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About the caps

Bad caps do not need to look bad to be bad. They slowly, over time loose their capacitance, the effective value becoming smaller. Do a Google search on E.S.R., 'effective series resistance', to see why.
As regards the temperature rating, VERY important in an under-hood application.
I do 2-way radio work and have a bag full of trashed 47 mic 25 volt caps, all rated for 85deg C. These came out of radios installed in school busses which are big yellow pizza ovens in the summertime.
They would loose value and eventually leak their guts all over the place. The leakage IS CORROSIVE to the copper land patterns on the board actually etching circuits open! And under all that sealant... well, if that happens there's no way to replace the lost wiring, just buy a new one!
Incidentally, the manufacturer of that radio eventually did get the message... the third production run of that box was using the 105C guys.

Good article, thanks!

Stan
 

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Hi there.

My XJ (though it's papers state it's a 97, it has the pre-97 trim, marker lights and dashboard, go figure) seems to have this exact problem. I've had it for 4 years now and it has performed admirably, but developed this problem since around 3-4 months ago:

The check engine light (which should come on for a couple seconds before starting) doesn't come on. If I try to start it right away, it will crank forever and won't start, but if i leave the key at "on" for a LONG while (it varies) without giving it start, turn key to off, then on several times, it'll get to a point when the check engine (and a relay under the hood, i guess the ASD) begin to click rapidly, then progressively slower until the check engine light stays on solidly for a while then goes to off. After a couple more magic passes I crank it and it starts right away, and it runs like a champ after that.

Fuel pressure doesn't build up until the "clicking" phase is over, after that is plenty. Spark is good. It has brand new pump, distributor cap and rotor BTW, all of them changed part because I was diagnosing this problem, part because it was already time for new parts.

Another clue I think points to this same cap problem: whenever i try to start it on a sunday afternoon, after it's scorchingly hot inside from all the sunlight (Venezuela, coastal town, sea level), it starts much more easily... Maybe the the busted caps' remaining electrolyte complies a bit when it's hot?. Monday mornings? No such luck. I need to get up early to do some extended "foreplay" with the XJ :(

My XJ has the dull metallic grey type PCM, the one with 3 connectors (1 each gray, white and black), fixed by 3 5/16 screws besides the air box near the radiator.

I'm proficient enough to change a bunch of caps like in this writeup, but I'd truly appreciate to know if anyone here has done this kind of repair on that newer type PCM. Logic indicates it shouldn't be much different, but surely the circuit layout and/or components have changed. You know, some intel before messing with it. ;)
 

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I had the exact problems described in this thread in my 98 zj (3 plug pcm) and I pulled it apart with no luck. There was only one cap in it and it tested good.

Ended up replacing it and everythings been bliss since.
 

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Yup, was a PITA trying to find out what it was though. Best part too was that I just bought the truck and drove it out of this guys driveway and about 2 hours later it dies and wouldn't start. Didn't have fuel, nothing back there, so I bought a fuel pump on the way home from work, and luckily figured I'd try to see if there was power back there for the pump, and saw it wasn't getting power.
Took the pump back...
I didn't know what to do at that point, I check fuses and wires etc, thinking something electrical went.
Decided to see if I was getting spark, and I wasn't getting that either. Decided it was the cps (crank position sensor).
Bought one of those, and when I was about to install it, it dawned on me that it couldn't be that, because it wouldn't cut fuel when you turn the key on (when it normally 'primes' the fuel pump). As even if the sensor was bad, when the engine is stopped (as in, before you turn the key to start, having it in the on position) it doesn't relay anything to the pcm anyway, so it wouldn't matter.

Decided to get pcm ordered.
Whats odd though, is it was on/off a few times. Twice it randomly started, we thought it was just a loose wire or connection somewhere from messing in the engine bay. It would run like a champ, and then come out of the store and it wouldn't start.
Put the new pcm in and she started right up though. No problems since
 

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There has to be a way to dissolve that damn potting material. If I could get that stuff off, it would be easier to try to track down problems on the board. Most likely, a majority of all PCM failures are from a component failure, not a IC failure....
 

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Ok Ive gotten to the point where I have cut away at the gel around the old Caps and where they are soldered to the board. I have the Radio Shack caps in hand. My question is, how do I determine which side is negative, and which is positive?
 

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One of the two leads to the cap is longer. This is usually the positive.
 

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Polarity

Also, the caps have the polarity printed in their plastic sleeve. Usually a band of different color that runs through the cap and has either a bunch of dashes (indicating that side's lead is negative) or pluses.

Use what's printed on the cap to be replaced as reference (assuming you haven't pulled it out yet) and just stick the new cap in the same orientation.
 

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Also, the caps have the polarity printed in their plastic sleeve. Usually a band of different color that runs through the cap and has either a bunch of dashes (indicating that side's lead is negative) or pluses.

Use what's printed on the cap to be replaced as reference (assuming you haven't pulled it out yet) and just stick the new cap in the same orientation.
Ok thanks! My OBD1 board is slightly different then the one originally pictured. It has a steel heat sink instead of an aluminum one.

I think I can figure out the +/- on the replacements, but is there any way to tell on the board if I have already pulled one out? If I have the board in one position, and lets say determine the left side of cap #1 is positive. Can I assume the 2 others are left side positive aswell???
 

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Ok thanks! My OBD1 board is slightly different then the one originally pictured. It has a steel heat sink instead of an aluminum one.

I think I can figure out the +/- on the replacements, but is there any way to tell on the board if I have already pulled one out? If I have the board in one position, and lets say determine the left side of cap #1 is positive. Can I assume the 2 others are left side positive aswell???
Not really. You have to evaluate each cap you're replacing individually.
 
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