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

First thing I wonder is... how much of a spark. A lead melting, smoke producing zap or a tiny blue snap.

There are a few capacitors in your system that could cause the latter, the caps that come to mind are in the clock, maybe in the radio if you have anything more recent than the base model non-digital display AM-FM stock unit. Unless they fully discharged you'd probably not get that little snap again, when you reinstalled the 503.

The 510 is absolutely compatible with the 503. I've swapped them in 83s and 84s without any difference noted in the performance. Though I don't know the differences in fuel mapping, there are a few slight differences like the bias on the ECT sensor and the fuel relay driver transistor output that look like improvements to me. Aside from those tweaks, the only reason I could tell for the 510's release was the heated oxygen sensor added to the late late late 84s.
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Art Benstein near Baltimore

Art,
From what I described, does it sound like the 510 I tried out has a problem? I sort of suspected that it was simply running a different fuel mix, etc, that I might be able to adjust for. What I would adjust, I'm not sure. From your experience, what symptoms have you seen with defective ECU's? I have a one wire oxygen sensor, as you probably know.

Thanks, Ron Peltier, Bainbridge Island, WA

Ron, I kinda dodged that part of your question because I don't know. Certainly is possible, and I'd say if you could swap it in and out several times and repeat the symptom, you'd still have the oxygen sensor to suspect: if a good 510 expects the heated sensor to be telling the truth about which side of stoichimetric the mixture is within a half a minute, and your sensor is still dead after 10 minutes, yes you'd see a difference, I suspect.

Then there was another thought that kept me from answering. Suppose your AMM is at the far end of spec and the in-tolerance differences in the two ECUs you have are enough to cause the fuel injection to come out of closed loop when you test the 510. Both the AMM and the ECU are precision analog devices on that particular part of the feedback; the ECU uses multiple parallel precision resistors selected in final testing, and the AMM uses multiple series laser trimmed resistors in its DC offset trimming - done in final test. That's tough for a digital yes-no good-bad guy to fathom, but you could see where the addition of tolerances could make a combination of one out-of-spec device work just fine with another in or out of spec. You might be able to correct for that with the mixture pot located on the AMM.

Advice? Test your oxygen sensor (crude fashion) using your original 503 and a high impedance voltmeter on the sensor lead. See that it changes from lean to rich 5 to 10 times in 10 seconds when running warm. You may have to add a few rpm over idle. The sensor still may be a candidate for replacing, but you can at least prove it isn't stone cold dead. Meter the pink test wire. It should flip between 0 and 3.5V roughly, and you can adjust the AMM mixture screw to even the duty cycle as much as possible. Note the voltage at the mixture pot output (pin marked 12 on the AMM plug), then try with your 510. The voltage range is 0 to about 2.7V; if very close to either end, you may be looking at a problem with combined tolerances.

Symptoms of a defective ECU? Only seen open relay driver transistors myself, meaning in my own cars. I've found ECUs in the junkyard that need R56 replaced, a .47 ohm 1/2 watt resistor used to isolate the ground return current of the air mass meter. Mostly I've had AMM troubles with B23F cars. Know 'em well enough now it is the car of choice for my family. Wanna part with yours?
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Art Benstein near Baltimore

Art,
Thanks for the high quality info and advice. I will do the tests you suggest. I appreciate you taking the time to answer my posting. I, too, like these cars, even though my 13 year old son loves to make fun of it. My 83 is actually it quite good condition, with only minor rust that I keep working at with corroless and touch up paint.
Regards, Ron Peltier

Will the 510 work with a non-heated O2 sensor? I think my harness has the extra wires for the heated one, what are the advantages?
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'83 244GL 210K

I believe both would work with the heated sensor, just the 510 will probably begin allowing the oxygen sensor to modify mixture sooner in the warm-up-- 15 seconds instead of the first few minutes. This function is programmed in the read only memory, and I have no details myself or dissasembly.

My guess based on this belief is the advantage of adding a heated sensor is more complete combustion during warm up and maybe extended idle, although your sensor location is still high up on the manifold junction, not down by the cat. So you'd be polluting less. Am I making any sense?
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Art Benstein near Baltimore