Got caught in a storm on my way back from the National Matches and ran into high water. Volvos are not amphibious vehicles, especially not when they have holes in the floorboards! Water came up high enough to get into the bottom of the ECU. It ran about a mile down the road then quit. Cranking it only flooded it. I took the ECU to my electronic repairman cousin and he found a shorted transistor at the bottom that he had an equivalent for. However, it still just floods, like the injectors are constantly spraying. I wanted to see if there were any codes and discovered there's constant power to the #2 port of the OBD, the light stays on and it won't flash anything. The ignition side of the OBD flashes 1-1-1 though. I've already ordered another Bosch 951 ECU, but figured I'd see if anyone else has encountered the constant light on the OBD or gone through the wet ECU experience. Is there information available that my electronic repairman could make use of? I wouldn't mind having a spare if the wet one can be repaired. All suggestions appreciated, I only have a few days to get it out of the guys very busy parking lot!

 

sounds like its fried, all right. the ECUs are moderately complex little beasties, with an old school Intel 8051 industrial control microprocessor, and an eprom full of a few K bytes of assembly language and fuel map tables, plus all the drivers and control electronics to sense and run things. if the ODB signal is always-on, odds are the CPU itself isn't running any code.

AFAIK, there are no internal wiring diagrams or anything to aid in trouble shooting

 

Thanks Pierce, I'm discovering more about them. I've found Bosch driver ICs at a reasonable price and I see there are equivalents for the Intel 8051 you mentioned. I might be able to get something together that will work. I'm not waiting for it to be repaired though, another 951 will soon be here. I should go check that tracking number right now.

 

I got the used ECU installed. Some things got better, but it still won't start though it has both spark and fuel. Now there isn't too much fuel, and the light that was stuck on the OBD#2 has gone out. Checked for codes after a bit of cranking, both sockets 1-1-1. I thought it was going to take off when I first cranked it, fired a few times, but no go after that. At least I got it moved to where it won't be towed away. Found a cousin a couple miles away that has a big lot. I also installed a new Ignition Control Module. I've got some time to ponder any advice received, won't be able to get back to it for at least a week.

 

ignition POWER module, or ignition control UNIT ? the ICU is another computer (much like the ECU) under the dashboard, while the Power Module is under the hood and serves as the power trigger for the ignition coil. IIRC, on a 740 the ICU is on the left side behind the dashboard.

 

This thing. On my car it's behind the air intake box on the driver side fender. I've seen the illustrations showing it around the pedals, but there's nothing there on mine. I also didn't note the whirr from the fuel pump when the ignition was turned on yesterday. There was some gas on the plugs, not much, might have been left over from previous flooding. Could be I've gone from too much to not enough. I'll have to get it hauled the 50mi between me and it. The water might have been high enough to have seeped into the timing belt cover too allowing mechanical timing to slip. I suspect that strongly, but won't know for awhile. I'll keep digging, found the way to get around the fuel pump relay, and I'm printing out a lot of other info. I don't think the used ECU I bought is bad, since it straightened out some of the problem, I think either the timing has slipped, or some electrical device downstream of the ECU has gone bad too. In my digging, I have noted a wire common to the fuel pump relay and the radio suppression relay. Wonder if that could be a part of the problem too.

 

ok, yeah, thats the 'power module'.

I don't have a wiring diagram for the 89 740's, my 7/9 diagrams are for 1991-1995.

 

I got the 951 ECU in last week, still couldn't keep it running, but the fuel pump was coming on like it was supposed to. This week I put a new radio suppression relay in along with a new set of plugs, and it took off. Never saw a car put out so much oil smoke in my life, and I hung around a garage when I was a kid! I kept it going until it warmed up, expecting the fire department at any second, but the smoke soon cleared. Still no idle. So, I adjusted the throttle plate a bit more open, until I could keep it running through downtown Newark, Ohio. It was running rough and I was getting some black smoke. Once out of town I ran the snot of it for the 50 miles home as it preferred the higher rpm. After I got home I got codes, 1-2-1, 1-1-3, 3-2-2, 2-3-3, and 2-3-4. I didn't change the 02 sensor, just cleaned up the connections. I'll get a 3-wire sensor before attempting to change it. Why go through the bother of splicing the 4 wire in when it can be returned? I'll try cleaning the AMMs I got before giving up on them, but it looks like about everything on that circuit got fried when the switching transistor got wet! I've got another idle air control valve and TPS and they may or may not work, but I'll try them before buying more.

 

TPS can easily be tested with an ohm meter, its just a simple switch, closed at idle.

clean the throttle body, best done if you remove it from the car, spray it with throttle body cleaner, and use a toothbrush to clean any gunk from the throat, and air plate. make sure all the vacuum pinhole openings are clear (poke them with a thin wire or whatever).

the IAC should just need cleaning, take it off the car, spray some throttle body cleaner in it, clip-lead it to a 12V battery so you can pulse it, its a simple open-close mechanism. keep spraying it and pulsing it til the shutter moves freely if its gummed up.

make sure all the rubber hoses like the ones to the IAC are in good condition, replace if they are hardened or cracked.

with the IAC hoses pinched off, and all accessories (headlihts, a/c) switched off, it should idle about 500rpm, you adjust the idle bypass screw til it does. re-enabling the IAC and it should idle at 700rpm regardless of a/c or headlight loads.

 

Now that it is home, the electronic tech cousin can give me a hand. The battery suddenly won't hold a charge too.

 

The new battery went in and the car immediately ran better! Most of the codes were gone with the exception of 1-2-1 for the AMM. A quick peek showed the platinum wire burnt. I had another AMM, but for some reason it makes the car idle fast once it's warm, 1000rpm. It's a newer one, with the slingshot shaped do-hickey rather than the airfoil shaped thing-a-ma-bob. I can't get the throttle body to close anymore than it is. I think all the other codes I got were in the memory from when I was cranking it and it wouldn't go. The shorted cell of the battery was at the + post, and the connection of the two red and one brown wire to the bolt of the terminal was a bit corroded. If I'm following the wiring diagram right, they are the power source for many of the things the diagnostic unit said had a problem. No juice in, no signal out to the ECU. Very glad to have it going again! I am tempted to put the ECU cousin repaired in to see if it will run the car now, but afraid it might cause some harm.