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I've read the ECU thread pretty thoroughly, and I'm not seeing any method of identifying if my caps have been previously replaced or not, nor if they need to be.
I have a 1991 and a 1994. The 1991 has serious electrical issues. The 1994 was not performing well and the airbag light is on; now it's refusing to start. So I pulled the ECUs for inspection and both are super clean. The 94 has yellow and blue paint pen writing, so I'd guess it was a used replacement.
I know the issues I'm having are often attributed to failing ECUs. I understand that it's an age and manufacturing issue and not often verifiable. I guess I'm mostly worried about wasting my time replacing capacitors on ECUs that are failing for other reasons.
I guess what I really want to know is should I go ahead with the cap replacement or is there something else to do first?
The way to tell is to inspect the board to see if there are signs of previous soldering work on the capacitor leads. Replace the ECU capacitors if you intend to keep and drive the car, this is one of the key steps in eliminating running issues. Putting money into other parts may be unnecessary if it comes back to a failing ECU.
As I have written, "I came across many failed Nichicon's PF(M) type capacitors" at the post #8 here, you can judge seeing the outer label.
Those QAS capacitors have ID mark PF(M) or PR(M). This photo can tell how you can recognize those printings. Those capacitors had had sold only for big auto mobile industries and most repairing small business companies had no idea to obtain those. So those capacitors are identical to see by your eyes and you can judge easily.
Leaked or not leaked, simply replace if printed PF(M) or PR(M).
But does that mean this one from my 1991 has had the work done? These caps are neither of those and some are marked US!
But this 1991 stil has weird electronic problems.
OH BUT WAIT!!! I THINK I FOUND THE PROBLEM!!! Your post on says that it us ok to be OVER on voltage, but this 220uF is only a 10v where there is supposed to be a 16v! The one recommended is a 50v.
Thanks Yamae! I must have missed that.
Ok then so this 1994 needs caps:
But does that mean this one from my 1991 has had the work done? Tese caps are neither of those and some are marked US!
But this 1991 stil has weird electronic problems.
OH BUT WAIT!!! I THINK I FOUND THE PROBLEM!!! Your post on says that it us ok to be OVER on voltage, but this 220uF is only a 10v where there is supposed to be a 16v! The one recommended is a 50v.
You are welcome,JonnyO78. Seeing is believing.
Yes, your 94 ECU definitely needs to work. It has infamous QAS capacitors. Some may be already leaked at the bottom side. If not leaked yet, I recommend to replace those because 24 years are long enough to fail.
Your 91 ECU seems to be re-capped but not fully. I can see capacitors printed Marcon which are the original capacitors of initial productions. Denso has used Marcon's capacitor when LS400s were started to assemble but they have quickly changed to use Nichicon's QAS capacitors due to the reason that Nichicon's showed better results at lower temperature.
The 220uF/10v capacitor is strange to me. I wonder why such low voltage one was used. Some re-builders might have done incorrect jobs. Marcon's capacitors don't have the QAS liquid inside and they seldom leak but due to the age, the capacitance is decreased and the ESR is bigger. It would be better to replace those and others.
Yes, your 94 ECU definitely needs to work. It has infamous QAS capacitors. Some may be already leaked at the bottom side. If not leaked yet, I recommend to replace those because 24 years are long enough to fail.
Your 91 ECU seems to be re-capped but not fully. I can see capacitors printed Marcon which are the original capacitors of initial productions. Denso has used Marcon's capacitor when LS400s were started to assemble but they have quickly changed to use Nichicon's QAS capacitors due to the reason that Nichicon's showed better results at lower temperature.
The 220uF/10v capacitor is strange to me. I wonder why such low voltage one was used. Some re-builders might have done incorrect jobs. Marcon's capacitors don't have the QAS liquid inside and they seldom leak but due to the age, the capacitance is decreased and the ESR is bigger. It would be better to replace those and others.
That's exactly what I needed to know. I'll just go ahead and swap out all the caps on the 91 as well just to be on the safe side. I can't see any sort of leaking on either, so I'm really pleased about that.
This is gonna be my first major soldering project! Wish me luck!
All done! Here's a pic of the 1994 after being repaired. Several of the capacitors were starting to leak, so I'm super glad to have caught this now.
The 1991 was much easier and pretty much cleared up it's weird electronic issues. Aside from a failing alternator.
But 1994 still won't start, but the OBD tells me it's time to replace distributor #1. I'm going to pull the ones off the 91 for now because I am suddenly without a vehicle!
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ECU Inspection?
