rkt68411

In my haste to jump start my 1991 XJ I mismatched one end of the jumper cables. Went so far as to try and start it.(insert dumba$$ here). Got out, got the cables on correctly, went to crank it up and no start. It turned over just fine. Started to smell that burnt electrical smell, jumped out again and disconnected the jumper cables and the negative battery cable. Along with the smell there was white smoke coming from the ECM. After making sure it neither vehicle was going to burst into flames I began kicking my own a$$. I think I read that my 91 is in the years that the ECMs are plug and play.
Having said all that to ask this...What else should I check/replace after this boneheaded stunt? The salvage yard I go to has a good supply of early 90s jeeps.
Thanks in advance.

tech

Yup once you let the smoke out you can't put it back in . just get another ECM and it should be fine if it didn't fry any sensors .

boxburn

Your alternator may have taken a hit too. One thing I do know on ECM compatibility is they are different for emissions (Federal and CA) and Auto and Manual. Those are the 2 things I'd make sure your Jeep and the ECM donor have in common.

rkt68411

Installed an ECM I pulled from a 1993 XJ. No obvious physical damage for what thats worth. Now its cranks but wont start. I did notice that I cant hear the fuel pump run for that 1 sec after you turn the key. I suspect the crankshaft sensor took a hit. Anything else I should consider replacing?

Cummins93

Can you manually jump the fuel pump to get it started?

country2

Sound's like you may need to replace the jumper cables with NOCO Genius Boost Plus GB40 to where they wont let you jump off with the cables reversed? Sorry man.....

Steve Hayes

Well that sucks! Nothing worse than self-inflicted repairs.

rkt68411

1 aw sh!!t erases 10 attaboys. Nearly broke my hip kicking my a$$.

rkt68411

So now I have no spark from the coil and no fuel to the rail. Cranks like a champ, tho. I can jumper the fuel pump and it runs. I have replaced the battery, the fuel pump relay, the CPS, and 2 60 amp fuses. One for the ignition system and one labeled Alt. pwr 2. I installed an ECM from a '92 XJ. All other fuses are good. The only other item that is different is the gauge cluster. I swapped out the original with one from a 93 XJ. It has gauges and not idiot lights. I'm at a standstill.

RockyMtn96XJ

Mmmmmmmmm.... Good ole' Kentucky Fried ECM...

awg

this doesnt make complete sense.

Are you saying providing 12v to the fuel pump will allow the vehicle to run...or just the fuel pump ?

rkt68411

awg
i can jumper the fuel pump relay contacts and i can hear the fuel pump run and I get fuel to the rail. I suspect that the ecm from the 93 XJ is not compatible with my 91 XJ.

rkt68411

I had to back away from the Jeep for a bit but now I'm back at it. I have replaced the following:
ECM from same year, engine and transmission
fuel pump relay
ASD relay
battery
60amp ignition fuse
distributor cap and rotor
plug wires and plugs
I checked the voltage at the distributor pick up and its 4.8volts
Has good spark at the plugs
I can here the fuel pump prime on key turn
It has fuel at the schrader valve on the rail
Poured a tiny bit of gas into the intake manifold and it ran for 2-3 seconds but at very high rpms.
I am 100% stumped.
I'm considering re-indexing the timing but I'm leaning toward a bad fuel pump or fuel pressure regulator.
Any thoughts from the group?

rkt68411

I checked the fuel pressure at the rail and it showed 35psi after prime and 40psi while cranking. How much is needed to start? Could the injectors have gotten fried? Or possibly the TPS?

Noah911

If it runs when you put a little gas down into the intake, then it sounds like you have a fuel delivery problem to me.

The fuel injectors are linked electrically very closely to and with the PCM. An electrical problem with a fuel injector can certainly toast a PCM. Maybe the reverse could somehow be true in a way too? Maybe the newly installed replacement PCM is not providing an intermittent ground to the fuel injectors circuit, how it needs to be doing in order to make them work?

I think to test the fuel injectors sounds right. You can hook a noid/12 VOLT test light to the fuel injectors circuit. Look to see if the test light stays illuminated while you are cranking the Jeep. With how I understand it, the fuel injectors circuit is not correct if the testing light does not flash on and off for you when you do this procedure.

You can narrow it down by testing at the fuel injectors side of the circuit. If there is an electrical grounding problem found at the injectors.. from there, you can try testing at the PCM side of the circuit.