90 F150 4x4
302 AOD

51 (ECT possible open) has been coming up ever so often for awhile. I replaced the ECT once but I still get the code occasionally. I also have checked it's resistance per Haynes and it seems within range as well as the reference voltage.

54 (ACT possible open) & 63(TPS possible short) have just started coming up. I noticed today that the Engine light didn't come on until after I turned the AC on. Go figure. It didn't do it today, but when it started acting up the last time I drove it, it had a hard time idling and wanted to stall at low RPM's.

I just find it hard to believe that the ACT and the TPS are both acting up at the same time. Any ideas? Maybe they are on the same power wire or something. I really hate to change out all three sensors unless I know for sure that is the problem.

Thanks,

BroncoBasher

 

I have determined that I do not get the codes unless I turn the AC on. I also noticed that the idle gets real bad when I turn the AC on. Reckon the wire that tells the PCM that the AC is on is open? I still do not understand why I am getting those codes, they totally seem unrelated to the AC.

Thanks,

BroncoBasher

 

I had the idle problem when I turned the A/C on too......took my IAC off and sprayed LOTTTS of carb/TB cleaner inside, worked it back and forth, cleaned it good......fixed it.

I didn't have any of those codes though.......can't help you there.

Good Luck
Bob

 

I believe the Saga has come to an end. I was continuing to get the 51 code every now and then. A couple of weeks ago, it started running worse and I got a 51 code with a TPS code and a pickup coil code. I replaced the Pickup coil and it took care of that code and began to run much better. Ran great for a few days and then stalled out on me and wouldn't restart on a busy road. It finally started again after a couple of minutes of blocking traffic. In my head, I was sure it was the TFI, so I replaced it and the ECT. Still having a hard time starting. I got the 51 ECT code again. So I checked the voltage at the plug to the ECT and found it in the 1v range. I began to wiggle the wires at the computer and found I could make the 4.6v or so that it is suppose to have come and go. I narrowed it down and pushed my probe in the back of the PCM harness and then in the other end of the wire. No continuity. I pushed harder, and now I can't get it to fail. I think the wire in the harness must have not been snapped in all the way. So ECT problem fixed and no more bad idle or stalling. It now runs and idles great. I then noticed it was having a hard time starting. I checked codes and got a 87? fuel ckt failure, a code that I never got when I was having the ECT code set. The ECT code must override the FP ckt error code. I then noticed I didn't hear the FP running. I would cut the ignition off and on until I heard it run, then it would start and run fine. That settles it, it must be the FP relay, so I went and bought one. I was figuring that turning the ignition on and off was making it make contact eventually, and then it was fine until it cut off again. My hypothesis was that when it stalled in the road, because of the ECT losing signal and wouldn't restart because of the FP relay. 2 separate problems that I thought was 1 problem. Anyway, I decided to test the old FP relay. (tangent) I replaced this relay right after I bought the truck because it run continuously. The salesman said the previous owner was a mechanic because he had put a battery cutoff switch under the passenger wheel well. Obviously not a good mechanic, because he fixed the relay problem with a switch to keep it from killing the battery instead of fixing the real problem. Idiot! (/end tangent). I tested the old relay with my portable battery jumpstarter and multimeter. I couldn't get it to fail on the bench. I went out to put the new one on anyway and decided to check the EEC relay. The top of it came off exposing the insides. Not good. I took it in and found out it had a bad ckt across the coil. I couldn't get a consistent resistance reading across the coil. I compared the relays and determined the EEC is the same architecture as the FP relay. So I replaced it with the new FP relay. I think some of the other miscellaneous codes I was getting may stem from the EEC relay. I think that VPWR that that relay feeds on the EEC is what the EEC uses to power all of the other sensors on the engine. I now notice the FP seems to run better and the vehicle startup response seems better. I think I have it licked. Will have to drive it alot the next few weeks and see if it acts up anymore. I'll keep my fingers crossed. For good measures, I may buy another FP relay and go ahead and replace it too.

One thing that still has me puzzled. The check engine light still came on even when the FP didn't. If the EEC relay kept my FP from running, why did the check engine light still work? Or is that light independant of the EEC relay?

I just thought I would give you guys an update and hopefully my story will help some other poor soul figure out their problem.

BroncoBasher

 

The computer grounds the check engine light circuit to activate the light. If that circuit defaults to closed when the computer is not powered on, then the light would activate even though the EEC relay was not supplying power.