1994 850 Diagnose a failed ECU?

[shaker_chi]

I had an ECU fail on me a couple years ago on my 1994 850. It took me about a month of racking my brain and checking fuel, spark, etc. over and over, until I picked up a used ECU on ebay and the car started right up. I see Ozark just had the same scenario on his V70. Is there an easy way to diagnose a bad ECU? Could I have taken my to the dealer for a check or a reflash? Just a thought.

[tryingbe]

It took me about a month of racking my brain and checking fuel, spark, etc. over and over, until I picked up a used ECU on ebay and the car started right up

Other than this, I don't see any easy/cheap way to diagnosis.

[cn90]

I had the same issue with a 1983 BMW 735i Bosch ECU many years ago.
The diagnosis was made by swapping a good ECU from junk yard.

Sometimes the solder joints develop tiny cracks with time/heat, and can only be detected using microscope or magnifying glass. So the short answer is: get an ECU from junk yard for cheap as a test.

Another source for used ECU is ebay.

[misha]

So...the tales that ecu(s) on these cars don't go bad changed.It's very rare but happens.

It seems that they actually can go bad and can't be diagnosed.When investigation lead to the point that everything is ok but still the car will not start or run,the only solution is to try with a replacement ecu.

I have a list of trouble codes in specific Volvo format (and also in Pxxx format)which have a code for bad ecu.I don't understand why it will not appear when you scan the codes?

I know that if the car will not start or run, you don't know if the Cel is on,but when you scan for codes it should appear.

[Atis]

The usual development target for engine ECU lifetime is 15 years. For this duration they should be really robust. Therefore the urban legend that they never fail.

During the 15-20 years they are on the road they got more than enough heat cycling and vibration.
In our cars the engine bay can get really hot although the ECU has good cooling from the passenger compartment. The vibration load is really depends on the road you are driving, how unbalanced your tires are, how did you upgraded your suspension, etc. Every minute of driving (and parking as well) is taking away from the lifetime of the ECU, but it depends on the circumstances how much it takes.
So even if you kept every component on your car in the best shape and repaired everything with OEM parts, your ECU will fail one day.

There are (were) special devices for ECU diagnosis as at the end of the production lines there is an electrical check for functionality. I do not think these are available today (for Motronic 4.4), and they are not available in dealerships.
So no, they cannot be diagnosed 100%, not even 60%.
The only way to tell that is what cn90 and Misha mentioned. Rule out everything else.

Every electrical unit has its own lifetime and the load what they get during their lifetime is really depends on the circumstances. So even if you find a good looking ECU in the junkyard, it could be that it has just a month left.

As from mechanical buildup the 4.3 and the 4.4 is similar, the first failures should pop up at the 850s.

If any of you has a failed ECU, it would be interesting to see the internals. I mean whether the vibration was the root cause (loose component inside) or something else.
The magnifying glass method could work if the root cause was temp cycling and you are lucky enough to catch the broken solder joint. In this case there is a chance that re soldering will solve the issue (if the open circuit did not caused the failure of other component).