I recently was told by a local used Volvo parts supplier that my 503 ECU could be swapped out for a 510. I was told that the 510 is a more "robust" unit, which I immeadiately found appealing. Wanting a spare ECU for diagnosing future problems, I have obtained a used 510 and tried it out today. Before installing it, I dissconnected the battery. Note that when the 510 was installed and I reconnected the battery, there was a spark. This did not occur when I reinstalled my 503 original unit.

When installed, the 510 did not work as well as my 503. Engine did not start right up, and when it did seemed to idle oddly. It would purr along and then seem to almost stall before smoothing out again. My question here is twofold: are the 510's comptable with the 1983 240's that came equipped with a 503?, and if so, does it sound like the 510 I have is defective?

Thanks, RDP