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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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#2 ·
sasquachYJ said:
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
You my friend.....

Are ALOT smarter than me.

Strong work. :thumbsup:
 
#4 ·
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


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.

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.

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)

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.

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.


Click Free. Looks good, definitely cheaper than replacing the thing.
 
#6 ·
This is awesome, I had a 94 ECU do the EXACT same thing. Its sitting in my basement and I had no idea what to do with it... Think I'll try this out and either sell it or sell the one in there now... props on the writeup :wave:

I second on the sticky (or add it to the FAQ)
 
#7 ·
when you solder the capacitors, make sure and double check the connection, because if it is bad the it will not work correctly. Just to let everyone know, me and my buddy are not very good with wiring, so dont be afraid to attempt this! It isn't as hard as it seems.
 
#8 ·
yeah, I just did this a few weeks ago. Worked awesome. I'm not that great with soldering either. In fact, I didn't need to use any solder on this job. I was just very careful not to pull too much of the old solder out with the crappy capacitors. Then I just heated up the old solder points and slid in the new caps. Took me about twenty minutes.
 
#9 ·
The clicking relay monster

I think I try this. I have traced the fuel pump wires. I changed both battery cables. I changed the battery. I changed the ignition switch. I changed the relays. I looked at and dismantled the fuse and relay box. I bought a new fuel pump. I changed the fuel filter. I check for ohm resistant in the crank position sensor. AND DO NOT FORGET I PRICED A NEW PCM. WOW! I'm still not broken. I once read some where that a man that owns a jeep is always ready to work on it. I think it went some thing like that
 
#12 ·
Being an electronics engineer for the last 30 years I had the same question as jay-h. How in the world would you know to change some electrolytic caps instead of any one of a hundred other parts? I suppose electrolytics are bad about the electrolyte drying out after a long term exposure to heat but I would think it was pure luck for changing those to fix the whole circuit board.
 
#14 ·
I am an Idiot

4.0 caps are 22Uf and 32 volt. Radio Shack 1.29 each.

I will provide this information provided you do not let another soul know.

Be carefull with what kind of sealant is used to re-seal the caps/ solder points and the circuit board to the case. Make sure there is no water or water based solvent in it what so ever. "a guy I know" used caulk, didn't check that it was latex based. Bad for everything including solder, circuit boards and traces on them. :brickwall :brickwall :brickwall

Everything is better now, "he" got real good at fixing circuit board traces.

Just a little humble advice. :(
 
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#21 ·
4.0 caps are 22Uf and 32 volt. Radio Shack 1.29 each.

I will provide this information provided you do not let another soul know.

Be carefull with what kind of sealant is used to re-seal the caps/ solder points and the circuit board to the case. Make sure there is no water or water based solvent in it what so ever. "a guy I know" used caulk, didn't check that it was latex based. Bad for everything including solder, circuit boards and traces on them. :brickwall :brickwall :brickwall

Everything is better now, "he" got real good at fixing circuit board traces.

Just a little humble advice. :(
wait is it 22 or did u mean 220?
 
#15 ·
the way i was told of the capacitors could be the problem is my friends 94 started doing acting up when it was cold out and the elecrolite fluid or whatever its called freezed up and messed up the capacitor. my friends brother works on sound and circuit boards for a living and he told us that could be the problem and we tried it and it worked. it was pretty much hit or miss you just have to get the right capacitor.
 
#16 ·
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.
 
#17 ·
Just had the same problem, and used this post to fix it. Everything worked great and fired up 1st try. I'm currently at school and don't have a solder gun so I prepped everything and brought it to a local computer/electronics repair shop and had a nice solder job done for $15. Only trouble I had was trying to pry the ECU out of the plastic casing itself without damaging it.
Thanks for the post!
 
#30 ·
Probably not. Disconnect it before putting the ecu back in.

The issue is when you have partial contact and you have arcing electricity that could go where it shouldn't. Usually you just get a light show but even with the battery connected most of the time installing an ecu you wont even realize that there is current around, it really is just a safety measure.
 
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