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Crash Course In Brain Surgery Replacing Capacitors On ECU

255K views 314 replies 163 participants last post by  fishadventure 
#1 ·
This is for a '94 Yj 2.5 liter ECU repair,
Symptoms: turn key but no check engine light but after a few minutes of the key on, you hear clicking and then the check engine/shift light come on and you hear the fuel pump prime and it starts right up.

-I've been dealing with this for about 3 weeks now since the weather has gotten cold. when the problem first arose I thought it was a bad fuel pump so I replaced it and the strainer still same problem. I was really getting fed up with the problem of leaving it and coming back and having to wait 5 minutes before it would start, i searched all over jeepforum for an answer i found sentinel02's thread about checking all the plugs leading to the ECU very complex but informative but i didnt feel like checking all the electrical components. so I either had to send my ECU out to get rebuilt or rebuild it myself so I decided to take it step by step to show in detail how its done, due to the fact that i havn't seen a step by step version of this and also I noticed lots of '94 YJ owners having the same problem.

Tools/Equipment:
Small needle nose plyers
Soldering gun(including flux, wire)
x3 220 uf 35wvdc capacitor (radio shack) the stock spec is 25v but 35 seemed to work also.
utility knife
silicon/sealer
8mm socket

STEPS:
1. take ECU out of Jeep you'll need to remove the washer fluid container held on by one philips screw on the side and then it just slides right out.
2.now for taking off the ECU disconnect the main wiring harness leading into it using a 8mm socket, when loosen bolt and pull off gently being sure not so break any contacts.
3. once the harness is off there are 3,8mm bolts just undo them and the ECU will pop right off the firewall.
4.Take the cover off by prying at the prongs that hold the lid to the casing.
5.once cover is removed run a utility knife along the edge close too the plastic once you have cut alll the way around gently pull the ECU itself out of the casing.
6. now heres were it gets tricky. locate the 3 capacitors on the top of the ECU
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301732.jpg

7. After locating them you have to dig out on the underside of the ECU to expose the ballistics encased circuit board. Its a tidious task but it has to be done to get the capacitors out.
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301729.jpg
8. now that the contacts are exposed take a soldering gun and melt away the factory solder points, have a friend grab the contact on the top side and pull as you melt the solder.
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301731.jpg
9.After you get them out take the gun and put it were the contacts went through to make a hole for the new capacitors to go through.
10. now its time to put the capacitors in, make sure that the side with the stripe is facing the big metal tower (reference picture)
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301732.jpg
11.push them all the done and solder them on the other side back in the original place making sure the stripe is facing the correct way.
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301734.jpg
12. once they are soldered in place take nailclippers and clip the extra wire.
13. take putty or silicon to reseal the holes wait for it to dry and follow the take out procedure in reverse order.
http://i259.photobucket.com/albums/hh315/ahmedthecamel/S7301740.jpg
 
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#90 ·
Read this thread and did it last weekend,starts everytime and runs smoother and in past few days even noticed better fuel mileage..thanks for the write up!..THIS is exactly why i love this forum:thumbsup:
 
#91 ·
Another satisfied customer. I was experiencing all the same issue's and like an idiot i started replacing everything i could. Even a new PCM(which didn't solve the problem kinda funny). Did this little swap of the capcitors on my old PCM and it worked. About an hour of time and $5.00. Thanks for the write up.
 
#95 ·
could this be the reason I have had a problem? vehicle ran fine a month ago. cold winter, have not started it in this amount of time. turn key, see cel, cel goes out after a second, hear click up front, no fuel to the rail. replaced relay, replaced pump/sender unit, fuel filter. still nothing. clicked ignition on 3 times, codes were disconnect battery, a/c fault, and that was it. i shot starting fluid in the intake and got fire so it is a fuel prob. any ideas if this will work?
 
#96 ·
i doubt it. if it fired with the starter fluid then you've got spark which means the computer is most likely working. disco the fuel line at the fuel filter (pump side of the filter, not engine side) and then run it into a cup/can/container. have someone turn the key to run, wait a second for the pump to prime, and then to start to get the engine turning. you should get fuel out the hose into your container on both key positions. the first one will shut off after a second or two (priming phase) and then continuously while trying to start. if you get one, but not the other, I suspect your ignition switch may be bad. if you get nothing, then test the pump harness for 12V power and ground to see if it's getting power. if it is, then chances are either your new pump is bad or maybe you pinched a line or something installing it.
 
#97 ·
I have just got thru replacing the caps on my ECU. 93' 4cylinder manual. I am waiting overnight for the silicon to dry. I will let you know if it works. My symptoms were no CEL,No Fuel Pump, NO spark. I had good power going to ECU. No power going to CPS.

It took me and a helper about two hours to replace the caps. I suck at soldering and have a crappy solder gun. If this works for me then anyone can do this. I promise to update tomorrow.:cheers2:
 
#98 ·
Just put the ECU in and blew a cap. The cap that blew is marked in red. The black arrows show the way the stripe was facing. I didn't notice how the old ones came out. I guess I'm going to try and replace the cap that blew. If anyone can tell me if my caps are not pointed correctly I would appreciate it.
 
#99 ·
looks like you put that one in backwards according to this>
Well electrolytic caps have a gel in them that dries out over time and it can leak out. It is probably corrosive too. Like the original post mentioned, you have to install an electrolytic cap one way. If the cap has a long line on one side like the one in the pic then that is the negative side. Sometimes they will have a + on them. The +, of course denotes the positive side. So just look at which way the cap is installed that you take out. Electrolytics will also have a max voltage stamped on them like 25V or 16V. Make sure you buy caps that have a max voltage at least as high as the ones you are pulling out. If you take out one that says 25V then you can put one back in that says 25V or higher. Just as an FYI, if you install and electrolytic backwards the metal cap on the top of it will blow off like a little light weight bullet when it is energized. Just a little fun with electronics 101.
 
#100 ·
From that quote I still cant tell which way the stripe is supposed to go other than the fact that the cap blew. I am about to replace it as soon as radio shack opens. I'm going to put it the opposite way and hopefully no more blown caps and engine starts.
 
#104 ·
The electrolytic caps do not like reverse voltage. The will blow almost immeadiately. The caps protect the rest of the module from reverse battery feed if you hook up the jumper cables backwards. They also surpress voltage spikes that come in on the battery and ignition feeds. Unfortunately they wear over time when exposed to extreme voltage changes in the vehicle. The diodes in the alternator surpress some of it but not all. That is why it is so important to becareful with connecting and disconnecting the battery leads with the jeeps electrical loads on and making sure the battery cables are securely connected.
 
#111 ·
#114 ·
dam good right up! im having the same problem with my 92 dodge pickup i knew it was the computer and now i dont have to buy one ill just fix it.Thanks a million....SOME ONE GET THIS MAN A DRINK!!!
 
#115 ·
PCM Caps replacement on a /95 4.0 L Manual YJ

Having satisfied all 4 criteria mentioned in a previous post of pcm problems in a hard starting jeep, I replaced my pcm capacitors last night. I have a /95 4.0
manual YJ. My motherboard has 2 capacitors at the top and 1 near the black tower which I take to be a heat sink. Others have posted photos of this configuration. I had 3 identical caps, all being 220uf 25v 105 degrees (not 22uf as someone had suggested in a thread here). There are not 4 screws holding the motherboard in the case as someone else had stated in a thread here. The board is just held in there by a lot of silicone like gelly material. After removing a quarter of an inch of this material around the edges of the board, I used small curved dental scaler instrument to get under the edges.
A wax carving tool available at hobby stores would fill the bill here. Once the board is out, the job is relatively easy. I noticed when I was removing my pcm from the jeep that the one 8mm bolt holding the wire connector was loose........something to check before proceeding with a job that may not be necessary. At this time, my silcone filler around the motherboard is curing inside the house. I won't reinstall the pcm in the jeep for a day or two as it is minus 10 degrees Celsius outside right now. I hope this post is helpful to the next jeeper who needs this 'fix'. Note that I used caps that are 35v as they are more available near me.
 
#116 ·
Followup; I just tested my pcm caps replacement; turned the key, CEL came on immediately and jeep started right up. Now that's better! Those 3 caps cost me all of $1.50 Now the pcm is back in the house (it's minus 5 degrees celsius outside now) so the silicone can finish curing in comfort.
 
#117 ·
Great thread! I originally thought my PCM wouldn't be the issue as this was a replacement from autocomputerexchange.com. When I removed the ECu from the case, it appeared to just be a junk yard PCM in a new black case. It took me about 1 hour to remove the old caps & install the new ones. After finishing up, took it out to the Jeep & the CEL came on instantly. The Jeep fired up with 0 issues. Now I'm waiting for the silicone to cure to reinstall tomorrow morning. With all the work I've done prior to this, the Jeep is purring like a kitten now and idling like a champ. Thanks a ton for this thread.

Off-topic, I had several eclipses/Talons that also suffered from this same issue. Only difference is I paid someone to replace the caps in those ECUs.
 
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