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Unread 03-21-2011, 04:18 PM   #1
LagartoJuancho
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Join Date: Oct 2008
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Fixing bad caps in sealed type XJ PCMs

Just repaired my PCM, and decided to document my experience, in case anyone else needs help.

DISCLAIMER: 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...

The problem:

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...

First Attempt:

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):

Disassembly:
  • 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

Electronics:
  • 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:

Backside


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.

First Results:
Meh

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...

Second Attempt:
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.

Results?:

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...

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Unread 03-21-2011, 04:48 PM   #2
Carves
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Good for you.

You solder like some people weld tho ...

.... as do I ....

Its a shame that shop repairs on electronics are so labour intensive making the replacement of 25cent parts a million dollar exercise .... damned stuff ought to be big, bulky and easily get-atable


BTW ..... Looks like you have the first of the OBDII engine management systems ...
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Unread 03-22-2011, 09:00 AM   #3
JS97ZJ
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Interesting info....
That one looks to be a reverse manufactured PCM. Much easier to work on. I don't understand how those caps went bad in such a short period of time, being that it couldn't be too old.
Hey but if it works I managed to repair mine which is the old style and it wasn't the caps, it was that the pins to the PCB needed to be re soldered. A pretty intensive fix, but worth it when considering the cost of replacing it. It also looks as though they used a 2nd rate wafer to mount the components to. The print came off the board when you soldered it? That will usually happen with cheap indoor electronic stuff.
Were the caps you put into it rated for a higher than normal operating temp?
I like the way they mounted the 3 gang connector to the cover, thats a big plus considering the old style is made in a way that the weight of the harness is being supported by the circuit board. Technology...Gota love it, Good luck!!!
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Unread 03-22-2011, 12:47 PM   #4
_litz
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Ok ... I gotta ask ...

1) is this the same PCM that would be in a '97?

2) who on earth though 85c caps in an engine compartment was good idea?

(I'm guessing the same people who put the airball in the bumper)
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Unread 03-22-2011, 02:10 PM   #5
LagartoJuancho
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Quote:
Originally Posted by JS97ZJ View Post
Interesting info....
That one looks to be a reverse manufactured PCM. Much easier to work on. I don't understand how those caps went bad in such a short period of time, being that it couldn't be too old.
As in 15 years? it's a '96. AFAIK it's the original PCM.

Indeed, now that you mention it, it's easier to work in this one than the old type with the jello/resin thing covering the entire board.

Quote:
Originally Posted by JS97ZJ View Post
Hey but if it works I managed to repair mine which is the old style and it wasn't the caps, it was that the pins to the PCB needed to be re soldered. A pretty intensive fix, but worth it when considering the cost of replacing it.
Indeed!

Quote:
Originally Posted by JS97ZJ View Post
It also looks as though they used a 2nd rate wafer to mount the components to. The print came off the board when you soldered it? That will usually happen with cheap indoor electronic stuff.
No, actually if you look close, it isn't a circuit board per se, but a flexible film board, remember those matrix printers of old, that used to have an orange plastic tape with copper tracks embedded inside to connect to the printing head? the whole circuit is made of that stuff, pretty unforgiving to work with

The track that came off actually was in the zone where the cap electrolyte flowed in, so it might have dissolved whatever was holding it in place. Not a deal-breaker issue but it had me a bit worried.

Quote:
Originally Posted by JS97ZJ View Post
Were the caps you put into it rated for a higher than normal operating temp?
Nope, the same, 105 degs C. The first one (the big, yellow one) was rated to 85 C only, that's what i had available atm, then i changed all 3 with the proper ones.

Quote:
Originally Posted by JS97ZJ View Post
I like the way they mounted the 3 gang connector to the cover, thats a big plus considering the old style is made in a way that the weight of the harness is being supported by the circuit board. Technology...Gota love it, Good luck!!!
Indeed. Actually the plastic the whole 3-way socket is made of is pretty strong and resilient. You might think after 15 years of oil, gas, heat and water it could have cracked, but its in perfect condition!
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Unread 03-22-2011, 02:23 PM   #6
LagartoJuancho
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Quote:
Originally Posted by _litz View Post
Ok ... I gotta ask ...

1) is this the same PCM that would be in a '97?
Not sure, but chances are it is. Has yours developed the same issue?

Mine has a big "298" printed in the tag, you got to check if yours is similar. If it's even remotely like this one it might have the same structure / circuit inside. From what I've read around the net, the different number models mean they were meant for different options (stick, auto, 2wd, 4wd, etc), and judging from the missing power components in the board, this PCM i got might be a "crippled" one, meant for a 2wd as it is.

Quote:
Originally Posted by _litz View Post
2) who on earth though 85c caps in an engine compartment was good idea?

(I'm guessing the same people who put the airball in the bumper)
Well, actually the caps that were there were rated for 105C. I just changed one first, the cap that looked the ugliest, to try to check if the problem was there, later I switched all 3 to caps of the proper ratings.

If i had access to them, i might have even use the polymer type electrolytic caps, the military grade ones used now in PC motherboards and video cards, as i understand they're even harder to kill that regular caps.

On the other hand, being that the XJ runs so freaking hot, it might call to change caps to ones rated for toaster ovens
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Unread 03-22-2011, 03:43 PM   #7
LagartoJuancho
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Quote:
Originally Posted by Carves View Post
Good for you.

You solder like some people weld tho ...

.... as do I ....
Ours is the correct way to solder, let no one tell you otherwise

Quote:
Originally Posted by Carves View Post
Its a shame that shop repairs on electronics are so labour intensive making the replacement of 25cent parts a million dollar exercise .... damned stuff ought to be big, bulky and easily get-atable
Indeed, that's a pet peeve of mine as well

They're actually made to be disposable, so after they break down, you go and buy another one, or even another entire car...

Quote:
Originally Posted by Carves View Post
BTW ..... Looks like you have the first of the OBDII engine management systems ...
I think it is, I've never plugged a code scanner in the socket, but it looks the correct type, located under the steering column (mine is LHD, i guess you guys down under have RHD cars, am I right?)
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Unread 03-22-2011, 03:54 PM   #8
_litz
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Nah, mine is (so far) just fine .... just marking stuff down in the "so when it does this, fix this" log ...

Replacing caps is nothing to me, I used fix arcade games for a living, and still do game and PCB repairs on the side, now that I have a nice comfy "real job" job ...

I do know what you mean about small cheap parts making stuff not work ... I've replaced caps in computers, video cards, TV's and ... of course ... just about any make and model coinop arcade game you can think of ... :-)
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Unread 03-22-2011, 04:25 PM   #9
cheese_man
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it was not the caps themselves that went bad
the chemical composition in side the caps that was defective
( computer motherboard manufacturers lost millions from defective voltage regulator caps )
some corrupt korean lab guy stole the chemical recipe from a japanese cap manufacturer and then sold the recipe

but they the criminals did not get the recipe just right
( that is why the motherboard manufacturers use credible+reliable japanese voltage regulators now )

it is best to let a circuit board expert to replace the caps , as the copper on the pcb boards can easily melt from the wrong solder temperature

then you will be replacing the PCM
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Unread 03-22-2011, 04:45 PM   #10
_litz
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The weird thing is ... the Bad Caps time period was around 99 to 02, in that range.

I changed so many of those things in bartop touchscreen games, it's not funny ...

And yeah, when they fail, they spew electrolyte ... yuck

Not worried about doing the work, if required ... I'm one of those "circuit board experts" you refer to ... ;-)
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Unread 03-24-2011, 01:50 PM   #11
LagartoJuancho
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Quote:
Originally Posted by _litz View Post
The weird thing is ... the Bad Caps time period was around 99 to 02, in that range.
This certainly was manufactured several years before that. I think other factors might have conspired: The XJ's usual running temp coupled with the weather here (not too far from the equator, summers here are scorching). Maybe some short circuit at some point decreased their life?
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Unread 03-24-2011, 04:34 PM   #12
JS97ZJ
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Quote:
Originally Posted by LagartoJuancho View Post
This certainly was manufactured several years before that. I think other factors might have conspired: The XJ's usual running temp coupled with the weather here (not too far from the equator, summers here are scorching). Maybe some short circuit at some point decreased their life?
The PCM was indeed original to the jeep.
If you look at the processor, it's stamped Chrysler "94".
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Unread 04-27-2014, 06:47 PM   #13
The Shadow
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Size?

What size are these caps? Dia. ?
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Unread 04-27-2014, 10:25 PM   #14
dmill89
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Quote:
Originally Posted by The Shadow View Post
What size are these caps? Dia. ?
The one he linked was an 8mm (though it is 85C), since he said the replacements were 3/4 the size of the original I would guess that the originals were 10mm (maybe someone else can confirm this, I haven't had to open my PCM so I am not 100% sure of the cap size). If the originals were 10mm I would get 125C rated automotive-grade caps like these: Nichicon BT 220uF 35V =$0.90/ea.



If they "cages" will only fit 8mm dia. caps, get quality 105C rated Japanese caps like these:
Rubycon ZLH 220uF 35V = $0.41/ea.
Nippon Chemi-Con KY 220uF 35V - $0.45/ea.
Nichicon HE 220uF 35V = $0.51/ea.

Of those three I would go with the Rubycons, they have the highest rated life (8,000hrs.@105C vs. 7,000hrs@105C. for the others) and are the cheapest, but all are good quality parts.

Also make sure to use leaded solder, lead-free solder is more brittle and prone to cracking in applications where it experiences a lot of heat-cycles like under the hood of a vehicle.


Some words of warning on capacitors:
Do not get cheap Chinese, Taiwanese, or Korean caps, these will only come back to bite you (you will be replacing them again in a year or two), stick with quality Japanese (Rubycon, Nichicon, Nippon Chemi-Con/United Cemi-Con, or Panisonic) or American (Cornell Dubilier) caps.
Do not buy caps on Ebay almost all of them are counterfeits (especially any that come from China).
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Unread 04-28-2014, 05:19 AM   #15
The Shadow
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Great post

Thanks dmill89, that was a very informative post. You gave the exact info I needed. I guess you've been doing this for a while? I hopefully can get the PCM soon and get it open with no problems then I can figure out what size to get , but your opinion if I go with a smaller size and these cages don't hold them as securely, is there some way to true them up? Like would I be able to use some kind a silicone in a tube to help them being jarred around? This company DigiKey, do you trust them to order these parts from?
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