Why does an ecu fail? - Alfa Romeo Forum
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(Post Link) post #1 of 11 Old 04-08-09 Thread Starter
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Why does an ecu fail?

After my airbag light lit up one day for the first time, my (trustworthy) garage was surprised to find they couldn't just reset it. Further investigation revealed they couldn't even access the airbag ecu with their "official" alfa computer thing. After much messing about they managed to access it with an ancient disgnostic computer they had "in the back room" but this revealed various fails that could not be re-set. Their diagnosis (after trying a new ecu) was my bella needed a new ecu. And it hurts the wallet.

I can't help wondering what would cause the ecu to break. Isn't it just a bunch of electronics? I mean, we're not talking about mechanical parts here. This ecu isn't even associated with "engine stuff" is it?.. where voltage spikes and other nasties might occur?

I'm just curious.

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It probably isn't broken. ECU failures are pretty rare.

More likely one of the connectors under the seats for the pretensioners have come loose, which is preventing it from being able to be reset.

The only long term solution is to solder them as far as I am aware, then get it reset.
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Originally Posted by symon View Post
It probably isn't broken. ECU failures are pretty rare.

More likely one of the connectors under the seats for the pretensioners have come loose, which is preventing it from being able to be reset.

The only long term solution is to solder them as far as I am aware, then get it reset.
Sorry but they are not,i see a lot of failed ones every month to know.
Airbag ecu's in particular are very sensitive to current drops more so than engine ones.
Most common causes are jump starting or turning the engine over on a nearly flat battery in which case the starter motor struggles to turn it over and tries to draw current from everywhere.
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I remember someone telling me that 80% of ECUs sent back for repair don't actually need repairing...who knows if they were right. My mum once paid about 400 for a replacement ECU to resolve a problem, didn't fix it and didn't get her money back or the old ECU....
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The fact that they can read from the ecu is pretty weird if they say it's broke.
I use a snap on modis diagnostic unit, its only a couple of years old and that reads my 97 gtv ecu's fine.
I'd get a second opinion....
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For info my Airbag Light stayed on whilst I was trying to start the car on a nearly dead battery.

Local indy reset it for me.

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ECUs like any other box if electrics can fail but in the same way they should be easily fixed. It is often just the connections that cause the problem so when the garage swaps the unit it cleans the contacts, the new ECU works, diagnosis , bust ECU loads of If it really is the ECU and you can wait there are places that fix ECUs, problem is not to fix but as ECUs are sealed , taking them apart without damaging them. I think they are built as "sealed for life" hence why garages will always say replace
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I think Symon makes a very good point here which seems to have been by passed.
When an airbag light comes on it seems everybody just tries to reset it, have they checked to make sure there isn't a problem with the airbag system which is what the light actually signals,

I may be looking at this too simply, and I have no idea of the electronics involved, but to try and solve an alarm issue my car a new body computer fitted (under warranty), it made no difference at all.
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Airbag light came on in my wifes 145,hand book said air bag should be replaced after 10 years. Age of car 1996 light came on 2007. Could it be programed into the system ?.
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I've never opened up a modern ECU but the older ECU's will open up easily and they are made up of very basic components. Heat from basic use can destroy electronics by drying out the capacitors but they can be replaced for a few quid and a soldering iron. I've never repaired a car ECU but I have repaired other electronics such as video recorders, cassete decks and an old SEGA Gamegear by simply replacing the caps. Spikes in tension are highly unlikely and you would need your battery to go in a big way to cause a spike big enough to blow a circuit on an ECU in a 12v system. It would probably have to reverse polarity on itself or something like that. Current drain form a starter motor pulling from a duff battery would do nothing more than drain power, not spike it and burn your ECU.

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(Post Link) post #11 of 11 Old 05-08-09 Thread Starter
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As a follow up I had a good chat about it to my trustworthy mechanic.

It seems he worked for a fair few hours on the fault.

His alfa, up-to-date diagnostic computer, which usually works on everything, could not access the ecu. He'd connect up and get messages like "disconnected" or "can't access". He explained how the software would give some sort of serial number (ISO number?) for an ecu and when if it managed this for mine he noticed it would give different numbers every time followed by a message of incompatibility.

He tried disconnecting every sensor to the unit to rule out a bad connection or bad sensor but same result.

He then tried an old diagnostic computer which to his surprise, did access the unit. It showed lots of bad sensor readings (which perhaps reflected the fact he'd disconnected them!) amongst other stuff but nothing could be reset.

He tried a new ecu and everything worked properly.

It sounds to me that he went through everything he could think of. He checked all obvious electrical connections like the earths too. As he said, he didn't want the problem to bite him in the future - if he just replaced the ecu and the fault lay elsewhere I'd be back with a complaint that his work hadn't solved the issue.

One point I learned is that the airbag ecu has the accelerometers inside - the things that detect a crash. There's more to this unit than some simple logic circuitry (ie a chip or two). Its also not the sort of thing you want to be pocking about inside with a soldering iron - probably good way to make your air bags go off unexpectedly. Those tamper-proof screws are a good idea me thinks!

Anyway, I'm pretty confident the work and replacement was needed. and I think the bill wasn't too bad (450) considering the labour and the cost of a new ecu.
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