Fried 2 ECMs on a 1996 cherokee - Pirate4x4.Com : 4x4 and Off-Road Forum

kjkrsch · 07-07-2007, 09:08 PM

Some help please....I own a 1996 Jeep Cherokee SE with the straight 6, 4.0L engine, automatic, and of course four wheel drive.

6 months ago I pressure washed the engine of my cherokee and parked it for a week or two (bad idea in retorspect). I return to start it and no joy. After some considerable trouble shooting it was revealed that the ECM had gone bad. So I ebay a used one and toss it on. She runs like a dream....for about 3weeks. Now it appears I have FRIED another ECM!

So here is what I know
-I have a check engine light with the codes reading for a bad rear O2 sensor which can be either the O2 sensor or the catlytic converter.
-I know I have no spark,
-I have fuel,
-it turns over strong,
-fuel is reaching the injectors.
-No blown fuses, unless there is a super secret secret squirrel one hidden some place I can't find and is not in the haynes or owner's manuals.

So I have bid on another ECM but I don't want to just plug it in and fry it, at least before I can sell the damn thing...or figure out the problem and keep it.

So weigh in here. What are my possible causes of frying an ECM aside from the obvious "sprayed it with water".
Note; it does have a lot of after market wiring done to it like an amp and fogs lamps...etc.

I am at the end of my know how, toss me a line here.
Thanks ya'll
-Josh

Diluted · 07-08-2007, 08:51 AM

gotta make sure all grounds are good and nothing is shorting to ground before the load in the elec circuits, otherwise it will fry the pcm (so long as it runs through the pcm) ....

MudderChuck · 07-09-2007, 11:25 PM

Try putting in the old ECM and test the supply voltage at the sync sensor. Do you have any check engine light at all? Does the fuel rail *keep* pressure while you crank it?
I have a hunch, which may be right or wrong but worth checking out.

98 xj of death · 07-12-2007, 10:27 AM

ive got a 98 and i fried 2 ecms as well...

the first one was the original ecm.. gave out at about 85xxx

replaced it with another one and ended up frying it within 2 weeks

turned out i had a bad cps which caused it to short out

so check that little bastard

MudderChuck · 07-12-2007, 06:45 PM

Quote:

Originally Posted by 98 xj of death

ive got a 98 and i fried 2 ecms as well...

the first one was the original ecm.. gave out at about 85xxx

replaced it with another one and ended up frying it within 2 weeks

turned out i had a bad cps which caused it to short out

so check that little bastard

Same thing I was thinking, but a couple of easy tests before you run down and buy a new CPS would be prudent. I was thinking a short in the 5 volt sensor supply circuit someplace. I had a partial short (low resistance) in the other two legs (they never mention) in the ohm test for the CPS.
The partial short was (in the 5 volt supply circuit) in the CPS, to the sensor ground. They never mentioned anything about a resistance test between these two pins, in the CPS test procedures. Sucked the sensor 5 volt supply down to around 2 volts and caused multiple gremlins. The MIL lamp wouldn't work at all and the fuel pump relay was intermittent, sync sensor got low voltage and I imagine there was others that I didn't test for. My ECM survived, likely because I caught it fairly early, before it turned from a partial (low resistance) short to a true short.
But I'd guess a (partial or full) short in any sensor on the same 5 volt supply circuit, could cause the same symptoms or maybe fry the 5 volt power supply.
Just a possibility.
You have a check engine light, I didn't on my 96.
The rear 02 sensor wires may have come loose and are frying on the exhaust. Few people replace the, use once, clip that holds the connector and harness up. I always put a real nut on there.
If the CPS is bad the fuel rail will build (some) pressure, but as soon as you start to crank, the fuel pump relay opens back up and the pressure disappears. Bad CPS=no spark.
Wish you luck, been there done that.

Broke That · 07-12-2007, 07:20 PM

Cherokee electrical problems.....gotta' love em.

Other items to check, there is a very small blade fuse called the 'Automatic Engine Shutdown' its located in the engine compartment fuse box.

Also, check your injector wires, look closely as they make thier final turn into the injectors, any chaffed wire?

And another...

Check your rear or second O2 sensor, are the wires good?

Those checks don't cost a dime and you just might find your problem there.

You might be able to guess why I might have some insite on this one!!

07-07-2007, 09:08 PM	#1 (permalink)
kjkrsch Registered User Join Date: Jun 2007 Member # 94708 Posts: 2	Fried 2 ECMs on a 1996 cherokee Some help please....I own a 1996 Jeep Cherokee SE with the straight 6, 4.0L engine, automatic, and of course four wheel drive. 6 months ago I pressure washed the engine of my cherokee and parked it for a week or two (bad idea in retorspect). I return to start it and no joy. After some considerable trouble shooting it was revealed that the ECM had gone bad. So I ebay a used one and toss it on. She runs like a dream....for about 3weeks. Now it appears I have FRIED another ECM! So here is what I know -I have a check engine light with the codes reading for a bad rear O2 sensor which can be either the O2 sensor or the catlytic converter. -I know I have no spark, -I have fuel, -it turns over strong, -fuel is reaching the injectors. -No blown fuses, unless there is a super secret secret squirrel one hidden some place I can't find and is not in the haynes or owner's manuals. So I have bid on another ECM but I don't want to just plug it in and fry it, at least before I can sell the damn thing...or figure out the problem and keep it. So weigh in here. What are my possible causes of frying an ECM aside from the obvious "sprayed it with water". Note; it does have a lot of after market wiring done to it like an amp and fogs lamps...etc. I am at the end of my know how, toss me a line here. Thanks ya'll -Josh

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07-08-2007, 08:51 AM	#2 (permalink)
Diluted Registered User Join Date: Jan 2007 Member # 86077 Location: Conway, SC Posts: 162	gotta make sure all grounds are good and nothing is shorting to ground before the load in the elec circuits, otherwise it will fry the pcm (so long as it runs through the pcm) ....

07-09-2007, 11:25 PM	#3 (permalink)
MudderChuck Registered User Join Date: Jul 2007 Member # 95255 Posts: 20	Try putting in the old ECM and test the supply voltage at the sync sensor. Do you have any check engine light at all? Does the fuel rail keep pressure while you crank it? I have a hunch, which may be right or wrong but worth checking out.

07-12-2007, 10:27 AM	#4 (permalink)
98 xj of death Registered User Join Date: Aug 2004 Member # 34476 Posts: 1	ive got a 98 and i fried 2 ecms as well... the first one was the original ecm.. gave out at about 85xxx replaced it with another one and ended up frying it within 2 weeks turned out i had a bad cps which caused it to short out so check that little bastard

07-12-2007, 07:20 PM	#6 (permalink)
Broke That Registered User Join Date: Aug 2006 Member # 78387 Location: Borrego Springs Posts: 793	Cherokee electrical problems.....gotta' love em. Other items to check, there is a very small blade fuse called the 'Automatic Engine Shutdown' its located in the engine compartment fuse box. Also, check your injector wires, look closely as they make thier final turn into the injectors, any chaffed wire? And another... Check your rear or second O2 sensor, are the wires good? Those checks don't cost a dime and you just might find your problem there. You might be able to guess why I might have some insite on this one!!