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Electrocuted my jeep

ryan

ryan

NAXJA Forum User
Location
logan utah
I have a 92 4.0 and I did something stupid. While jumping a dead battery in my jeep, I must have screwed something up and either reversed the polarity, or grounded the wrong cable from the good battery. Unfortunately I don't know exactly what happened, which probably makes things a little more difficult. Anyway, I am trying to get this thing running again and I need some help.

Current symptoms:
-engine turns over and cranks fine
-aux. fan immediately starts running
-no power to the fuel pump (I don't hear it priming and no pressure at the rail)
-no spark
-check engine light comes on, but I can't recover any codes

I checked and replaced any blown fuses (two maxi fuses for the alternator, and 2 interior light related fuses). I have not replaced any relays.

So, where do I begin? Do I need to get a new computer for this thing? I have searched, but it appears that not many people do something this stupid!
 
How do like your XJ? Regular or Extra-Crispy?

Go for the basics. It cranks--good. Run a 14 gauge wire from the positive side of your battery, or from another known good circuit of at least 14 gauge, direct to the fuel pump and make that sucker pump. Once you have fuel, then check for spark.

Keep us updated.
 
dagr8tim said:
Why not check the fuses under the hood. It could also be the fuel pump relay. I'd start with that stuff first.
Click to expand...

Said he checked the fuses.
 
joe_peters said:
Said he checked the fuses.
Click to expand...

Not to split hairs, but as I also said, check the relay's. It's alot easier to replace a >$10 fuel pump than rewire the fuel pump or do anything drastic.

Personally I'd play musical relay's and swap the fuel relay with a known good relay and see if the fuel pump primes.
 
Yeah, I checked and replaced the fuses. I also had a spare relay that I swapped in for the aux. fan (and other relays where the number matched) and it did not seem to change anything. I have never really messed with relays...are they cheap and can you pick them up at the local parts store? If so, I might just replace them all.

As for the ECM, from what I have read, it seems like it might be toast. I was going to hit the junkyard tomorrow and try to pick one up. Will any ECM from 91-94 work?
 
I swapped my spare relay (left over from when I disabled the abs a few years back) in for the fuel pump relay and the pump still did not prime.
 
You can also test relays yourself with an voltmeter. One circuit closes the switch, that activates the other circuit. Do you have voltage in to the ECU? If you have voltage in, and don't get voltage out, it is cooked. You are aware of course that the fuel pump only runs for three seconds in "run" key position unless the ECM sees cranking signal from the CKP (crank pos. sensor). With the key in the crank or start position, it should run all the time.
 
My pump is pretty noisy so I am used to hearing it right when I turn on the key. As for the relays, I guess I will check them all, check the ECM, and get back. Thanks for all the replies.
 
Last edited:
Guess I'm just too Old School. If I had done something like what the OP did I guess my first job would be to use a couple dollars of 14 gauge wire to jump my fuel pump and see if the vehicle would run. If it runs, being Old School, I would confidently set out to repair the fuel circuit. If it didn't run, at the cost of a piece of wire (that I can reuse anyway), and there was no spark, I could confidently start checking the ignition system.

For the cost of the wire.
 
joe_peters said:
Guess I'm just too Old School. If I had done something like what the OP did I guess my first job would be to use a couple dollars of 14 gauge wire to jump my fuel pump and see if the vehicle would run. If it runs, being Old School, I would confidently set out to repair the fuel circuit. If it didn't run, at the cost of a piece of wire (that I can reuse anyway), and there was no spark, I could confidently start checking the ignition system.
Click to expand...

Your system is good. Troubleshooting is as much art as science. We both just have different starting points.
 
i think the key here is that the fan immediately starts running with the key on. i'll bet you fried the ecm/pcm/computer/whateveryouwanttocallit. make sure the ecm fuse is not blown, then try swapping computers.
 
joe_peters, I totally understand your point and appreciate your reply. In the past I have gone down the wrong path and thrown parts at the jeep, to find in the end that it was a simple and cheap fix. However, I feel the problem involves more than just power to the pump. I also do not have spark, and further, the computer will not flash any codes at all. Not even the disconnected battery (12) and end sequence code (55).
 
ryan said:
joe_peters, I totally understand your point and appreciate your reply. In the past I have gone down the wrong path and thrown parts at the jeep, to find in the end that it was a simple and cheap fix. However, I feel the problem involves more than just power to the pump. I also do not have spark, and further, the computer will not flash any codes at all. Not even the disconnected battery (12) and end sequence code (55).
Click to expand...

Starting to sound serious. As Winterbeater said, do you have power in to the ECU? Power out?
 
ryan said:
joe_peters, I totally understand your point and appreciate your reply. In the past I have gone down the wrong path and thrown parts at the jeep, to find in the end that it was a simple and cheap fix. However, I feel the problem involves more than just power to the pump. I also do not have spark, and further, the computer will not flash any codes at all. Not even the disconnected battery (12) and end sequence code (55).
Click to expand...

Check your CPS, had a grand 4.0, guy did the same thing and in the end it turned out to be the CPS and a few fuses. You might get lucky.
The CPS shorted in my 96 (OBD-1 export) the CEL died, not codes, no light at start-up, nothing. The supply voltage to my Sync sensor was also low (like 2 volts), I'd guess from the short in the CPS sucking the 5 volt supply voltage down.
They had a rash of wrong pole jump starts over here, it did fry a few computers. The poles are on the opposite ends of the battery for a typical US car than for a normal European car, they'd try jumping US cars without checking the polarity.
 
ryan said:
I swapped my spare relay (left over from when I disabled the abs a few years back) in for the fuel pump relay and the pump still did not prime.
Click to expand...

the abs relay is not the same as other relays. when i pulled my abs the relay had ABS set into the plasic on it. not the same. go pull the relay from something that you know works- like the e fan you mentioned or the horn (check the horn to see if it works first). the shock might have fried the fuel pump before the fuses blew. try swapping out the fp fuse anyway to be sure.

if still no, then time to start tracing wire with your multi meter...
 
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