Zolly

My 1994 Jeep Grand Cherokee (ZJ) does not start up. About 30 seconds after turning on the ignition, the SECURITY light appears on the dashboard. Previously I charged the battery and forgot to remove one of the car’s clamps so the car was energized while charging. As another mistake, I forgot about it and I was cranking it during the charging process. The syndrome is exactly what I found in the VTSS section of the owner’s manual: ” If the Security Lamp comes on after ignition ON and stays on, the CCD bus communication with the PCM has been lost.”

I am glad that I found this hint about this specific fault, but I do not know how to go ahead (the explanation is not very clear for me). In my interpretation it might mean that the security alarm module cannot “see” the PCM and blocks the ignition (or the PCM is blocking it). CCD bus means for me that the PCM is maybe not energized (although fuses seem to be OK), so I tend to go into the following direction:
- checking whether the PCM is energized or not. How can it be done the best way?
- If the PCM is energized, then the communication is lost due to another reason. Unfortunately, the PCM connector is a 60-way connector, with many options to measure. What should I focus on?
- Is it possible that any of the sensors is ruined? (crankshaft position sensor & CAM position sensor, engine coolant temperature sensor, intake manifold air temperature, manifold absolute pressure sensor and throttle position sensor) If so, how can I find it out? Which one is wrong most likely?
- If the PCM is faulty and has to be replaced, how can I find it out? I do not want to buy a new PCM just to find out later that it was not needed at all.

It is clear that the charger ruined something, but I do not know whether it is the PCM itself or something else.

Any help would be greatly appreciated!

Thanks in advance,
Zolly

Zolly

I am just back from the car mechanic with some potentially important news. I was trying to carry out the ON-OFF-ON-OFF-ON self-diagnostic test to get some trouble codes but it was not possible. The check engine lamp did not come on, there were no flashes to count. Even more important, the check engine light does NOT appear for a short while when turning on the ignition (as I have just noticed it). Seemingly, the regular engine test does not take place (or the check engine bulb could be blown out, but it would not explain that the engine does not start).

My mechanic checked the PCM’s 60-way connector’s battery supply (cavity 3), and it is available (energized), the grounds are also OK (11, 12). We have checked the security alarm module’s CCD connections, but no current flows to the PCM module. That seems to reaffirm the service manual’s description that the CCD bus connection has been lost. On the other hand, the security module seems to work.

What is the fact indicating that the check engine lamp does not come on? Is it the failure of the PCM, or a sensor, or the security alarm module is faulty? I assume that the check engine lamp should work if any of the sensors is faulty, so guess that the PCM is faulty most likely, but how can I prove or test it? (Check engine light does not appear even if the SAM is disconnected.)

OverlandZJ

I had similar issue with the wifes ZJ. 93 5.2L

It would crank over and not start. If you turned the ign, but not fully to start and wait... i got the security flash on dash. Then i would hear a series of clicks underhood, which i found to be the fuel pump relay.

Once i hear the relay clicking i would hear the fuel pump prime. Then it would start right up.

I searched and found this is somewhat common of a PCM going. I swapped it out with a reman'd unit and i have had no further issues in a few years now.

Theres been one or two others here who went through this as well.

Zolly

Thank you for your informative reply. I’m afraid I am experiencing the same story (with my ZJ 94 5.2L Limited). I also hear the series of clicks, others confirm too that this is because of various relays are working (e.g. fuel pump relay). Anyway, in the nice old days (up to 2 weeks ago) the green beast started immediately after turning the ignition to the start position, there was no need to wait for any clicks in the on position.

According to all indications the PCM is the faulty part. Firstly, the check engine light does not appear at all. Should a sensor be faulty, the regular engine-check should take place (indicated for 1 second by the check engine light appearing), and eventually the check engine light would stay on. It is more likely that the check engine lamp’s fault is associated with the PCM rather than with any sensor located one level further down in the chain. Secondly, the same happens even if the security module is removed, with the difference that the security light does not appear after 30 seconds. Seemingly, SAM detects that something is wrong with the PCM (e.g. CCD bus connection has been lost) and issues the security lamp signal.

I take your advice. As the first step I will try to locate a used PCM or a PCM I can borrow just to install it and try whether it solves the problem. Then I can consider buying a reman’d or original unit or just stay with the used but functioning one. I will keep you posted on the developments.

I would buy a PCM right away only if I knew a way of test proving that it is definitely the faulty part, but I do not know such procedure. Ironically, the corresponding trouble code cannot be pulled down because the check engine lamp does not work.

OverlandZJ

I feel your pain... i searched and read many threads till i stumbled on the thread i had. It all made sense from there.

Theres an auto dismantler on ebay who i got the used reman PCM from for $80. It was cheap enough for me to just grab it with some confidence that was my problem.

Good luck..

Zolly

Thanks a lot for your encouraging words. Initially it was pretty provoking that the fault was not obvious at all. My mechanic assumed the alarm system broke down and the car was not disarmed appropriately. Today I noticed the missing check engine light and it is rather obvious that the check engine light is much more associated with the PCM rather than with the security system. With check engine light I mean the yellow engine symbol and not the red CHECK ENGINE warning. The appearing security light is also kind of misleading at the first sight.

I think you made a very good deal with your PCM for $80. For such a price I would grab it too, without hesitation. Everything happens for a reason, and a PCM will mostly not break down by accident. By mistake, I was cranking my car while the charger was in operation and my car’s battery clamps were not removed. I am confident your PCM will operate flawlessly for many years to come.

I wish you all the best.

OverlandZJ

This is the guy i bought mine from. He has several listed currently..

http://cgi.ebay.com/ebaymotors/94-95...Q5fAccessories

Zolly

Thanks a lot for the info. Currently, I see only different engine computers and PCM's listed, but I will monitor it. Anyway, I will first look around at the local dealers and dismantlers.

OverlandZJ

Call your local Jeep dealer parts dept. I did and was given a list of compatable part #'s.. there are numbers that succeed prior ones.

I then used this to find a succedded number and was a reman.

Mater of fact.. i think i saved the list and it might cover your vehicle. I'll take a look through my files.

Zolly

It is very generous from you, thank you! But do not make much effort with locating the list.
You are right, it is very important to make sure you buy the right component part when making a deal over the Internet. It is somewhat easier if you buy it from a local. I can even disassemble the PCM and take it with me an compare it to the other one I intend to acquire. If we are not on the alert when dealing over the Intenet, we are running the risk of ordering a wrong part.

Zolly

Dear CF Supporter,

Finally, the case has been solved. Due to your supportive feedback the assumption has been confirmed that the PCM was faulty. Fortunately, I found an electrical mechanic who could spot and repair the problem, so actually there was no need to buy a new PCM. It was a tiny IC in the PCM that busted. The most unambiguous indication pointing to the PCM’s fault was that the self-diagnostic test (ignition on-off-on-off-on) could not be carried out because the check engine light did not flash, and the yellow engine symbol did not appear either.

Again, thank you very much for your invaluable support which made it possible to narrow down and solve the problem efficiently! I wish you all the best.

trucker

similar effect happen to me. I tighten the battery posts and problem went away.