96-98 PCM repair procedure W/Pictures
This writeup has been a long time coming and is sort of a give back for all the info that has been a help to me in the past. Thanks.
This procedure is not for all, but if followed, will fix the “infamous dying while driving” problem that many had believed to be caused by the 2 torx screws or the 3 bad connectors on the PCM.
You will need to know how to solder correctly, so if you decide to take this on, and don’t have any experience soldering, you can practice with just about anything that plugs into the wall now a days. Just about everything has a circuit board in it.
Just to save typing, if you have been successful with getting your Jeep to respond well to the problems of back firing, stumbling, stalling at driving as well as idling speeds, by tie wrapping the PCM connectors to the PCM, all indications are telling you that the soldering connections within the PCM are bad. This is the procedure for reflowing the bad connections that have occurred over time due to the expansion and contraction of the potting compound, that the circuit board is encase in, caused.
I will explain the whole procedure in detail just so you won’t screw up anything as I did on the first PCM I attempted to repair.
If you screwed up the PCM connector pins and need to replace them. The following link has all the info on getting replacement pins, and what you’ll need in order to replace them.
Supplies needed for the repair,
Regular hand tools,
A pocket knife
A thin razor, snap off blade type utility knife
A torch (propane or map gas)
A brass utility brush (a steel one is too aggressive)
A can of Acetone (for removing traces of potting and cleaning up the pins and circuit board)
A soldering iron (25-40 watt) A Weller iron is a decent brand
Solder (.020”) you might get away with .032” gauge, but the thinner the better
Solder wicking (for removing excess solder, if too much has been applied
A tube of silicone chalking (not the latex type)
The first thing that needs to be explained is how the PCM is assembled so nothing other than what is needed to be done is disturbed.
In the picture below, you’ll see that the top cover is actually a circuit board, laminated onto an aluminum plate. The stuff on there is a gel potting compound that is contained in a foam retainer, and acts as the protective coating and also wicks away heat while the PCM is operating. It sticks onto the cover when the cover is taken off. We want to leave it this way. Do not remove or disturb this stuff when you have it apart. There is no substitute that I have found as yet that doesn’t come in 2 one gallon jugs. $$
Don’t do anything yet,
You’ll also notice, on the cover, that there’s a connector bonded to the cover, which plugs into the (MB) mother board, which is totally encased in a much firmer potting compound. When it’s time to remove the cover, you will need to pull up on the right and left sides (torx screw holes) of the cover so you don’t harm the bond that holds the connector to the cover. Don’t worry it’s held on pretty good. Don’t try taking the cover off by pulling up from the bottom end. It might break the bond that holds it there.
So much for the adjectives……
Disconnect the battery
Remove the over flow reservoir
Remove the PCM
Remove the 2 torx screws that hold the two circuit boards together
Bend up the 8 case tabs holding the cover on. (These tabs will break if bent a couple of times)
The foam potting retainer is glued to the top cover, and needs to be freed up, so that the retainer remains with the MB.
With a screw driver blade, create a space between the case and the cover, and with the snap off razor knife, cut through the foam on the right, bottom and left sides, inside the case though the crack, to free its bond to the cover. The top portion of the foam by the connectors can’t be reached without disturbing the gel. So leave it attached. It will tear when removing the cover but won’t affect the outcome of the repair. Remove the cover by the torx screw sides. Inspect the connections where the cover connector is soldered to the cover. This isn’t one of the suspected bad areas but I checked it all the same.
Next, cut around the sides of the case, to loosen the firmer potting compound, with the pocket knife. Make sure you get it loosened in the corners down to the back of the case.
Put the case in a vise (gripping it by the two PCM mounting eyelets) to hold it while you heat the back of the case with a torch. You’ll need to do this in a well-ventilated area.
You aren’t going to damage anything by heating the back of the case. It will melt just enough of the compound, in order for you to pull the MB out of the case. Heat the back evenly! With a pair of channel locks, grab the 3 gang connector (by one of the plug’s separating ribs). The inside of the connector has a waterproofing layer, so don’t destroy the seal. Grab the outside of the connector. Be careful, this melted compound is as hot as hell.
Let the MB cool completely before trying to remove any of the compound. It comes off much easier. Remove any melted or clumps of potting from inside the case. Don’t clean it out completely. The left over potting in the case will help to bond the MB back in.
Looking at a side view of the MB, figure out the area that needs to stripped of compound, in order to access the pins on the back of the board.
Using your two thumbs, roll off the compound, start on one side and work compound off to the other.
The film that is left over will need to be removed by using the Acetone and the brass utility
brush. The Acetone will break down the active agent in the compound and the brush will pull off the film like crumbled cheese. Don’t use a steel brush. It’s too aggressive.
There are 96 pins that are soldered to the MB from the 3 gang connector housing and 23 pins from the connector housing for the cover board that need to be reflowed. Put your good set of eyeballs on. I found that, when reflowing the pins, lifting up on the iron after soldering each connection prevented any accidental shorting of an adjacent pin. Check your work with a magnifying glass.
I would recommend that you test out your repair by putting the unit back together and running it for a week or so, without ty wrapping the connectors or caulking. I still have mine in running this way for almost a year now because I want to do further research on it. Just assemble it and secure the whole unit back together with thin ty wraps securing the covers and the case. Don’t bend the case tabs. Until you have caulked up the back of the MB with silicone evenly with the OEM potting compound and run a bead around the MB and case, leave the tabs straight until you are satisfied that it’s working correctly.
Just a little more info for anyone that might be interested…
The shot below is a picture of the frequency modulator that sets up the speed of the processor that I haven’t figured out as yet. There is no numbers on the chip that can be cross reference in order to get a replacement. I don’t know at what speed the microprocessor runs at. I don’t want to over clock it or have it running at a slower speed, so if anybody is familiar with these ancient chips and knows anything about their speeds, I would be interested. I destroyed the JY PCM while doing the complete potting removal and need this chip in order to repair it.
Great write up, seems like a ton of work for a $40-$50 PCM.
$40-50???? When did they lower the price?
Just bought a PCM from a 5.9 a couple of weeks ago for $40 with a 90 day warranty.
Someone PM'd me on when I was going to do the write up. Most guys here seem to be paying upwards of $250. just thought I'd help a little for those that aren't as lucky as you. I sure wasn't. I made my own luck w/lifetime warranty for nothing.
Don't get me wrong it's great and worth while write up. I'm sure that folks will find it very useful. 5.9 and even the 96-97 4.0 PCM's are hard to find.
A year or two ago you could find loads of JY's carrying a full array of replacements (cash for clunkers?).
I think all of these re manufacturers bought them up.
In the south, $40-50 PCMs are very scarce. I suppose it's because vehicles tend to stay on the road longer.
Great work JS:cheers2:
Doing this writeup was like doing my first TC overhaul. If I ever do another one, it will take a fraction of the time. ZeeJay, How's your new ride?
Regarding your missing chip: There is a company called KDS that make crystal oscillators: http://www.kds.info/index_en.htm
8M4566 could refer to the speed, as in 8.4566 MHz. Is quite common that they replace the decimal point with the SI prefix: k, M, n etc.
Yeah that could be true. I'm from Canada with salty roads and more cars then people. Oh yeah and no cash for clunkers here, that was a shame to see.
I had spoken to one of the engineers at KDS and if they could have answered me in a simple explanation as you have it would have been too easy. They weren't that helpful, but I think your concept would be a good starting point. Sounds very plausible.They had told me that the number was just an in house # that they couldn't look up.???
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