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How to Diagnose a Bad ECM/ECU on a 2001 XC70

MVS Volvo Forums member jcb went to great lengths to isolate the ECU in his 2001 XC70. He did work to make sure it was the culprit. This forum topic is how he diagnosed a failing ECM/ECU on his P2 Volvo.

After two days of troubleshooting, I’m ready to send my ECM out for repair (2001 XC V70 – 98,000 miles). Although I’m 99% certain that the ECM is the problem, thought I’d run my experience by the experts to make sure I’m not missing something. If nothing else, perhaps it will help others with troubleshooting things to do, or perhaps NOT to do. After writing the third paragraph, my wife looked at it and said it was way too long and nobody will read it. I agree, and while it is also probably hard to follow, I don’t know of any other way to describe a frustrating experience.

Then the next day:


Just completed doing a repeat of tests #1 & #2 as described above, this time with a ground jumper between the engine block and the bolt hole metal attachment plate on the coil pack (as suggested by abscate). Unfortunately got the same results – #1 cyl not firing the test light or the spark plug when held against the block. Moved to #2 cyl and the plug tester light pulsed on correctly when the plug fired. As before, it fired correctly even after removing the ground jumper, so it’s questionable how important that ground connection is. I did not do the “spare spark plug laying on the block test” because the engine runs pretty badly on three cylinders.

Perhaps more telling is the repeat of checking the ECM signal wires to the coil pack plug. In order to perform the above tests, I had to reinstall the ECM. Before reinstalling I ohm checked the signal wire on all 5 cylinders and all showed open as would be expected. After putting the ECM back in, I ohm checked them all again with the same results. However, after reconnecting the battery, I used a test light (attached to battery+) on each at the plug end. Even with the key off, the light glowed brightly on #1 cyl, and showed nothing on #2 thru #5. It is that difference that leads me to suspect the ECM. It seems my ohm meter doesn’t pickup a ground, but interjecting a small test light current into the ECM does. Perhaps this suggests something like blown diode, but I hate to speculate on such things for lack of electronic knowledge.

So if there are any other thoughts or options anyone would like to suggest, please please let me know a.s.a.p. – I’m just about ready to box the ECM for shipment (not certain to where at this point).

Thanks !

About To Send ECM Out For Repair – 2001 XC70

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