I have a 1990 Bronco with the 5.8 EFI and it bucked and surged going down the freeway then it quit with a smell of burnt wire. Found out that it was the ECM pin 46 the common line to the MAP, EGR, Throttle Position and Knock sensors. Now the truck will run but very rich with the MAP Sensor disconnected but plug it back in and the engine dies. I have taken the harness apart looking for a short and found nothing. All the sensors have the 5.0 VDC and also 12.56 VDC on them. I hope this gives enough information so I can get some direction to get the truck running.

 

Welcome to FTE and the Big Bronco Forum.

Every sensor has battery voltage to it? That's not right... or good. The only sensor that needs battery voltage is the O2 sensor... the power heats the sensor when it is cold. The +5VDC reference voltage is required by all of the other sensors. Somewhere something has shorted. The ONLY common voltage to all of the sensors should be VREF (+5VDC from pin 26 of the ECM). Each of the sensors you mentioned however sends a modified VREF back to a dedicated pin of the ECM. The TP sensor sends to pin 47. The EVP (EGR Valve Position) to pin 27, and the MAP to pin 45. Each of these sensors has one additional wire in common...guess which pin of the ECM is the signal return common... Pin 46. (There is no knock sensor on a 5.8). This is the most likely location of the short to battery voltage. You can test this theory buy checking voltage at pin 46 of the ECM. Voltage should never be any higher than the VREF output at pin 26. It can be lower.

If you find that the voltage at pin 46 is higher like oh, say +12.56 VDC, its probable (especially with the smell of burnt electronics present) that this common has somehow gotten shorted to a battery voltage positive somewhere. Where? Most likely where ever you find the burnt components or wires. Correct the short to battery positive, and you can try running the same ECM but I'd be willing to bet that even if it DOES work, its not going to work for long or properly after experiencing that kind of power surge. But I could be wrong about that. With the fact that the truck will still run with certain sensors disconnected says that you might get away with this. (And the truck should run stupid rich with the MAP disconnected because the ECM has NO idea how much air is in the mix with it disconnected).

 

well we took the plenum off and took the whole harness apart and pulled all the wires apart, then I soldered a jumper across the burnt trace now the truck runs but needs timing. No check engine light with all the sensors connected. I hope I can make it home now and put the timing light to it and pick up a new ECM.
Will let every one know as soon as I know.
Thanks for the input.

 

Well after all we did on the way up the mountain this morning the check engine light came back on and then the truck died...just cranks. It does have good spark but wont fire. HELP!!!!

 

Pull codes and see what caused the CEL to come on. If the truck won't start, the best you can do is a KOEO test but the continuous memory codes are still retrievable. From there you can better determine where the problem has originated.