I have a 94 850 with a 96 engine I installed three weeks ago. After installing, I tried to start but the coil was arcing to itself so no spark. I replaced the coil and it started right up. Ran great for 5 days and then died in our driveway as my son pulled up to park. Cranking but no start. Also, the battery was losing charge over the 5 days. Here is my status:
- when the battery is connected the ecu and the idle motor both heat up and stay that way, even with the key out.
- The fuel pump will not pressurize when the key turns, nor run as I turn the engine over.
- If I remove the ecu, fuel pump will pressurize and idle motor does not heat up, but injectors will not activate (even with the ecu in) I have spark and pressure with ecu out but no injector activity.
- I have replaced the ecu twice and no luck ( as well as the fuel pump relay)
- I tried to pull codes from no.2 socket but get nothing (other sockets are working)
- With the ecu out, I pulled one injector out and let gas splash its way into the inlet - the engine would fire on that one cylinder.

Any ideas would be incredibly appreciated!!!!

So...I have replaced the wiring harness and it at least solved one problem - the ecu and Idle Air Control don't heat up anymore. But still no start! Just cranking. One thing that is also different is that the main relay buzzes loudly with key in position II. I tried three different relays and all the same. Also, I get a code in A-6 (Ignition ecu) that indicates no signal from injectors. Does any of this mean anything to anybody??

you're likely not starting because those codes cut off the injector (you didn't specify which cylinder(s)). If its more than one, I'm gunna doubt you'd get it to start. Usually it will start with 4 of 5 working...

According to volvo when the 1-x-5 (where x is whatever cylinder) is set these are the possible sources:

signal lead shorted to ground/supply
open circuit (broken wire somewhere, or ground not connected)
Rust in connections
Faulty injector.

I assume there's more than one code, which would rule out injectors. If its just one, it might be worthwhile to swap that one with another cylinders and see if the code moves with it... Otherwise start looking at wires and connections between the injectors and the ECU...

Buzzing isn't a good thing, just leads me to believe you have electrical problems (something is shorted or open around the ECU that shouldn't be...)

Did you make sure all the grounds were attached?
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If you're not driving it "like its stolen," are you really driving?

When you pull/put in the ECU, how have you done it?

How long does the IAV stay on after the key comes out?

I wonder if you've got a shot relay (or two) which is no fun to diagnose when the car's not running itself..

The ECU's you've replaced with, were they junkyard pulls? Were they also same? 4.3 and 4.4 do not like each other.

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If you're not driving it "like its stolen," are you really driving?

I just pull them straight up out of the box - I'm pulling the 6.2 which I believe is engine/fuel management. That's the one that's heating up with the key out. The IAV stays hot as well for as long as the battery stays connected - hence the discharge of the battery during the 5 days it was running after engine replacement. Last night I replaced crank position sensor and cam position sensor - no change. The ecu's are from a Volvo only salvage place that I trust implicitly. And I've tried three more beyond original so I think the odds are against all being bad. But then, the odds are against what's happening now! I have a good used harness to try so maybe that's the next step?

I just wanted to make sure you were using the same ECU. For the 96 there's the Fenix 5.2 Motronic 4.3 and 4.4. I don't know what the 6.2 is...

Lets see, if it stays running then something isn't telling the main relay to shut off (or the relay has gone bad). When you pull the ECU, if you are leaving the battery connected, you have to wait until the main relay shuts off power (usually within 2.5 minutes of turning off the engine.) The relay's on the fan shroud on the battery side.

You say you can get no codes from A-2, but can you run diagnostics? With the key in Pos II push the button 3x, it should test a bunch of components, including the relay.

What happened to the previous engine?

I'm also wondering if there's a problem with the car realizing its off. I can't remember if a faulty ignition switch would do anything like this however if the ECU thinks the car is running, it won't turn off the relay (and therefore keep draining power). I don't know why it would keep one thing (the IAV) on but not run the other (fuel injectors)..

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If you're not driving it "like its stolen," are you really driving?

Okay - I'm learning here... Is the main relay above the radiator? I haven't checked that yet. We bought this car knowing that it had been overheated and had a blown head gasket. Pulled the head and had it planed and pressure tested. Turns out that did no good as the block was warped as well - dropped in the middle (I'm told that's not uncommon). Anyways, at that point instead of planing the block I decided a new engine was better (actually a tested used engine) because that much heat would do damage elsewhere.

Yes, above the fan attached to the radiator is a line of relays and switches. The one closest to the battery is the main relay. As far as I know its not prone to failure. I don't know exactly how that relay is controlled though, so I can't help with diagnostics on it. Perhaps someone has a wiring diagram that would help...

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If you're not driving it "like its stolen," are you really driving?

Maybe put a current meter in series with the battery cable and see how much current is drawn with everything shut off and the doors closed. Should be less than 0.5 mamps I believe. If more then try pullig fuses one at a time until you find the one(s) that eliminate the drain. Go from there depending on what you find. Sounds like there's a short somewhere.

Don't have a meter but when I attach the positive lead to the battery you can hear the idle air control valve click (I've replaced that also). So I went through all the fuses to see if any would stop that and none did. I believe I'm on the verge of a new wiring harness?