First I would like to say Hi and everything being I just registered.

It seems my 91 F150 is giving my little brain a run for its money. I am hoping someone can help.
Its a 91 4x4, 5.0 with and auto trans. Right now its really acting up. The trans isn't shifting outta first, it dies and is hard to start, etc. It set a few codes - for TPS and MAP but it also set a 98 which is for the computer. I don't think the TPS and MAP codes mean there is a problem with either of those sensors but just that the computer is unable to make sense of what is happening.
So I took it up to the shop and they are stumped as well. They hooked it up to the scanner but can find no real problem. They said they tested the map sensor and didn't see any voltage coming out of it. I replaced this about 2 year ago cause the engine light came on with a map sensor code. I went through the diag on it and didn't see any voltage coming out of the map then so I replace it. I tested the new one and there was no voltage coming out of the new one either. Beats me, the truck ran ok and no codes or running problems after that. But that was 2 years ago.
So we are stumped, my mechanic and I. We don't see any blown fuses in the block on the drivers side of dash near the bottom. I was wondering if there was a power dist. box somewhere else. I see the one on the drivers side engine compartment but nothing pertinent in there I don't think.
I am think its a ground problem but all visuals seem to check out unless I am missing something. Any help would be appreciated, thanks.

 

98 can also be a transmission code if you have the E4OD. The computer supplies power to those two sensors so it could be bad, but I would check for a broken splice first. I had a factory wire splice come apart on my truck which killed power to the coil.

 

Its an AOD. I am sure its a bad wire or ground. What freaks me out is having no return signal from the MAP. I replaced the old one for that reason and the new one still had no signal but the truck ran OK. I hate to start throwing parts at it, it can be expsensive and when it don't fix the problem then it gets depressing.
The new Map ( and TPS) are from Napa so I know I am sitting in Nowheresville with that crap. In fact I just replaced the front calipers from Napa that where less than a year old, leaking like a bitch. They tried to charge me for the bottle of brake fluid I would need but I was having none of that.

 

You won't get a return signal from it if it isn't getting power. I would check for the broken wire, and if I didn't find one then I would try a new computer.

 

I beleive it is getting power, just no return. I will double check that. I think its a 3 wire Map so I guess 1 of those wires is a ground. Maybe that wire is the culprit.
If the map is bad that would cause the trans problem and the running problem.
I am just thinking aloud here and thanks for your input.
Like I said, I had the check engine light come on about 2 years ago with a map code. I check the map and found a good input signal but no output. The engine was also racing to 4 grand.I replaced the map and rechecked the inputs and outputs. It had the input (.5volts) but still no output. I then realized the high rpms was caused by the cruise control cable popping out of the retainer on the throttle body and jammimg the throttle open.
After I fixed that the truck seemed to run Ok for a few years. No engine light or running problems. Wierd I suppose with the map not sending a return signal.
The truck is a the shop and they told me about the condition of the map. I will head up there and check it out myself cause I gotta know what in the heck is going on.

 

If you can afford it, take the truck to a shop the specializes in automotive electrical work. They should be able to track down the short for you. I had a 93 Dakota that had the emergency flashers not working. Wouldn't pass inspection without them. I took it to an electrical specialist shop and they found a shorted wire that ran under the washer fluid reservoir. Only cost me $125 and saved me a lot of headaches.

Good luck.

 

Try disconnecting the Battery for a minimum of 5 mins. This clears the KAM & allows it to rebuild. Sometimes the ECU locks out a sensor that continuously reports out of range. After replacement, it may take 40-80 completed warm up cycles, for the input to once again be utilized. This method skips that lag time. Afterwards, you should drive your truck 15-20 miles w/ multiple completed warm up cycles, prior to rescanning.