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Discussion Starter · #1 · (Edited)
Lazy PCM? A writeup on how to replace the capacitors in the PCM on your 93-94 Grand Cherokee. The OBDII ZJ's (95-98) contain different PCM's and this repair does not apply.

All the information on this subject I've gathered from the forum and a post can be seen on the topic here:
http://www.jeepforum.com/forum/showthread.php?t=501574

I'm sure there's others…

The PCM contains three electrolytic capacitors that overtime will bulge and leak, presumably due to the constant temperature changes in the engine bay. As the capacitance of the capacitors goes down your heap my produce the lazy start symptom.

What you'll need:

-Soldering iron, unless you have all day I recommend a 40W or greater. We're going to be dealing with a fat Motorola circuit board.
-Possibly some solder but I only used it for one contact point. The manufacturer was pretty generous with solder.
-Razor or sharp pocket knife.
-Three radial capacitors rated at 220 micro Farad's at 25 volts and + or - 20% tolerance. Radio Shack sells some 220micro Farad at 35V for a buck something. However, the Radio Shack versions are rated to 185F while the originals are rated to 221F I believe. But that difference won't really matter unless your running really hot all the time. In any case you can always order online the important thing is to match the capacitance value of 220, you can safely go over the 25V rating but it won't do anything in terms of service life/performance.
-Silicone rubber sealant.
-Basic hand tools to remove coolant reservoir and PCM from engine bay.
-Small flathead or dental pick.
-Needle nose pliers, they better have some teeth on them for grip.


Might be a good idea to remove the negative battery cable… Something about computers.



Remove the three bolts holding down the coolant reservoir. Carefully unclip the level sensor on top of the tank. While you're at it check for any cracks or leaks on the bottom and especially near the bolt down points and inspect the hose. Carefully place the tank out of the way without over extending the hose.


Use a socket to remove the single bolt holding the connector in place to the PCM. I forgot what size it was but I'm sure you can figure it out. Disconnect it carefully and make sure the dirt sitting on top does not fall on the pins. You can just use a q-tip to clean them later if some does get in. There are three bolts that I did not show. Notice I said bolts not screws! Two towards the passenger wheel and one towards the driver side, you'll need some light to see the ones by the passenger side. They screw into white plastic into the firewall mount.


Remove the five Philips screws holding the cover in place. If you wheel the heap watch out for some collected dirt to rain out.


You should see something like this. If you have the newer model you won't see the ginormous heatsink there. You will, however, see the three capacitors. The board is covered in a rubber silicone, actually I think it would make for some good ballistics gel.


This rubberized board needs to be removed from the back case. It should just pop out, well the Jeep Gods don't allow for such easy things. Use a razor or thin knife to cut around the outside, probably not a good idea to use a hunting knife for this task... Then grab hold of the pin connector (big black rectangle) and pull slowly but firmly. Work it side to side and you should hear it begin to separate. Don't use a knife or flathead to pry the board up from the side as you may risk damage.


Remember those capacitors? Well flip the board and find the location where they connect to the board on the back side. Be warned that the rubberized coating smells like rancid onions that came out of a horses *** when touched by a hot soldering iron. Carefully use a razor to cut a rectangle around the contact points. Don't cut hard, so as not to damage the board, and watch out for those tiny resistors.


So where are the contact points for the caps? Lucky you… I made a map just for you! The blue shaded areas represent traces on the board. The red dots are what you need to desolder and the yellow symbols show polarity of the contacts. Remember that for new capacitors the short wire is negative!


It's hard to take a picture and solder at the same time. Anyways there are two points on each capacitor and both require a decent amount of heating. I would suggest a solder sucker to make this easier. Or you can do what I did and grab the cap with pliers on one side and heat one point on the backside and rock that side out. Repeat for the other side so that you are basically walking the cap out heating each point back and forth. Take the new cap and line it up with the points, you will more than likely need to reheat the point to open up the hole to stick the leads of the new cap back in. Use the same method if necessary to walk the new cap back in, if a hole was there and the leads just slide through then take a bit of solder on the tip and touch the point to resolder the hole.
 

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Discussion Starter · #2 · (Edited)

Slide or walk the new cap all the way in and trim the excess leads on the backside. Keep track of the trimmings as last time I checked circuit boards don't appreciate tiny loose conductive pieces of metal.


Cut some of the gel away from the circumference of the cap, if you haven't already, and refill it back in with silicone sealant. Don't cover the top of the caps with the sealant.


The square you cut out the back to get at the contact points should be filled in as well.


Trim the outside of the board from any dried or cracked gel. Go around inside the backcase with some silicone and place the board back in.


Purrrty… If yours has the heatsink clean it to allow better heat dissipation.


Lastly apply silicone around the edges on the bottom and sides to prevent any water from getting under. Not really necessary unless you plan on driving through the Mississippi, but then again the worst stuck thread explains it all. Apply dielectric grease to the connector pins and...

As the Hayne's manual says it best "Installation is reverse of removal."

Hope this solves your problem. Good luck! :cheers2:

UPDATE (7/22/2013): So it's been 5 and half years of running with the Radio Shack 185F rated capacitors in my heep, no trouble yet. I had the PCM apart this May and the caps did not show signs of bulging.

JF member ijusgottajeep has found a source for the higher temperature rated capacitors for those that don't believe me which I've linked here. http://www.parts-express.com/pe/showdetl.cfm?partnumber=020-1746
 

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Good job and I like how you identified the positive and negative sides (a very important part of installing the new Caps!)

It was John-in-MA who first showed us pics of the PCM guts and the 3 Cap culprits.
 

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I just replaced my PCM and will now refurbish my old one do to the great help here.
Just one question I noticed that John-in-MA replaced the transistor on his,

what would be a symptom of this being bad and would it be recommended to change will you already have it apart?
 

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I do not believe that John replaced the transistor. I think he just labeled it. It does not say dig at the transistor either.
 

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so why would you do this? is it basically just for no-starts ?
 

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so why would you do this? is it basically just for no-starts ?
It is for more than no starts. Once you have troubleshot everything else and replaced what you need to replace and that doesnt work, this is another cheap fix to try. It will cost roughly $700 to replace a PCM with the dealer, it will cost less than $4 to fix it yourself with parts from radio shack. We had this issue (without knowing it) We replaced the fuel pump and filter, the ignition coil, the CPS, all of the relays and several fuses(roughly $600) before trying this fix. We found a few threads related to PCM probs before we found the fix from Gomes512. The probs described by others were indentical to ours. We had no CEL light at all, even after replacing the CPS. We figured why not and gave his write up a shot, she started right up and has been fine ever since.
 

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Any more follow ups on this repair??? How long ago have some of you done this and still have no prior symptoms??? I am sure there are other things in the PCM that can fail, is there anything else I can replace while I am in there as more preventitive maintance?? Thanks in advance for any replies
 

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Discussion Starter · #20 ·
I did mine whenever the post was made. No problems, Jeep runs like a top.

The rest of the board should hold up through nuclear winter. I'm curious as to what's under that big 'ol heatsink. Obviously something that puts out heat, I could see that as being a future failure point.

The reason the caps are prone to failing is due to the harsh environment they are in. Constant heat/cool cycles are the problem. The caps in my vehicle information display on the other hand look great.
 
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