Ecm keeps going bad. 96 xj

I have a 96 xj I have replaced all senors. Came down to a bad ecm. Put one on it ran about 2 hours. And ecm bad again was wondering what is causing this?

I would think a short in the harness.. Don't know how to go about checking that though lol

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I was think that ( short in harness) or the alternator regulator part of the ecm burning it out because of the alt. But the short in the harness/ bad plugs is more likely.


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what about it is failing?

What are the symptoms?

Is it unreadable with a scan tool?

Good question. A thousand different things can make it seem like the ecu is dead. I'm assuming it's noticeably deceased, but...


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Can't read it with a scan tool. When the ecm fails it dies all at once no warning signs at all. When it dies it just cranks. I was wondering if it was a short. I'm new to the Jeep world was just wondering if I was possibly looking over something. Or if it was a common problem.

To many (most) of the sensors the ECM supplies a voltage lower than battery voltage. Typically 5 volts, but sometimes a little more. The sensors supplied are grouped (a sensor loop), anywhere from a couple to half dozen sensors.

Typically the Check Engine light you see when you initially turn the key to the run position, fails to light at all if a sensor is shorted. The check bulb at start stops functining. Which usually indicates a shored sensor or a partially shorted sensor. The ECM shuts down to protect itself.

When the Check Engine Light fails on my 96, I unplug sensors one at a time until the light (CEL) comes back on, when I turn the key (initially) to run. You remove the shorted or partially shorted sensor from the loop and often the ECM will function again.

Someplace to start, unlikely to help if the wiring is shorted, the power to the ECM is flaky or a ground is bad.

Usually when an alternator fails the voltage regulator part of the ECM will fail, it seldom affects the rest of the ECM functions. The exception being reversed polarity at the battery or a serious voltage spike.

Low voltage is usually temporary and doesn't hurt anything long term.

Don't confuse an ASD relay issue with a sensor or ECM issue. I found a flacky connection under the dash near the gas pedal that would sporadically de-energize my ASD.

Just some ideas.

yeah.. Jeep ECU's are pretty hardy against shorts and it takes a lot to actually kill them.

As suggested above, it just may be a case of a bad ground or a faulty sensor grounding the 5v reference causing the ECU to go "brain dead." Once the cause of the short is corrected the ECU will return to normal.

Digger87xj said:

yeah.. Jeep ECU's are pretty hardy against shorts and it takes a lot to actually kill them.

Click to expand...

Replacing the ECM is the last thing you should do, I have never found a dead one.
Since the Jeep started working when you replaced the original computer, I would suspect a bad plug, a broken wire or another defective electrical part.
If the computer has not set a code, scanning it with inexpensive scanner will not show anything.

Get a FSM and use the wiring schematic and plug pin-outs to trace the wiring. This isn't an easy process but it's what needs to be done.
Welcome to the world of antique Jeeps, be happy that the electronics are relatively simple compared to modern cars.

The reason I know the ecm's are bad is if I put them in my buddy's jeep they do the same thing as mine.

Any aftermarket equipment on it? Amp or something? I'd look into what exactly is dieing on the computer and go from there ( is the coil driver burning up or what).