Electrolytics in ECUs
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  1. #1
    Contented Peugeot Driver addo's Avatar
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    Default Electrolytics in ECUs

    The ECU pictured (XM w/PRV) is a pretty typical one from what I've seen in that it has a smattering of electrolytic caps.

    My assumption - and I love starting with assumptions - is that these caps are chosen because of their specific impact upon a signal path or input form.

    What is the possibility that, after twenty years, they are well past their prime and may not do their job so well?

    I'm raising this because of personal observations on a couple of cars that seemed to defy thorough examination of the inputs and general conditions. Good compression, good sensors, good connectors, no corrosion in the harnesses, no change with sensor swaps - yet the engine might display slightly poor running when you'd think it shouldn't.

    So - has anyone experimented with replacement, and if so were there noticeable changes?

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  2. #2
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by addo View Post
    The ECU pictured (XM w/PRV) is a pretty typical one from what I've seen in that it has a smattering of electrolytic caps.

    My assumption - and I love starting with assumptions - is that these caps are chosen because of their specific impact upon a signal path or input form.

    What is the possibility that, after twenty years, they are well past their prime and may not do their job so well?

    I'm raising this because of personal observations on a couple of cars that seemed to defy thorough examination of the inputs and general conditions. Good compression, good sensors, good connectors, no corrosion in the harnesses, no change with sensor swaps - yet the engine might display slightly poor running when you'd think it shouldn't.

    So - has anyone experimented with replacement, and if so were there noticeable changes?
    The quick and dirty diagnosis of faulty electrolytic capacitors is bulge. If the mercedes star on the top is has lifted then they are definitely faulty. Even if the top has just lifted a tiny bit.

    Sometimes they explode or split which gives positive id of the fault.

    In Microprocessor based equipment electrolytic capacitors are usually used for power supply filtering or smoothing. And less often for filtering of sensor inputs. Usually Input sensor filter caps s are smaller in size and lower in value and probably more reliable than electros.

    Replace the electros and see what happens. I'd think the rest of the caps would be OK.

    And.. the answer to your question is no.

    EDIT:

    Those little yellow b...s on the LHS could be tantalum capacitors. If they are expunge and replace ASAP.They are known cause of headaches for technicians! The yellow polyesters on the RHS should be fine.
    Last edited by robmac; 24th July 2012 at 05:15 PM.

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    Contented Peugeot Driver addo's Avatar
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    Ta for the hints. I didn't know tantalum caps were so dodgy.

    My exposure to crook electrolytics has usually been noticing signs of leakage on rather larger units.

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    1000+ Posts PeterT's Avatar
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    The bigger problem is ECU boards are multi-layered, even 20 years ago, and your ability to solder a new component in successfully is very slim!

    '92 205 Mi16
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    1000+ Posts robmac's Avatar
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    Quote Originally Posted by PeterT View Post
    The bigger problem is ECU boards are multi-layered, even 20 years ago, and your ability to solder a new component in successfully is very slim!
    Afaik multi-layer boards came into mass production around the time of SMD.

    Components with "pigtails" are fundamentally unsuited to multi-layer boards.

    Double sided boards have around a long time before multi-layer.

    Multi-layer boards are serviceable, you just need a very hot small iron, a pair of tweezers and a magnifier. Better still a re-flow station.

    Double sided boards (which seems to be what is in the image looking at the through plates) are a walk in park. You do need small hot iron, solder wick, tweezers and a solder sucker.

    Any circuit board is good to leave alone unless you are quite nimble and have done a lot of soldering.

    EDIT: The board shown is in fact an early double or triple layer. It still should be serviceable. The chips are SMD and the
    components pigtail.


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    Contented Peugeot Driver addo's Avatar
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    Just as an amusing aside, the surface mount EEPROM at lower right is where the keypad code is stored. Theft of an early XM or 605 is a snack provided you carry a spare EEPROM with known code, the soldering tools nominated above (plus 240VAC power), and are able to get in the car and open the bonnet... My point? Shannons don't recognise this coded ECU setup as an "approved security system" :/

    That board is double sided; can't recall if double layered. As pictured it had been for a swim in fresh water... I've had good luck with hot air soldering using the plastic welding tip of my heat gun, but lack really steady hands.

  7. #7
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by addo View Post
    Just as an amusing aside, the surface mount EEPROM at lower right is where the keypad code is stored. Theft of an early XM or 605 is a snack provided you carry a spare EEPROM with known code, the soldering tools nominated above (plus 240VAC power), and are able to get in the car and open the bonnet... My point? Shannons don't recognise this coded ECU setup as an "approved security system" :/

    That board is double sided; can't recall if double layered. As pictured it had been for a swim in fresh water... I've had good luck with hot air soldering using the plastic welding tip of my heat gun, but lack really steady hands.
    There are "tools" that reset such stuff (via the obd socket). If you even have a car with transponder and lost keys I can point you to a reliable supplier

    The software and interface for a pc are not all that expensive.

  8. #8
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    As mentioned in a previous post it looks like there are a couple of tantalums there, so replace those also.

    In older stuff (15 years+) that we work on electrolytic capacitors are replaced as a routine task whenever a repair is done. They may look ok from the top, but once removed you can see them starting to weep electrolyte (corrossive to the board) from the lead end.

    Cheers
    spiz

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    I've had a gas ducted heater unit fail after only a few years.

    If you cycled it through enough times (after waiting for the thermal trip to reset, it would eventually work and work all day.


    I yanked the circuit board and as described above found a bulging electrolytic cap.

    I bought a replacement from Jaycar for 10c and soldered it in. Problem solved.

    Same problem with the original stereo in my E32 BMW. Replaced the caps and all good.

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