Fixing lazy PCM. New caps writeup.

[gomes512]

Lazy PCM? A writeup on how to recap your PCM. Yay!

All the information on this subject I’ve gathered from the forum and a post can be seen on the topic here:
Lazy PCM, Capacitor replacement how-to?

I’m sure there’s others…

Anyways what I gather is the electrolytic capacitors get beat up and eventually come apart and leak. I recently recapped an old tube radio so the same idea applies. My heap recently started doing the lazy start thing as well and upon removal of two of the three caps I found they were leaking slightly.

What you’ll need:

-Soldering iron, unless you have all day I recommend a 40W or greater. All I have to say is it’s an old Motorola circuit board.
-Possibly some solder but I only used it for one contact point. There is enough of the old on there.
-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 motor starts overheating. 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.



Another good idea is to 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 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. An extension might help or something. 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 my Jeep being my Jeep doesn’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 the separate. Don’t use a knife or flathead to pry the board up from the side as I can tell you what that’s going to cost.


Remember those capacitors? Well flip the board and find the location where they connect to the board on the back side. This ballistics gel stuff smells like rancid onions that came out of a horses *** when touched by a hot soldering iron. Yeah, gets real old real quick. Carefully use a razor to cut a rectangle around the contact points. Don’t cut hard 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 the short wire is negative!


It’s hard to take a picture and solder at the same time. Anyways there are two points and both require a decent amount of heating. I would suggest a solder sucker. 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.

[gomes512]

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 like 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!

[greasefingers]

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.

[gomes512]

Yes I knew I saw it before somewhere but could not find it again. Thanks.

Credits goes out to him for the info initially!

[Virmagicus]

Nice writeup. I was having a problem with my computer also at one time. Turned out to be a little corrosion on the pins. I used deoxit d5 and deoxit gold. Has been working like a charm ever since.

[cundkeys]

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?

[greasefingers]

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

[gomes512]

Yeah good info about the pins, I noticed that mine had some old lithium grease on them that dried out. I'm not to particular about the connection design either.

[gidkid]

Do you have the capacitor part numbers from Radio Shack? Great article! Thanks!

[gomes512]

Yep, radio shack part # 272-1029

[MrSVT]

This fix is amazing!!! Pictures are good. Thanks guys, my JEEP is now starting as it used to be.

[JesterMX6]

just did this on my 93 ZJ 4.0 that was rendered start-less. thanks a bunch!!!