how can you tell if the computer is fried?

A

austinaubinoe

NAXJA Forum User
Location
Montgomery County, MD
Ok so I posted this on jeep forum, but no one responded in 3 day....


Basicially I have a non running 94 Grand cherokee Laredo that I just picked up cheap from my buddy. It turns over easily, but will not start. I was doing some research and I finally figured out that this is a common issue with the 20 amp ECM fuse blowing. This is caused be a bad ground to a O2 sensor or a worn through wire. My buddy was pissed off so he bridged the fuse with a bit of copper wiring.

He claims he heard a pop and the 20 amp fuse no longer instantly blew when it was replaced.

How different could a old 4.0 ZJ be from a 4.0 XJ? If you could help me out it would be appreciated.
 
thanks, Ill check it. Im gonna hook a code reader up. online it said CPS= P0320.

But any info if what my friend did with the wire would kill a CPU?


Appreciate it!



Edit: Whats a good brand of CPS?
 
He probably melted some wiring or blew up a relay. Inspect your wiring harness, especially where it comes close to hot or moving things like exhaust pipes or the driveshafts.
 
:doh: I didnt even think about that. the popping sound probably WAS a relay. Thanks for the suggestion.


If the computer was fried, would the engine even turn over? I was just thinking, doesn't the CPU control the dizzy?
 
austinaubinoe said:
:doh: I didnt even think about that. the popping sound probably WAS a relay. Thanks for the suggestion.


If the computer was fried, would the engine even turn over? I was just thinking, doesn't the CPU control the dizzy?
Click to expand...

it would still turn over, the starter solenoid is not computer controlled.

hook up a code reader, if you get codes the ECU is not likely fried. If you get nothing then the ECU has no power or is toasty.
 
oh ok! yeah that makes a lot of sense.

Well I took off the throttle body for a good cleaning. I plan to clean the CPS tomorrow and see if there is any improvement. I forgot my Scan Gauge is only OBDII, I gotta find a cheapo OBDI gauge that will shoot me the codes.

If the computer is cooked, ill replace it before anything else. Otherwise Ill probably get a new CPS to start, then a TPS and a MAP sensor.
 
Check your wiring first... fuses blow for a reason. Usually, in automotive stuff, it is the wiring between the sensors and other components that causes fuses to blow. In fact I have never, ever seen a fuse blow because of a bad ECU. The computers and sensors don't draw enough current to blow fuses generally, it's the actuators that do - alt field coil, injectors, fuel pump, etc. I would still check the wiring first.

My bet - when he got frustrated and put in a piece of wire, the wiring that was shorting out against something finally reached its breaking point and burned out. Now the fuse doesn't blow because the short circuit no longer exists, but the device that was being powered by the wire before it shorted out is no longer getting any power.
 
austinaubinoe said:
oh ok! yeah that makes a lot of sense.

Well I took off the throttle body for a good cleaning. I plan to clean the CPS tomorrow and see if there is any improvement. I forgot my Scan Gauge is only OBDII, I gotta find a cheapo OBDI gauge that will shoot me the codes.

If the computer is cooked, ill replace it before anything else. Otherwise Ill probably get a new CPS to start, then a TPS and a MAP sensor.
Click to expand...

don't forget, a shorted sensor can cause the ECU not to power up, especially a bad CPS.

It might also be possible that some wire was rubbing on the exhaust or something. Best to look for any damage to the harness.
 
there seems to be alot of oil covering the harness on the passenger side of the block around where the CPS plugs in. Other than that the harness looks good.

Would probably be a good a idea to clean every electrical connection and fill em with dielectric grease.

Oh and I hooked up a 5 year old red top (that left me stranded at rausch about a year ago) to a trickle charger for 2 days. IT STILL holds a charge! I had never bothered with it until now because the Die hard in the ZJ was dead.

Any brand of CPS to stay away from? or is this a part where all the brands are the same?
 
so just hooked the battery up. NO CEL......

Im guessing I have a bad CPU then because its is definitely an electrical issue. The wiring harness is really simple under the hood, and I cant find any bad wiring. Would the problem be at the distribution box? Should I remove it and check for issues underneath it.

Thanks for the help, This is the first jeep ive brought back from the dead.
 
check the wiring harness down along the rear of the drivetrain, i.e. going to the rear O2 sensor. Also yes, you should lift the PDC up and pop the bottom off it and look over the wiring for burn marks, melted insulation, etc.

Then, pick up an FSM for it (you will need it sooner or later anyways) and a DMM. Plug in a new fuse, look in the FSM for a map of the power distribution from that fuse, and check if you're getting power at every device powered by it. When you find one that isn't getting power, you found the problem (or a symptom of it, this just means somewhere between the fuse and the part not getting power there is a fault.)

My bet is still on a wiring harness problem simply because I have seen more of them than ECU failures.
 
magic smoke: the material that makes electronics and wiring harnesses work correctly. When they fail, you let it out, and they no longer operate as desired. It is very difficult to get the magic smoke put back into the parts properly.

It's somewhat of an engineering inside joke.
 
oh ok, never heard of that before! Think I saw it at vato zone today next to the flame decals LOL.

Anyways, dismantled the distribution box, all the wires look clean. All the fuses check out, and non of the relays are burnt. Checked a spark plug and the rotor and they are SHOT. worst I have ever seen. HUGE gap and the electrode is barley visible. Also, the ceramic looks strangely worn on the one plug I pulled.

I also degreased the side of the engine with the dizzy. Once the engine dries its a new cap/ rotor, and 6 champion coppers. Funny thing is the MOPAR wires look almost new, and the cap is way newer then the rotor?!?

Gonna pick up a can of MOPAR throttle body cleaner tomorrow when I pick up a part for my sisters 04 Grand. heard that stuff works the best.

So, after it has new plugs, cap, rotor, a clean throttle body, a fully charged battery, and dielectric grease in all connections Ill be able to better diagnose the issue. Thanks alot for all the tips.
 
http://www.naxja.org/forum/showthread.php?t=1044073

After reading this thread I have a feeling its the CPS sensor. I also read the OEM sensor from the dealer is $125, but this would throw my whole budget for this "cheap Jeep" off. Whats the best sensor they sell at rock auto? They have a Wells sensor for like $45 iirc, thats really the only brand I recognized.


can I just say it sucks when it gets dark at 5!! Cant wait for my dad to buy a new house with a HUGE garage in a couple months.
 
Hahaha, that joke is older than dirt :roflmao: I don't even know where I stole it from.

Austin, if it is the same sensor from an xj 4.0 (crossref it) just go to the jy and buy 4. They are easy to pull if you're a bit of a contortionist and/or have a stubby 11mm gearwrench and some time. Id rather pay 5 apiece for 9 than 45 for one, chances are, 8 of them will work. Toss a couple in the glovebox and a couple in the trail kit and bring your tools.
 
kastein said:
Hahaha, that joke is older than dirt :roflmao: I don't even know where I stole it from.

Austin, if it is the same sensor from an xj 4.0 (crossref it) just go to the jy and buy 4. They are easy to pull if you're a bit of a contortionist and/or have a stubby 11mm gearwrench and some time. Id rather pay 5 apiece for 9 than 45 for one, chances are, 8 of them will work. Toss a couple in the glovebox and a couple in the trail kit and bring your tools.
Click to expand...


You know I would do that, but the nearest pick and pull is 45 min away. Its kind of an all day thing. But If I know for sure that I need a new CPU, that where im getting it, so ill pick them up at the same time.

I was readying a right up where a guy took apart his YJ computer and re soldered a connection that had gone bad. If all else fails, should I open the old CPU up and visual check for damage?? Any idea what years are compatible CPU wise? Ill look it up, but im mainly wondering if i NEED a 94 4.0L (exact year).

Also, Im pretty confused about the CPS. Not even the parts store knows if I need a one bolt or 2 bolt mount for starters. Did they change mid year? I have honestly never had an issue with a CPS before, so ive never removed one. Do the XJ's have a single or double bolt CPS?

Ok and the dizzy is pretty weird. There seems to be a wire running into it under the cap. just thought about it, so I dont remember where it goes, just that I have never seen anything like it. Ive worked on a 4 banger TJ dizzy and a 360 AMC dizzy.

Gonna really crawl up underneath it tomorrow to see if I can find anything wrong with the oxygen sensors. Did not see anything today, but maybe I didnt look hard enough.

Thanks for your help, thinking I should start an account on NAGCA. Jeep Forum is USELESS!
 
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