Everything was fine until I came out of the grocery store and fired up my 780 4-cyl Turbo. It popped and wheezed and idled rough--wouldn't take any throttle but got me home about 3/4 mile at maybe 10 mph. The on-board diagnostics gave me a 121 code (bad MAF or AMM signal). I disconnected the MAF harness and the car started and idled OK, and allowed me to get the RPM up. I believe this is the limp-home mode. So, went and got another MAF from the junkyard, and put it on. It started OK, but again wouldn't take much throttle. With my son I got RPM up to 2500 a few times and shut it down but we couldn't detect the voltage surge that's meant to clean off the MAF filament. Now it won't start at all with either MAF--just pops and misses.

I am getting 12 V to the MAF, also seem to have the right resistance between 6 & 7 on the Fuel System Control Unit regardless of which MAF I have connected, although my multi-meter is a cheapie and I wouldn't vouch for the accuracy.

I'm wondering if I'm just not getting the voltage impulse that cleans off the filament, but I have no idea what I would do about that.

Before I do anything I'll regret, can anybody offer some ideas? Diagnosis or next steps?

 

Been there! Junkyard MAFS are often bad. Last time I had the same problem and got THREE jY sensors which were bad, the fourth one worked. Pretty sure the MAF is your problem... Just get a good one..

 

Lev,
You're probably right, although I've had good luck with junkyard MAF's for other cars. I'd still like to be able to detect the voltage pulse that's meant to clean off the filament. Hmmm... I'll try to check for the pulse on another car that's working fine, so at least I'll be able to validate my test procedure. Back later.
Rob

 

Well, a new MAF didn't fix it. First it started, roughly, and stopped. Then it wouldn't start, then after a five minute spell it fired up and let me drive around the block. Encouraged, I headed to the freeway a mile away, and lost power on the ramp, and steadily slowed down and finally cut out on the next exit another quarter mile down the road. Same OBD code 121 (faulty MAF signal).

Could there be something that is malfunctioning when the engine warms up? Oxygen sensor? Or something else that would trip the 121 code?

Any ideas? Bueller? Bueller?

 

Dealer mechanic said one possibility was to look for any unmetered air getting into the system after the MAF and before the turbo. I can't find any gaps cracks or poor fittings, but I wonder if the valve connecting the hose from the flame trap into the main duct could be malfunctioning?

Still hoping for some other insights.

 

Got it back from the freeway ramp; MAF disconnected, no throttle, and blowing through stop signs at 5-1/2 mph. I did find that by keeping it in first gear I could ease the engine to 3000 rpm.
When I got home I had only one code:
232 – Adaptive Fuel Trim too lean or too rich at idle.
I reconnected the MAF, tried to start it but couldn’t. Turned up two more codes:
121 – MAF sensor signal absent or faulty (the original code), and
322 – MAF burn-off signal absent or faulty.
I’m calling it a night and hoping for some inspiration and input if anybody is out there. Bueller?

 

I am seriously vexed with this thing. The manuals seem to assume a lot and offer few procedural details. Checking the power supply to the Lambda Sond sensor, the manuals say I should be getting battery voltage, the power lead with the spring retainer has two connections in it. Checking voltage betweeen these two gives me only 5 to 8 volts, based on multiple checks over about two hours.

The next step then is to check the resistance of the power and ground circuits from the ECU. The circuit diagram shows terminal 24 on the ECU as one terminal, but the other end of the circuit is not identified. Assuming we are trying to understand the resistance between the ECU and the Lambda Sond connector I used a jumper and got resistance of the order of 2-3 ohms. If that is the prescribed measurement, this is not good news. The low resistance shows the wiring is fine and the low voltage getting the the lambda sond is apparently from a problem in the ECM. The manual says get a new ECM.

I don't know if this will fix my original problem, or whether this IS the original problem which caused the other issues. I suspect this might have been the case.

I would welcome a critique of my diagnosis, and also any suggestions as to recommended rebuilders. Victor Rocha is highly recommended for ABS units, and I'm hoping there's someone out there who will give similar quality and speed of support, repair, and service for ECM's.

 

Hm, strange... These things can get like that. I have several cars I'd swap parts around with or go to the junkyard and get parts for cheap. Have you tried the brickboard.com?

 

I've had some great help from swiftjustice44 that has helped me learn more about this car. I picked up an ECU from a 93 940 Turbo and installed it. It made no difference--my 1990 780 still idles rough and the quickly dies. I've finally found the suppressor and auxiliary relays (behind the left headlight forward of the battery on the 780 turbo) and will do some swapping and testing today. I'm also replacing the fuel injector system relay (commonly known as the fuel pump relay). My pumps are running but I'm not sure what other functionality this failure-prone relay provides, and since it's 22 years old...

www.davebarton.com has graphics, descriptions and photos showing the appearance and locations of relays. This is a major weakness in the Chiltons manuals which tend to use variable names for relays (I finally, and hopefully accurately, realized that the fuel pump relay, the "system" relay, and the fuel injector system relay are all the same thing), and then often fail to give any clue as to what the look like or where they are. Dave Barton filled that gap for me. Thanks Dave.

Does anybody know what the voltage should be at the Lambda Sond connector? Can anyone check on a 740 and tell me what you find? Chiltons says "battery voltage", but I suspect that's wrong. I think I saw somewhere else (I can't find where now) that it should be about 5 volts.

Any other ideas are welcome.

 

every O2 sensor I've mucked with, the heater power is 12V battery voltage. On my 1992 740T, that comes right off the fuel injection/fuel pump relay. The O2 sensor signal is something like 0-1 volts pulsing with a variable duty cycle, and often reads around 0.3V, give or take.

 

Pierce:
That's good to know. Chiltons seems to think the only reason for low volts at the O2 connector would be line resistance or defective ECM. I'll be installing a new fuel system relay in about a hour and will check the voltage again. That must be one of the other functions of that relay I wasn't aware of--I thought it just ran the pumps. I'll let you know what I find.

Thanks.

 

I strongly dislike the Chiltons books. Hayes aren't much better.

Its a shame Bentley never did a late 740/940 book as their 240 book is way better than either C. or H. I bought the "greenbook" Volvo electrical diagrams for my 92 740, and rely on those for all things electrical, and assorted internet sources for most things mechanical.