x" if you have a choice. Touch the meter leads together. The needle should drop to zero. If it doesn't, and it has a "zero" adjustment, adjust the meter to read zero with the test leads shorted. When you stop touching the leads together, the meter should read full scale (infinite ohms).
If this works, you are in business.

Measure the resistance of the TPS harness side connector to ground. You should see near infinite on Vref and TPS Sense. You should see zero on SIGRTN. If you see zero or near zero on TPS sense, you have found the problem.

 

vref is at zero. with the key on this shows 5 volts. with the key off measuring ohms its at zero. sigrtn is zero and tps sence is infinite...now what? is the wire fryed or something

 

OK. It looks like SIGRTN is OK and VREF is OK. TPS Sense is not shorted to ground. It might be an open circuit back to the computer. Corroded 60 pin connector, break in wire, etc.

A couple tests you could try:

Deliberately leave the TPS sensor disconnected and drive the truck around. Does it run about the same, or worse? Does it set the check engine light right away, or does the check engine light behave the same as when the TPS is connected? The idea here is to get a basic idea of whether the TPS functions at all from the computer's point of view. If disconnecting the TPS sensor entirely makes no difference to how the truck operates, it is likely that the TPS sense line is open.

For idea #2 you need a sewing needle or something similar. Pierce the insulation on the TPS sense wire, such that the needle makes contact with the conductor inside. Now connect the TPS sensor. Turn the key on. Don't start the engine. Measure the voltage between the TPS line and ground, using the needle as a test point. Operate the throttle through a range of motion. The voltage at the TPS sense line should start at 0.5 volts or above, and rise smoothly to about 4.5 volts as you open the throttle.

 

havent done idea #1 but #2 heres what happened. I put the pin in and it measured pretty close to zero volts. i moved the throttle by hand and the volts went evenly to 1volt or about half throttle, then i heard a click sound like a little mini relay or something and the volts dropped back close to zero. this didint happen every time i tryed it sometimes the volts wouldnt move at all. I also heard something (im asuming its the computer) the only way i can describe it was like if you ejected a cd. this happend twice. does this give you anything to work with? I think were getting close. thanks again

 

The TPS sense should read more than zero at closed throttle. There is definitely something wrong. My best guess is it is shorted to some other signal.

Two possibilities for the sound effects:

1) The computer is (belatedly) recognizing that the TPS is no longer at idle when you get to half throttle, and is opening the IAC all the way. Opening the IAC is done as a part of the normal part-throttle cruise strategy. Normally this happens at throttle openings well below half throttle. That could be the "CD eject" sound.

The relay click could be the cannister purge valve, or one of the other emission controls, changing state once the computer "thinks" the vehicle is no longer at closed throttle. The problem with theory #1 is that I would think the computer would factor in engine RPM, MAF, or some other indication of being off-idle besides just the TPS position. But then I didn't write the software. If you have a buddy with the same truck, try advancing the throttle under the same conditions and see if you hear the same noises.

2) Whatever the TPS is shorted to is being activated by the voltage from the TPS, once said voltage gets high enough. The problem with this is I can't explain both the relay click and the buzzing sound.

It would be nice to have a breakout box or some sort of made-up pigtail to splice into the TPS harness connector. I would suggest trying to isolate the TPS from the wiring back to the computer, and watching the voltage from the TPS side. It should behave more normally -- going from 0.5 to 4.5 volts.

I hesitate to suggest you cut the TPS sense line. I'm not sure you have the ability to splice it back solidly. But it would be the next logical step in figuring out why TPS is so low. If the TPS output behaves normally without the wiring going to the computer, then the next step is to trace the TPS wire through the harness back to the PCM connector.

One more test could be to repeat the sewing needle measurements on VREF and SIGRTN. We should see VREF stay constant at 4.5 volts plus, and SIGRTN stay constant at less than 0.5 volts, without regard to throttle position.

 

the engine light came back on,,and i didnt get the relay or cd noise. its no longer going to 1 volt its stays steady at less than half a volt ,no change when the throttle is moved. i did the last test you suggested SIGRTN stays constant at less than.5 VREF stays at 4.5. So youve narrowed it to sense. Now were is the pcm located? how large is it? im callin it a day. but if you reply ill hopefully connect with you tomorrow.....thanks

 

If your 1996 is like my 1991, the PCM (computer) is mounted on the driver's side of the firewall, about halfway down. It is actually inside the cab, the wires connect through the firewall. Mine has a 60 pin rectangular connector, yours may be different.

I suggest trying to follow the wire bundle from the TPS back to the connector. Look for chafed wires or other damage. Try flexing the cable to see if the problem clears up. Don't flex it too hard -- it is possible to break the IDM resistor if you get too vigorous with the bundle that contains it.

 

If u clear codes or not ,Check your tempture of your thermostat if it dont warm up to 170 say in a drive cycle your MAF sensor or Your TPS sensor wont figer it out for you tell thats achave....u have to do it at 40 to 100 degrees outside drive at 40 MPH untell it reaches 170...anyway all three need to work togeather to keep your idial at 850 RPMs