96 gc used computer swap - JeepForum.com
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post #1 of 22 Old 09-15-2017, 03:02 PM Thread Starter
azdave35
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96 gc used computer swap

i have a 96 gc...federal emissions...no alarm..i bought a used computer from a running 96 gc...fed emissions and supposedly no alarm.....when i plug it in my jeep..it will start and run for a couple seconds and die..anyone have any thoughts on this?

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post #2 of 22 Old 09-15-2017, 07:25 PM
jm8881
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I'm fairly certain that your security system is kicking in. There are many threads on here for fixing it the right way and many for rigging it to work.
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post #3 of 22 Old 09-15-2017, 08:47 PM Thread Starter
azdave35
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my jeep doesn't have a security system
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post #4 of 22 Old 09-16-2017, 02:55 PM Thread Starter
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nobody here has run into this problem before?
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post #5 of 22 Old 09-16-2017, 03:52 PM
Uniblurb
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While you may not have an alarm system on your 96 if that used PCM has it programmed in, even though it supposedly doesn't, that could be your problem. If a PCM activates an alarm system it will cut fuel off in 2-3 seconds if it's finding something amiss. You may want to check to see if you have fuel pressure at the schrader valve on the fuel rail when it shuts down.

Have you ever cleaned your PCM ground on one of the coil bracket studs with this ongoing problem? There's a link/photo of it in the dirty dozen thread in my signature in the 'grounds' section. A corroded/oily PCM ground on my 96 4.0 took out 2 PCM's and 2 Mopar crank sensors to boot.

Would have answered last night but guess you decided it wasn't worth answering my post at the end of your thread from last year.

http://www.jeepforum.com/forum/f13/9...hange-3839953/

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #6 of 22 Old 09-16-2017, 04:35 PM Thread Starter
azdave35
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i have had the jeep for a couple years and have done most of the obvious to it...crank and cam sensor...cap...rotor...cleaned grounds...my old pcm works but it had some corrosion on the terminals ...i've had the pcm apart and resoldered the connections...but it has corrosion down in a female terminal that i cant get to....if this used pcm i bought is programmed for an alarm..my question is..how do i reprogram it so it will work on my jeep....i know on ford and gm vehicles its possible but not sure on jeep....i'm sure i didn't ignore your post...i may not have seen it...my old wore out brain isn't what it used to be
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post #7 of 22 Old 09-16-2017, 07:27 PM
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Here's a thread talking about PCM's with and w/o security. In reading the whole thread towards the middle it says a non-security PCM can be installed in a similar year ZJ with security and it will run. But then it says if security is programmed into a PCM it's permanent and won't work in a non-security ZJ. This sounds more like your case if it is programmed with security. And unfortunately it says there's no way to tell if security is programmed in or not going by part numbers.

http://www.jeepforum.com/forum/f13/c...65/index2.html

It's my understanding the dealer will charge $100 for reprogramming or flashing a PCM. Even if they could remove the security from the PCM this price defeats the purpose in buying a used PCM to begin with. Difficult to figure out what to do unless you know exactly the ZJ that PCM came out of.

As was also mentioned in that thread you should have the CEL come on for a few seconds when first turning the key on. If not then the PCM is not communicating correctly. Have you checked for any codes?

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #8 of 22 Old 09-16-2017, 08:10 PM Thread Starter
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i haven't checked for codes...i'll plug the pcm back in tomorrow and see if any come up
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post #9 of 22 Old 09-17-2017, 01:27 AM
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If you aren't getting the CEL coming on for a few 2-3 secs when first turning the key on then the data link connector where you plug a scanner/code reader in won't work either since the PCM isn't communicating.

Hopefully that's not the case. But you can always try the 'key trick' for pulling up codes if need be. Turn key on-off-on-off-on within 5 secs w/o cranking then count the flashes of the CEL on the dash. Short pauses between digits and long pauses between different codes. 12 (battery has been disconnected in last 50 key strokes) is the beginning of the test while 55 is the end. You likely already know how to do this but just thought I'd mention it.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #10 of 22 Old 09-18-2017, 11:53 AM Thread Starter
azdave35
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i plugged the used pcm this morning..tried to start it..same deal..runs for 2 seconds and dies...then i did the key trick for codes...no codes came up..not even 55 or 12....the cel and the rest of the lights come on but no codes
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post #11 of 22 Old 09-18-2017, 01:54 PM
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That's an odd one the key trick doesn't work and the only ZJ you can't use it on is the 98. Since you aren't even getting the 12 & 55 start/end codes would bet a scanner/reader wouldn't work either plugged into the data port. And typically if that's the case the PCM isn't communicating correctly.

But you are getting the CEL to come on for 2-3 seconds when first turning the key on? Think I'd still push in the schrader needle valve on the rail after it shuts down after a few secs to see if there's fuel pressure there.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #12 of 22 Old 09-18-2017, 07:15 PM Thread Starter
azdave35
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i would think it has to have fuel pressure..at least for a couple seconds...it will run for 2 seconds before it shuts down
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post #13 of 22 Old 09-19-2017, 01:15 AM
Uniblurb
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That's what I'm getting at again. If that PCM does have the alarm programmed in and it's activated it's going to cut the fuel pressure off in 2-3 secs after start up. Reason I mentioned checking for fuel pressure after shut-down.

But it sounds like your PCM just isn't communication with not being able to use the key trick. That ASD relay may shut the injectors down along with other things if something is amiss in the PCM. Or maybe it just isn't right for your 96?

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #14 of 22 Old 09-19-2017, 01:21 AM Thread Starter
azdave35
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i pulled it from the exact same gc as mine..even the same color..lol...same year..same engine...same emissions...its got me baffled
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post #15 of 22 Old 09-19-2017, 02:25 AM
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Didn't realize you pulled the PCM yourself. One with an alarm would have had the small red light on top of the dash. I have heard of some PCM's going bad if they sat in the jy too long.

You could disconnect the fuel pump module connector and then run a hot wire from battery positive to pump power wire to see if it keeps running. On the pump ground you can just run a jumper from a ground back there to ground in connector. Wire colors/function are as follows.

Light blue/black - Low fuel sensor
Black/light blue - ground from PCM.
Dark green/white - 12v power from fuel pump relay.
Black - ground to pump from engine block or body

Don't think you need to do anything with that first wire. And it shouldn't matter if you go to both grounds in the fuel pump connector to ground back there. Then run power from battery to dark green/white cavity in connector. Just don't cross those wires back there.

Also was thinking about the fuses in the PDC. You may want to check those and believe there's one for the PCM.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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