Of course. It is the vehicle speed information to the brain, er PCM

The point is that the signal represents actual vehicle speed.

 

I saw a 1st gen years ago that acted the same after someone had rebuilt the transmission and turned up the line pressure. Has your line pressure been modified?

 

So your saying that the od works fine when manually activated with a switch, kinda rules out the hydraulics ans solenoid. Have you done any voltage drop testing on the ground side of the ecm/vehicle wiring? Maybe a poor ground ...they can cause odd mayhem, BTDT many times.

 

Yes, speed slowly keeps increasing.
It's not, it's lower.

Tried it, made it worse. Put it in the 5V line and it helped a little but not enough.

4 volts. I have the full throttle travel adjustment. Wasn't working right before that so it didn't cause it.

Both

It was doing this before the transmission rebuild.

I've been through everything more than once trying to find this little I've replaced wires, bypassed connectors, and added grounds. Every ground has been pulled, cleaned, and checked. If it were a resistance problem, you'd think I would have to turn up the TPS voltage, not down. All my resistance checks went from the ecm connector to the end of what ever circuit I was checking, fixing as I went.
The VSS does not produce any electricity. It is a switch that produces 8 on/off pulses per revolution. So both ecm and speedo are using the same signal.
There are 3 switch signals supplied to the ecm for OD operation: coolant temperature switch, transmission temperature switch, and the park/neutral switch.
I am starting to gather parts for plan "B", as in bypass.

 

Thats about as frustrating and aggrivating as it gets - to have checked and re-checked everything multiple times and still not find the culprit.

I'm at a loss because you're way ahead of me on all this. The only weak link I can see is that you checked all wires and bypassed the questionable ones. Could there still be a bad wire out there?

This is all compounded by the fact that you have to get out and test drive for a while at cruising speeds which adds a lot of time beween troubleshoots.

Wish I could be of more help.

 

HUH, I'm confused now.

If it is 4 volts, try putting a 270 ohm resistor in series with the 5V wire on the TPS. If that helps some, try 490 ohms.

 

Dumb question, as I don't have a FSM for a 92, is the cruise like the newer Gen. 2 trucks, I.E. Adaptive ( learning )?

Greenie all of the sudden one day started running away, she would go about 7-10 MPH over set point. Drove me freaking nuts trying to figure it out.

Finally found that little tidbit in the FSM, and once I re-taught it, has been fine ever since.

Just a thought if the first gen uses the same style, I never did figure out what scrambled its original learning, and hasn't happened since.

 

On second thought, put the resistor in the ground leg of the TPS, and readjust to 1.0V at idle.

I got shanghaied by the DTR database error routine and lost my train of thought. The TPS range is out of spec. Bring it in with trimmer resistors compensating for your mods before you condemn it to the junk heap.

 

First gen and a half , the PCM stores TPS last run low setting. You should disconnect the battery for a few minutes after a TPS adjustment.

 

The mystery is why the TSP range is out of spec. Was that way when I bought the truck and started tinkering. I tried a 500 ohm 10 turn pot on the 5V side (in 1st post) with no significant results. I'll try it on the ground side. I am usually very good with electrical, but this has me baffled. My biggest ? is what is going on in the ecm.

 

If the supply is at 5 volts, the pot is wrong, or the linkage is over driving it.

The reason I spec's the resistor in the ground lead is to compensate for either of those.

Don't forget to reset idle at 1.0V is you change the 10 turn pot.

 

I don't think anybody mentioned it, is your TV cable adjusted properly.

 

Not following you, the ecm supplies the power which is 5 volts from pins 4 (the return) and pin 6 (5V), the center (signal) of the TPS sends the varying voltage to pin 22.

 

Either something is wrong with the tracking in the pot, or it is being swung too far by the linkage.

In either case, the resister in the ground lead starts it out near the low end, and mechanically it shouldn't reach the high end.

The pot is 2.5Kohm, so 1V would be at 500 ohm, and 4V would be at 2Kohm.

With a 300 ohm resistor in the ground lead, 1V would be at 260 ohms on the pot, and 3.5V would be at 1660 on the pot.

Should be pretty close, maybe need to adjust trimmer downward a bit, but start there.

 

I happen to stumble across this post from my phone and thought I may be able to help you with some diagrams. If these get you somewhere but you still need more info let me know and I'll see what I can dig up. Hope this helps!

Engine Controller


Cruise Control Switch


Engine Compartment


Fuse Block & Ignition Switch


Instrument Cluster


Multi-Function Switch