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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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#35 ·
ok i know where the fuses are located but which ones do i check? should i just start pulling and checking?


so does that mean that switching from 220uf to 22uf is ok? b/c i just went out and bout 3 220uf caps...i gotta get her running b/c i gotta drive her to the shop tom morning to do the tie rods...im not going to take off the 22 uf ones yet. first ill check the fuses...please let me know if i should got with the 22uf or 220uf...i got both but right now i have the 22uf's on there. they are like half the size of the original ones....plus the original ones had 220uf stamped on it...should i have replaced with 220uf and not 22?....
 
#37 ·
if 220 uf were what came out of there, then that's what i would replace them with. obviously they're there for a reason. just to confirm though, you are seeing the right symptoms to be doing this fix right? namely no check engine light when you first turn the key on, no fuel pump priming.
 
#40 ·
if you haven't done it already, this thread will tell you if it's the computer itself or if the problem lies elsewhere:

http://www.jeepforum.com/forum/showthread.php?t=490788

at the very least, i would check pin 9 on the computer for power when the key is on. if you're not getting that then the computer looks like it's going bad but it's really a wiring issue. other than that, if you've been through all the tests and it still comes up as the computer going bad, then it's always possible there's another problem besides the capacitors. they're hardly the only thing that can go wrong in there, but they are the more common problem. where abouts in NY are you btw? i might be able to lend you a hand.
 
#42 ·
I wish I had read this a year and a half ago. I was going though all the described symptoms with my YJ 4.0 and after much frustration I brought it to a garage which replaced the computer with a rebuilt one for hundreds of dollars.
Recently I thought I was going through the same problem again but realized that the symptoms were a little different. When I turned the ignition the "Check Engine" light would light but the starter relay would just rattle. I would let off and try it again and it would start right up. This wasn't every day. It was very sporadic. I don't think weather was a factor. Now it is to the point where either it will immediately start or there will be no or very low power. If I turn the ignition on with out starting it, the "Check Engine" light will either be fully illuminated or very dim. I checked for trouble codes and I got code 12 which is "Battery has been disconnected within the last 50 starts" which it hasn't. I checked the volts on the battery and I get 12 volts. I checked from the battery to the fuse block and I get 12 volts. I checked from the batter to the starter and I got maybe 8 volts. I am tempted to just replace the battery but would rather be sure before throwing money at a guess. Electricity is like magic to me. Does anyone know a way I could pinpoint the problem? By the way, I live in Massachusetts and it's been in the teens here recently and I haven't had the problem in about 4 or 5 days.

Thanks
Dan
 
#46 ·
i was speakin to a friend of mine who told me that i might have overheated the caps if i took too long to solder them...one of them did take me a while...so im gunna replace again and cross my fingers thas the problem...ill let u guys kno wat happens...but just in case...wheres the cheapest place to get another ecu? Other than salvage yards? Cuz all the ones around here dont have any wranglers in em....i called around...
 
#49 ·
Has anyone other than Anonymous6915 tried this solution with the 4.0L? I have the same exact symptoms, but have yet to try leaving the key in the "on" position to see if it will start after that, and it's sitting at a friend's house at the moment since I had the problem when I went to leave to go home. Also, If I do have to replace the computer, I have read that the ECU from a Cherokee with the 4.0L will fit my YJ and any other 4.0, regardless of standard or auto tranny as long as it doesn't have cruise control, is this true? Anyone else have any similar problems with solutions? Help is much appreciated, it's my DD and need it back on the road ASAP
 
#50 ·
I've been encountering the same symptoms. 1995 YJ.
I put in a new distributor (complete) last night, I installed a new CPS on Christmas eve, previous to this I had new plugs and wires, new starter when it first started..and so on.

Will try this next.
Leaving the key and listening for the fuel relay to start click and start has become a habit in the two months.
 
#51 ·
Both the troubleshooting tips to determine if the ECU was the problem and this thread to fix the ECU worked awesome. I can't thank you guys enough, this forum is a life saver.

I have a 91 YJ 2.5 4cyl. The only thing I noted to be different on mine was that I needed (2) 220 uf's and (1) 470 uf. I'm not sure why mine was different (and in different locations than the photos), but Radio Shack had them all it it worked great.
 
#52 ·
can you tell me if this is the problem on mine? when i try to start, the fuel pump primes and the check engine light comes on, but the started does not engage at all. once the key is in the ignition for a while, it will evantually make a clicking noise when i try to start it but will not start. if i leave the ignition on for a while after that, it will evantually start. do my problems fit the ecu repair? thanks
 
#55 ·
Thank you, thank you, thank you !!!!

Thank you to all that posted on this thread !!!!!!! You guys are amazing . I had this problem on my 95 YJ Wrangler. I replaced the capacitors with the Radio Shack 220/ 35v capacitors and it's running !!!! My friend in electronics ran a test on the caps and found one reading 25 micro ferads when it should be at about 220 . The other 2 were pretty close to the specs. He told me these are like little liquid batteries that should only last about 10 years. There are caps. that are made of a dry chemical that should of been used for this application and has a life expectancy of 30 years. Firestone wanted $650 for the computer (ECM/ ECU) and $150 for labor . Ebay wants $250 for a rebuilt computer. It took 10 min. to take it out of the Jeep. Took about 1/2 hour to repair the board. This thread was right on the process. It cost me $6 at Radio Shack , some time, and I threw alittle cash to my friend. Thank you to all !!!!!!!!!!!!! :2thumbsup:
 
#59 ·
Thank You

Another Huge THANK YOU for this thread. I have a 95 2.5L 5 speed and it died the other night. It would start occasionaly for about 15 sec then die and not restart. I checked the fuel, did a complete tune up (knowing it MUST be the cap or button). No good. Then I found the thread in hear that took me through the ignition checks and determined that the CEL was not coming on so I hoped it was the ECM. After a $6 trip to Radio Shack and about an hour replacing the caps, the CEL came on and she started right up time after time.:2thumbsup:
 
#60 ·
So I've been having this same problem for a year but it never failed to start until last night after I played hockey til 12am and had to work at 7am and was 45 miles from home. While I waited for my wife to come pick me up I searched this forum from my blackberry and came across this thread. I bought the 220uf capacitors and 1 470uf just in case. I trekked back out to the rink with my tool box and capacitors. Radio Shack only had capacitors with axial contacts so if you use these make sure the arrows point to the same side as the stripes on the oe ones. $1.39 ea for the capacitors, $4.50 for the silicone. BWAHAHAHAHA to those selling used and reman ecu's. Anyway, an hour later the jeep fired right up and everything seems perfect. Count this as another huge THANK YOU from me.
 
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