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1991 F250 DTC Problems

Hello all,

I have an issue with my truck that I am hoping you can help with. I have posted in forums like this before and not had very good luck.

I have a 1991 F250, 5.8 L, 125,000 mi. I have owned the truck for about 8 months and would actually like to be able to drive it. I bought it with a check engine light on (almost a death blow in CA because it causes an automatic failure when it is SMOG tested). I ran codes with a jumper wire in the test port. I got a Code 63-TPS low voltage and Code 98-Hard Fault. On the recommendation of a mechanic firend I changed the TPS and IAC Valve. No change. I then went out and bought a tester at the parts store and ran the test again and got the same codes. I tried to complete the code reset procedures and the codes would not clear. I have also read that the Code 98 could also mean that the Neutral Safety Switch could be bad.

I have taken it out on several short test drives (around the neighborhood) and it seems to shift hard into second gear when moving from a start. When I bought it I had it towed home because it would not stay running when put into gear. When I started digging around I found that it was low on tranny fluid. I filled it with about 1.5 gal of tranny fluid and corrected that problem. Hoping that the hard shift is not because of the lack of tranny fluid. Previous owner had stopped using the truck and only drove it a couple of miles from home to service station that I picked it up from.

J
 

Fellro

Moderator
Staff member
Hard shifts are not due to low trans fluid, they are actually from high pressure shifting. That does play into the TPS, but remember, just because the sensor isn't the issue doesn't mean that the wiring isn't the problem.
 

Fellro

Moderator
Staff member
It will take a bit more diagnosis, in that you will need to take readings with a multimeter. You need to be sure it is getting signal reference voltage, and then you have to be sure it is getting back through to the computer. That is where the fun really is... Before you get too wild that way though, there have been some people have issues with corrosion at the computer itself, or could be at the sensor plug. You can get dielectric grease and put it in the connector to sometimes help with keeping moisture out and maintain a better connection. Look for any broken wires in the harness, mostly outside of the loom. Look for where the harness could have chafed through and is rubbing/shorting out. Also, be sure that the body grounds are hooked up to the engine. They will make trouble as well. If nothing more, you can make one like they used to have back in the 60's and 790's where you just take a wire, put a couple eyelets on it, then connect to a bolt on the firewall and a bolt on the motor.
 
Take multimeter readings at the TPS? What is signal reference voltage (I can turn wrench but electrical is not my specialty)? How do I determine if signal is getting back to computer? Where is the computer located so I can check for corrosion? I will go and spend a few pennies on dielectric grease and see if we have any changes. I will also check the wire loom and grounds.

I will post again in a couple of days when I have checked out all of these things.

J
 

Fellro

Moderator
Staff member
I would have to defer to a manual, but typically it is around 5 volts. You have to probe the wire (one amongst many) at the computer to see what the return is to make sure it varies with the position. This is where a service manual is the best way to determine, as they give the pin that the wire comes back into the computer at, as well as the appropriate voltages. Hopefully Vince will be able to give those details on the certified thread. He may have a better way to zero in on it as well.
 

dustybumpers

don't play well w others
TPS needs to be adjusted, not just changed. Use the voltage references as per the Haynes manual. there is an adjustment procedure
 
Hello all,
Hopefully there is somebody out there still watching this forum. I finally had some time off to test my TPS. I tested the TPS voltage- 0.03v; getting power from PCM-5v and have good ground. TPS was installed brand new. Thinking I got a bad one. Gonna take it off and swap it out at the store unless somebody out there has some pearls of wisdom.

J
 

Fellro

Moderator
Staff member
The TPS varies the voltage, that is how the computer knows where it is. The voltage signal back to the computer will go up an down depending on where it is at in resistance.
 
I replaced the TPS this afternoon. Fixed the CEL problem momentarily. I took it for a short test drive around the neighborhood. About halfway through it came back on again. I ran codes with a jumper wire in the test plugs and got a Code 11-all clear.

Any ideas/suggestions?

J
 

fatherdoug

Tonto Papadapolous
I had a similar problem. The check engine light would come and go. The voltage on the TPS should be between 0.96 and 0.99 volts. My TPS would drop below 0.96 volts, and the check engine light would come on, then go away.

Listed below is a really good article that one of the members on this site (Flareside Thunder) provided from another forum.(F150 Online Forum).

http://www.fordtruckfanatics.com/forum/showthread.php?t=20951

Replacing the TPS solved part of the problem. I had to drill the mounting holes on the TPS a little bigger so I could adjust the TPS to get more than 0.95 volts.
 
Got the TPS problem taken care of. Got a defective one from the parts store. After I put the new one in the CEL went off. Took it for a test drive and CEL came on less than a quarter mile from the house. Ran codes again and got a Code 33; EGR not opening properly. So I guess I am chasing problems now. Hopefully this will fix the problem(s) once and for all.
 

dustybumpers

don't play well w others
Take a vacuum line to the top of the egr, and apply vacuum, and watch to see if the diaphram is moving. If not, replace the egr.
If it moves, replace the egr gasket between the egr and the plenum, with a gasket with a 1/8" hole where the huge hole is. You will have to make your own gasket.
 

fatherdoug

Tonto Papadapolous
Got the TPS problem taken care of. Got a defective one from the parts store. After I put the new one in the CEL went off. Took it for a test drive and CEL came on less than a quarter mile from the house. Ran codes again and got a Code 33; EGR not opening properly. So I guess I am chasing problems now. Hopefully this will fix the problem(s) once and for all.

Thank you for the update!
 

fatherdoug

Tonto Papadapolous
Any parts store should sell a 8 1/2 by 10 sheet of gasket material of your choosing. You can also buy a variety pack of different materials.
 

dustybumpers

don't play well w others
There is a big carbon problem over the years, and no real solution. This was a "fix" that worked for me.
Shut the cel off, got rid of the code, and got rid of the "shudder" at part throttle.
 

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