Just repaired my PCM, and decided to document my experience, in case anyone else needs help.
: Try this at your own risk. I'm not responsible if you don't have the notions or patience and something goes south. You need at least basic knowledge of electronics. The PCM is full of components that might be damaged by just touching them and causing a static discharge...
My XJ ('96, 2wd, country trim) developed a starting problem. Each morning, when turning the key to on, the check engine light and the one immediately below it were off (they should be on for a bit, then turn off in normal operation) and no amount of cranking would make it start.
I eventually found out that if I left the key at "on" for up to several minutes, cycling the key from off to on several times (on, allow a couple minutes, off), until the check engine light started blinking (and at the same speed some relay under the hood clicking, i assume the ASD) progressively more "solidly" until it finally turned on solidly for a bit as it should, then off. A couple of magic passes later and the XJ would start...
This sounded more of an electronics problem rather than a regular electric one, and I dreaded it was the PCM. Finally, lurking around this forum i found this thread
confirming it, which details the same symptoms I was observing in mine.
I boldly went and opened the hood that night and to my surprise
my PCM is completely different than the ones depicted in the thread i just mentioned. Mine looked like this (after I removed the plastic snap-on shield):
Still, the symptoms were the same, it HAD to be the same problem, so I grabbed my tools and got to it.
You'll need these tools and materials(at least in this model and year):
- Ratchet with 1/2 socket, for removing the airbox (a must, save a bit of sanity and remove it first, remember to cover the hose that goes into the throttle body to prevent dust from getting inside)
- 5/16 one to remove the 3 bolts that hold the PCM to the driver's side.
- T15 torx driver, to open the PCM itself (more on that later)
- Couple of flat screwdrivers, one small one regular, to pry the aluminum sheet open.
- 3 x Metric tons of PATIENCE
- 3 x 220uf, 25v (i used 35v ones) electrolytic capacitors, rated to 105 C temp. For reference, they're like this one, only the one I linked is rated for 85C.
- Soldering iron, I used a 12w weller like http://www.amazon.com/Weller-WM120-120v-Pencil-Soldering/dp/B0000WT586/ref=sr_1_12?s=power-hand-tools&ie=UTF8&qid=1300734301&sr=1-12. Is quite enough to do the job
- Rosin core solder wire, fine gauge. Get a good brand.
- Small diagonal cut pliers
- Small needle nose pliers
- Small screwdrivers or tweezers, to CAAAREFULLY unlock the cap cages, the glue that holds them down can come off quite easily.
- Doesn't hurt to have a desoldering tool handy (like the blue with a yellow button one in a photo further down)
Couple more views:
The 3 plugs:
Plastic (heat?) shield:
REMEMBER: UNPLUG THE NEGATIVE BATTERY TERMINAL AND WAIT A COUPLE OF MINUTES BEFORE EVEN TRYING TO WORK ON THE PCM!!!!
After unplugging the negative battery terminal, i waited a bit for all electrical charges to fizzle out, Got the airbox out first to have some clearance to get the PCM out. It's not mandatory, but it allows you more comfort, and besides it's just a bunch of clips, a clamp, 2 screws and 1 hex nut. 5 minutes tops at sloth cruise speed
Next, the PCM itself is held by 3 long screws, 2 towards the driver, 1 to front. Remove the last one while holding the PCM so it doesn't drop. Unplug the 3 plugs that go into the PCM, they're one each black, gray and white, by GENTLY
depressing the tab at the top and GENTLY
pulling them out. Plastic this old and this exposed to extreme heat changes tends to be brittle. Fortunately i didn't break anything.
Out it is, now to open it. It's kept shut not only by the 7 T15 screws (3 on the back, 4 at front) that you see in the pics, but also internally by a continuous bead of gray RTV gasket silicone, which you must dislodge VERY
carefully, in order not to bust the electronics inside.
For that task, i used 2 flat screwdrivers and bit by bit, using one as a lever while the other pries, carefully inserting only just enough of the screwdriver's tip so you don't scratch the components or circuit inside.
Finally, I opened the aluminum "wrap" that carries the flexible circuit "board" (sheet?). When you have enough of it pried open, carefully bend it back (it's aluminum, it tends to crack if bent too much) to expose the circuit board:
Front (connectors, power supply)
Closeup of the bad boys (and my fugly repair work)
These two pics were shot after i replaced the most visibly damaged cap with the only one i had available at the time in my components box, which wasn't rated for the correct temp and as you can see was oversize (hint: upper right, the yellow one). You can also see here (lower part) the rows of pins of the 3 connectors of the wire harness.
The caps already leaked electrolyte into the circuit board. I cut the most offending cap's leads, cleaned the board to the best of my abilities, placed the new one and soldered it to the pads (actually to the remainders of the late cap leads) in the same polarity. There's a sort of coat over the entire electronics that you have to carefully peel in order to solder the components. WARNING
: some of the circuit tracks can come off and break if you're not careful, remember Murphy's law, plan accordingly.
A note on the cap cages
: they're the most annoying pieces of #$#% you can imagine, they're kept close by a small tab or hook on the outer side grasping a square hole on the inside. The caps closeup shows this. They're quite difficult to open without them breaking off the circuit board (happened to me, a chill ran up my spine i tell you). My advise? hold the cage down with a tiny screwdriver while GENTLY
pulling the upper sheet metal with a plier. You need to do this in order to securely remove the caps, or else you'll make a mess.
The problem persisted, but of course i hadn't replaced the other 2, so i wasn't disheartened. Next I ordered a bunch of caps of the correct type, this time they were even smaller than the originals, so they fit nicely into the cages, which are there, i assume, to protect the cap from vibration that over time could cause the leads to break.
After a few days the caps arrived...
Repeated the procedure, this time i replaced all 3 caps with the new ones i got, pried the whole circuit carrier off the chassis (to reseal it later):
The whole circuit (after the new caps, they're barely visible as they're about 3/4 the size of the old ones):
The chassis by itself:
I put it back together (without sealing it yet) and plugged it and the airbox to test it.
Like owning a brand new XJ!!!
As I saw the two idiot lights coming on right away i smiled, and let off a hoot when the thing started right away at the turn of the key without complaint. It's just a small repair, but it felt epic to me.
I went back with the PCM inside, disassembled it again, resealed it with gray silicone and put all back together for good. Now for (hopefully) 10 more years + without misbehaving
I hope this is of help to anyone with the same problem...