Got the truck back from the shop today. Basically to make a long story short, It’s thrown code 452 since the start. we’ve replaced VSS twice, PSOM, ICM, and while clearing codes on it one day, the computer flat out died. Wouldn’t even turn over. We figured that might have been the problem, considering it died and everything, but no it ended up continuing to do it. It doesn’t do it all the time, just occasionally. The most recent thing we did to it was put the new psom in, and then a new vss, for the second time. It ended up popping back up again, it seems to get confused when coming to a rolling stop or basically going at a slow speed and then getting back on the gas, and will rev up, then kick into gear. So back to today. The shop replaced the TPS, and I hope that fixes it (knock on wood). Buttttt, If it doesn’t, what should we do next? I’m thinking we test the wiring to the vss, or splicing a new connector on. Suggestions, comments, thoughts, are much appreciated. 1994 f-250 5.8 2wd e4od

 

Ooof. Throwing parts at it, wont fix the problem. These trucks are really simple and theyre ECU logic is easy to understand; It receives a signal, and it output a signal. Is your speedometer bouncy? Has anyone put a Oscilloscope on the test connector for the VSS and see what its doing? The VSS test connector gives raw VSS signal. If that signal is good, next thing to do is to test the connector on the cruise control servo. The cruise control servo received conditioned signal from the PSOM, same signal that the ECU receives. I suspect you'll find you have a corroded connector thats stopping the VSS signal from reaching the PSOM.
If the problem isnt apparent, youre gunna need to remove the computer, and probe each and every circuit and sensor, at the computer plug, and with the help of the Factory service manual, find out which circuit isnt happy.

 

Before replacing the VSS or the PSOM or any other parts you should electrically check the wiring. Now that you've wasted a bunch of money you should electrically check the wiring. I'll bet that is where the problem lies because the problem usually is in the wiring.

 

A false neutral could be a MLPS problem but like Mark said you need to be testing things especially since new parts can be defective right out of the box.

 

I understand, there’s just nowhere around here that has that kind of equipment or knowledge, so that’s why we put parts on it. Seemed like a simple fix, now it’s been a lot worse. Btw I do not have cruise control, and the speedo doesn’t bounce. We’ve already checked pcm when we got another. Psom looked good too, but we could be wrong. Either way, Clearing the codes seems to work for about 100 miles, but I would much rather fix the root cause, as it’s a nice truck and I hate to see it act up. The shop I took it to said checking wires is an absolute process, and it could be anywhere, which I agree with. Just hope a new tps may have fixed it.

 

Also forgot to mention OD light flashes some of the time

 

It will flash when the 452 code is set.

I have a question. Is this a real problem, or a hypothetical question?

 

Like I said, It just got out of the shop, and it could be fixed, Idk. But It only does it every so often, and I won’t know until then. But If it is not fixed I just wanted to know where to go, if that makes sense. This is a real problem, just don’t know the direction to take if it persists

 

Does that clear it up at all?

 

Unfortunately, you made the problem worse by throwing parts at it. I dont mean to beat you up over this, but new after-markets parts are sometimes worse than going to the junk yard and pulling parts off a junker. By randomly swapping parts, any of those new parts could be causing more problems.
What do you mean "We checked the PCM"? How did you check it? Did you open it up and look for leaking capacitors and water damage, or did you send it out to have it bench tested? You dont need to have Cruise control, you just want access to the connector to look at the signal from the VSS.
Checking the wiring on a vehicle is not a "Absolute Process", most mechanics just dont like doing it because they can make more money replacing timing belts than they can finding wiring issues. You can do it yourself if you have a multimeter. Theres only about 3 connector between the VSS and the PSOM. And only 2 connectors between the PSOM and the ECU. The PSOM isnt a suspect because its inside the cab which is usually weather-proof. But the ECU connector and the connectors under the master cylinder to the VSS are exposed to dirt, salt, and moisture all the time.

 

Replacing the TPS wont fix the VSS problem.
https://www.ebay.com/itm/14465342334...mis&media=COPY this may be the best $33 you've ever spent. Ive never used one this cheap, but all you wanna see is a wave form; you dont really care about the accuracy of it.

 

Thank you. What do I do if I get a bad reading? Do you buy new connectors? Where would I buy them? I’m not electrically inclined so it would be a challenge to check any connectors and replace them. And yes we checked for leaking capacitors on the pcm

 

If you get no signal or a bad signal, you start working backwards throu the circuit, disconnecting plugs or backprobing the connectors until you get a good signal. Once you find a good signal, you've narrowed down where the problem is and you can focus on the connectors or wiring in the small section of harness. I don't suggest replacing the connectors, just clean them by rubbing them with a small file, a pick, or sand paper. If you do need to replace the connector, you can find weather proof connectors at your local auto parts store. But you gotta ask them because theyre usually behind the counter or special order.

 

And you do this with a multimeter?

 

You check resistance with a multimeter. A good vss sensor is between 800-1600 ohms. That tells you the coil inside the sensor is good and wiring between the multimeter and the VSS sensor is good. But a multimeter doesn't tell you what the quality of the signal it's generating or if it's generating a signal at all.
To check the signal, you need an oscilloscope, which measures voltage over time. By spinning the rear drive shaft, the VSS sensor creates a signal that the o-scope can read and show you. You can do the same thing with your Throttle Position Sensor to find bad spots in the TPS windings. Handy tools for those who want to own older vehicles.