Questions w/ replacing computer

[skimmied]

I have a 94 4.0L Grand Cherokee that needs a new computer. I have come to find out that computers are a common issue in these, and have had several stories with buying used to replace. I need to know if a computer needs to be programmed to my Jeep if I buy one from salvage? Also are different model numbers interchangeable? When my VIN number was ran by my local dealership, it says it has a security system, but on inspection there was NEVER one installed, so when I bought a used computer with the same model number as the original, my Jeep started right up then shut off (was told that was because the salvage computer was for a Jeep w/ a security system). I was told by some, it needs to be programmed and told by others 93 and 94 are interchangeable as long as I get one w/out security...so I am at a loss and need advise before I shell out more $$ on something that my not work. (I DO NOT want to pay the estimate on new cpu and labor...)
Thanks!!

[Bob D.]

Yes the PCM module needs to be reprogrammed by the dealer.
I can't see any other problems as long as it's the same part# as your old one and the new one is good.

[dksob81]

I had a 94 and have an extra computer with it, both jeeps had a 5.2L engine and the extra computer I had was from a 93 larado no security, my jeep was a 94 limited with factory anti-theft and I used the other computer without any problems. I used it for quite awhile, Ijust used it to test it and everything worked great had no problems. The computer shouldn't need to be programmed.

Have you tried holding the gas pedal down slightly in attempt to keep it running, sounds like a bad IAC Valve. If the Anti-Theft was the problem your light (Dash Light and Headlights) would flash also.

[skimmied]

Yeah, when I had the used replacement computer and it kept shutting off, nothing would keep it going. I since took it back where I bought it since it wasn't working for me.
I have had issues with my Jeep since Feb.. With the original computer I "was" able to start it, and when it did start it ran perfect. When I turn my key on all the dash lights come on except for the Check Engine light...sometimes I would let it sit in On position, or turn the key off and on a few times and the light would come on. Once it did I was good to go...but after awhile it took longer and longer to get it on, and now it just won't work at all.

[dksob81]

Originally Posted by skimmied

Yeah, when I had the used replacement computer and it kept shutting off, nothing would keep it going. I since took it back where I bought it since it wasn't working for me.
I have had issues with my Jeep since Feb.. With the original computer I "was" able to start it, and when it did start it ran perfect. When I turn my key on all the dash lights come on except for the Check Engine light...sometimes I would let it sit in On position, or turn the key off and on a few times and the light would come on. Once it did I was good to go...but after awhile it took longer and longer to get it on, and now it just won't work at all.

Ussually when this happens it is caused by a bad CPS.

[bringselpup]

Originally Posted by Bob D.

Yes the PCM module needs to be reprogrammed by the dealer.
I can't see any other problems as long as it's the same part# as your old one and the new one is good.

On a 94 it absolutely does not need programming by the dealer.

When does the digital odometer show up? 99 and later? My impression would be that's when they need to be done at the dealership because I'm betting the vehicle mileage is in the PCM.

[bloodyprice]

i replaced mine twice

the first time i was dumb and paid $100 and got one from the junkyard, it ran for a month then shitted out

go buy a rebulilt one from carquest or somewhere because rebuilt is not as expensive as new, but the vin hasnt been programmed yet, which will program itself in about a week of driving.

when the vin is already programmed its meant for that jeep with those sensors using an already programmed cpu might fry certin sensors, both my 02 sensors when bad afterward.

anyways a reamufactured cpu will cost about $200, thats what i paid, or about 500 from the dealer.

but buying one from the junkyard is drffinately a bad idea, cuz the cpu is not gonna just reprogram the vin.

but the entire ZJ series does not need programmed by the dealer. they have a self programmer within the cpu (when not already programmed).

[hardisk_c]

Originally Posted by dksob81

Ussually when this happens it is caused by a bad CPS.

I'll second that vote, no CEL no starts are the trademark of the Crank Position Sensor (CPS) farking out. Your ECM is probably fine.

[dksob81]

Digital Odometer was used on 96 and later grands (ZJ and WJ)

[skimmied]

Thanks everyone for your input! I took the chance and bought a new computer (which was really my only option because I had a rare and expensive model), but NO PROGRAMMING NEEDED!!! My Jeep runs like new again!! Thanks again!!

[Jeepmaster1994]

Originally Posted by skimmied

I have a 94 4.0L Grand Cherokee that needs a new computer. I have come to find out that computers are a common issue in these, and have had several stories with buying used to replace. I need to know if a computer needs to be programmed to my Jeep if I buy one from salvage? Also are different model numbers interchangeable? When my VIN number was ran by my local dealership, it says it has a security system, but on inspection there was NEVER one installed, so when I bought a used computer with the same model number as the original, my Jeep started right up then shut off (was told that was because the salvage computer was for a Jeep w/ a security system). I was told by some, it needs to be programmed and told by others 93 and 94 are interchangeable as long as I get one w/out security...so I am at a loss and need advise before I shell out more $$ on something that my not work. (I DO NOT want to pay the estimate on new cpu and labor...)
Thanks!!

I also have a '94 Grand Cherokee. Its a 4.0 straight six. I am considering replacing the computer in mine. I was wondering what yours did when you had trouble? In my case, the engine would crank, but would not start. The fuel pump would not kick on to create fuel pressure. At first, if you kept turning the ignition on and off, the pump would eventually kick on, and it would start. Soon after, it got to the point where the only way you could get it to start is by pulling the fuse to the computer, and putting it back in. It almost took the effect of rebooting the computer. The same day, I checked for spark. It had none. I replaced the coil, no luck. I polished the grounds. I went as afar as pulling the coolant resivoir and lulling the computer. Still wityh no spark, I put everything back together. Just for curiosity, I tried to start it again after it was back together. It started right up. I shut it off, waited ten minutes, and it would not come back to life. I checked all grounds with a test light. They check out. These are the ones I found: two next to the battery on the wheel well, two behind the electronic brake computer, two on the alternator, and two on the coil. Am I missing any grounds? Is this what your Jeep was doing? Any advice would be greatly apreciated. Thanks

[skimmied]

That sounds EXACTLY how my Jeep acted!! At first we changed and triple checked everything that would affect the spark, except the computer (because mine also showed no spark...would just crank). Your example of waiting to hear the fuel pump and everthing sounds just like mine!

[Jeepmaster1994]

Originally Posted by skimmied

I have a 94 4.0L Grand Cherokee that needs a new computer. I have come to find out that computers are a common issue in these, and have had several stories with buying used to replace. I need to know if a computer needs to be programmed to my Jeep if I buy one from salvage? Also are different model numbers interchangeable? When my VIN number was ran by my local dealership, it says it has a security system, but on inspection there was NEVER one installed, so when I bought a used computer with the same model number as the original, my Jeep started right up then shut off (was told that was because the salvage computer was for a Jeep w/ a security system). I was told by some, it needs to be programmed and told by others 93 and 94 are interchangeable as long as I get one w/out security...so I am at a loss and need advise before I shell out more $$ on something that my not work. (I DO NOT want to pay the estimate on new cpu and labor...)
Thanks!!

I also have a 94 4.0. What did yours do when the computer went bad? What kind of symptoms will point in this direction? Mine has many problems, all of which I think point to the computer. Any advice is greatly apreciated. Thanks

[bringselpup]

I think the classic symptom for computer malfunction is the failure of the fuel pump to spin that initial burst at key on which is exhibited at the outset by extended crank time to start. What's really happening is you're cranking away and you don't hear the delay of the pump finally going 10 seconds into cranking.

A Jeep owner facing that extended crank time can begin to check and see by just turning the key to on and waiting for the fuel pump to go.

If the time to spin keeps getting longer, initially measured in seconds (the dedicated procrastinator can easily log times over 20 min) you are quite possibly looking at a bad PCM.

The vehicle always starts once the PCM spins the pump. But never before. Manually actuating the fuel pump relay in the PDC will help to prove the pump is working and not bad. But the pump still has to be fired by the PCM for it to start. I assume because other events controlled by the PCM occur also as the pump fires.