I have a 1990 780 turbo that won't start, I've traced it down to the crank sensor not receiving it's 12v (0v) to the red wire, and 5v (3.14v) to the blue one. I traced the wires back to the Ignition Control Unit, they have continuity.

Anyone have any idea what to check next? What tells the ICU to supply this voltage? Just the key being on? If that's the case, where does the ICU get it's power from?

Also, on a side note, the fuel pump runs constantly when the key is on, is that normal?

 

Fuel pump ground had been cut and grounded to a nearby bolt. I temporarily reconnected it to the original ground. Now the pump doesn't activate at all. Though I do have a good connection to ground through the original wire. Mind blown. Surely someone knows what's going on? Is it pulsed through the ground or something? In tank pump still runs on, I'm going to check if the same thing was done to it.

 

Fuel pump no longer works, as EM2 is no longer closing. Still not really getting anywhere. Control Unit seems to have power and ground. I ran through the REX self-diagnostic crap which I didn't think this car even had. Well, the TPS is good and it stopped me right in my tracks at the CPS... Go figure. It said to check the resistance between two pins at the ICM. Supposed to be 240 +/- 20... I got 180. But the weird part is that I switched back out to the other CPS that I had and got the same reading. So if they're both bad, they fail the same way. I'm thinking either the ICM itself is bad or it just doesn't put out voltage to the CPS unless it detects a proper signal???? Not a damn clue. Also can't figure out why **** keeps changing without me doing anything. These cars must have electrical gremlins. I have no codes at the moment. But I did have a coolant temp sensor code for a second.... Weird.

 

car wont go past a few mph ,creeper mode of some sort?

 

a turbo 780 should be identical to a turbo 760, which has the same engine, but different interior wiring than a turbo 740/940

a 1990 should have Bosch LH 2.4 injection, and Bosch EZ116k ignition. nothing Rex or Regina ever on a turbo. It should have the diagnostic block with both ECU (injection) and ICU (ignition) diagnostic codes.

your car sounds like its been hacked. the fuel pump should only run about 1 second when you turn on the ignition, then shut off until the engine is cranked over. The cranking over is detected via the crank position sensor, which is a simple magnetic coil pickup, it gets no voltage, it produces a tiny voltage when the holes in the flywheel pass under it, the ICU amplifies this small signal and sends a stronger version to the ECU. This timing info is used for both spark and fuel injection.

 

Hacked is an understatement. Lol. This weekend I went through and traced every single wire in and out of both ICUs. Everything checks out. I ordered a new RPM sensor, just because everything points to it. Even though I KNOW it isn't the only problem. So I found a used ICU on flea-bay for a whopping $40. If neither of those fixes it, I'll trace through every other wire in the system. I am using a 1991 Volvo 940 wiring diagram, for the turbo model. Everything seems to line up. Can't find my coolant temp sensor, the one the ECU uses, if it's mounted to the head under the intake manifold, something is wrong there, but everything else checks out. I also found an old thread stating that the -078 ECUs have been known to go out...??? I can find those on the flea-bay pretty cheap too.

The fuel pump issue was weird. It was grounded to a bolt next to the fuel pump mount, the both wires had been cut and re-spliced. My Fuel Injection Relay was activating both EMs at first. Then it stopped with the one for the pump. But I found good continuity to ground through the original ground wire so I put it back. One of the wires had fallen out of the back of the fuse box, I'm wondering if it was just touching a hot wire or something... I'm not drawing conclusions until I can at least get spark.

On a side note, anyone know what one of these is worth, running, in great shape, with 98k original miles? They must be rare, as nobody seems to know anything about it. Though I did find out that it weighs more than a 740 wagon, so that's cool. I only bought it because my 1999 V70 was a disaster and someone told me to buy a pre-99 Volvo. Maybe I picked the wrong one.

 

I had a 1989 780 V6 for 6 months when it was brand new. fantastic car. great ride, quiet, comfortable, great handling for a luxury grand touring coupe. sadly, I nodded off on the freeway at 3am and wrecked it, and bought a 89 jetta gli to replace it.

I'd be hesitant to take on a 780 nowdays as some key parts like the exterior lighting, windshields, etc are unobtanium and the original tail lights are quite likely crumbling.

 

I've been trying to find a windshield and taillights for a 1988 Scirocco for 2 years now. I know a little bit about unobtanium. My 780 seems to be good. Front valence has a crack, found 2 on eBay cracked in the same spot. Must be a design fault. Other than that though, it's in great shape. Small tear at the seam on the seat bolster, I don't know much about upholstery but I'm sure the whole square of leather could be removed, and another one thrown in.

 

there were a LOT more Scirocco's than there ever were 780's.

 

Whelp. Go figure, I was right, every troubleshooting FAQ was wrong. Not the crank sensor. The way I see it, the ECU has no idea the engine is moving, no codes, no nothing it can't even be bothered with starting. Anyone have a good answer as to what supplies power to the ECU? I at least know the grounds are good, and the power wires are going to the right places, wherever that may be, however, maybe a component is allowing power to be sent to the ECU? Radio suppression relay checks out, either way, I bypassed it and it didn't change a thing. I'm getting my *** kicked here.

Ignition switch? Could something be wrong there? Engine turns over, all accessories power and coil gets 12v, but ECUs don't? I doubt it, but you all might know more than me.

Still waiting on my ICU but I have a creeping suspicion that it's not going to fix it.

By the way, I am checking for spark with a timing light on the lead from the coil to distributor, anyone see a problem there? Works fine on the Scirocco. The fuel pump will run when I jump it to power, and fuel gets to the rail. No signal to the injectors, of course.

If everything points to the crank sensor, and it isn't the crank sensor, what the hell is it?

 

a 1990 780 turbo is probably closer to the 1991 940SE which was a one year only 'turbo 960'... it appears the ignition coil power comes from the radio suppression relay under the hood, and that radio suppression relay is in turn switched on by the fuel system relay... this is different than a 1991 740/940, where the ignition coil is powered directly by the ignition switch...

so, test light time. test 1) does pin 15 of the ignition coil have power when you switch on the ignition?

if test 1 is true, then test 2) connect the test light to coil pin 1, and crank the engine, does the light blink?

 

Ah, damn test light. Mine went south on me while I was testing this stuff earlier, I'll have to get another. I already know the coil has power, but I didn't actually get to that test.

I did just throw a new CPS and Igniter at it, mostly because the igniter was cheap and I was ordering parts anyway. Brand new cap and rotor to insure no problems once spark gets there.

But if I recall, that one tests either the CPS or the igniter, both of which I know are good.

Where I got to on the troubleshooting was checking to see if the CPS was receiving a 12v signal via the red wire, it's not. From there it said to jump a wire across the blue and black wire and check for spark....?? Which didn't make any sense to me, but I did it. Because I didn't understand it I went down both roads, one saying to chase the red wire back to the ICU, which I did, it checks out. The other route said to check grounds and power to the ICU, which as far as I know, also check out as every wire is going where it's supposed to without any resistance. It has to be the ICU, right? I've got one on the way.

 

if you do not have pulses on coil pin 1 but do have power on pin 15, then check ignition power module pin 5, which is a grey shielded wire connected to ICU pin 16, this is the ignition pulse that triggers the power module.

the CPS red and blue wires are a differential twisted pair, that generates a very low level signal, which probably can't be read with a volt meter or test light.

 

I'll run the tests, could I hook up my multimeter instead of a test light? Or is is too fast to possibly see?

So I shouldn't have 12v at the red wire anyhow?

 

I don't believe there's any power at the CPS, its an inductive pickup, it generates small signal A/C which you maybe could see with an oscilloscope, but I wouldn't vouch for that. as long as the CPS wiring and connector looks good, they are very rugged and don't fail internally.

a digital volt meter probably won't read fast enough to see the timing pulses at coil pin 1 or power module pin 5. a 12V LED would work, negative lead to ground, positive lead to the thing you're testing, I'd say you can pick one up for a $ or 2 at Radio Shack, but they've gone shut down the whole chain.

 

Hah! I can't even troubleshoot why this light won't work. But I pulled the bulb out, hotwired it, stuck it to the coil, turned the key and..... DEAD. I'll jump it tomorrow and give it a shot. Shame about Radioshack isn't it? Very useful store for us who like to tinker.

 

Still working on it. I was just going to slap in this new ICU because I figured if I have a brand new CPS, brand new power stage, and I know the coil is good (swapped it into my Scirocco), and my harness checks out. There isn't much else it can be. I'll be pretty busy for the next couple weeks, I'll try to get some more tests in and go back through everything. Check some relays in detail. Assuming this ICU doesn't fix it, of course. Would a bad fuel injection relay, radio suppression relay, or fuel injection ecu cause no spark? I'm pretty sure both relays are good, but I want to be sure not to count anything out. Also, the car is pretty low on fuel, when I jump the fuel pump it gets to the rail no problem. But there isn't some kind of safety start inhibitor for low fuel is there?

 

Just as I thought. ICU was bad. I now have spark between coil and distributor. But no fuel, so I'll be looking into that. I should probably just go ahead and replace the ECU right now and save myself a few hours of troubleshooting. Lol.

 

fuel pump relay is a strong candidate. pretty sure the 760/780 T uses a /different/ relay than the 740/940T as the '60 family uses a completely different relay panel than the '40 series.

note the fuel relay on the 740/940 and I *think* 760/780/940SE has two seperate relays in it, one relay powers the ECU, and also switches on the 'radio supression relay' under the hood which powers the injectors and possibly the coil. the other half of the main fuel relay powers the fuel pumps and only comes on when the engine is turning over.

the 780T, I believe, uses a special 'turbo+' ECU, its not interchangable with other 740/940T ECU's as it provides an additional few PSI of boost for max 10 seconds or something like that, this raises the peak horsepower from 160 to something over 180 HP, and it also raises the torque.