Hi all,


I have a 92 940 in the shop thats sat for 3 months at another shop. From what i know and have done. the both fuel pumps, fpr, injectors are soaking now, the whole distributor, spark plugs wires, fp relay has been checked by swapping with another. there is no change when the maf is unplugged. it is terribly hard to start and when it runs it lopes between 500rpm and 0 any higher or if the throttle is applied it dies no sputter or attempt to catch it just dies. there is white smoke out of the exhaust but the head has been rebuilt and a new headgasket is on also. its been compression tested and that turned out fine. i forgot to mention i adjusted the idle and it held at 500-525rpm without problem but one push of the throttle and its dead. im at a loss just wanted to see if anyone who deals with these on a regular basis has an answer

 

Have you run the diagnostic codes? How did you adjust the idle? The symptoms you describe point to a bad MAF sensor. I don't trust the UNPLUGGING trick much.

 

+1 On the MAF but its fail safe (unplugged) should be a 1000 ish RPM idle; it is at least on my car. Presume a vacuum leak investigation has been held; these cars will happily inject enough fuel to go with 200 RPMs worth of air passing over the MAF, eg not nearly enough, without the computer detecting hey somethings wrong.

Also check the idle switch; the diagnostic port has a mode for doing this.

 

how do you check the resistance on these mafs.

 

its not the maf we swapped it out today and this onw definitely works but it still acts like crap. i uploaded a video of it to you tube for anyone who wants to see and here it run. it gets dark when i go under the hood sucky camera phone. . vid sound turned out horribly.

 

Ok pulled the codes today and 2 1 4 came up which is rpm sensor. The boss told me that this has been changed already and it looked almost brand new. But o cleared the codes before I ran this check and this what came up. After researching the part it definitely seems it would be the problem. But what would make a new go bad this quickly

 

the wiring to the flywheel position (rpm) sensor can go bad too as its in a bad spot, behind the engine.... and that is the source for ALL the timing of LH2.4 cars.

 

We have put three brand new crank sensors on the car and are still getting the vode AFTER resetting the system and the car is acting the same way. Short in the circuit, warped flywheel? Any gurus have any idea

 

bad wiring would be my first guess.

 

new development. took a test light to the connector going to the ignition ecu. got a positive on ground continuity for all wires but nothing for power on the red or blue wire. the wiring diagram says the the next and only thing those wires run to is the ignition ecu. so the next step im looking at is pulling the harness and ignition ecu. pierce if you have anything thing else too add i would love for to add it now before i emabark on this pita job. by the way this shop does not have multimeter and refuses to get one.

 

I don't trust 'test lights', I do my tests with a digital multimeter.

I don't have my wiring diagram handy but the ICU (Ignition Control Unit) should have wires going various places, there's a couple to the ECU (EFI Control Unit), one to the ECT (engine coolant temp sensor), one to the crank position sensor, a few to the ignition power module that powers the coil, and a few power and ground wires. I'm probably forgetting a couple. So I dunno what you meant by your statement that the wires to the ICU only went to the ICU.

 

ok here is what im looking at VOLVO 940 1993 - Ignition Wiring Diagrams . if you look at it you'll see 7/25 is the sensor and c156 is the connector. two wires from the connector go from the sensor to the icu and one is obviously a ground. i tested all three and got ground continuity but no power even when i tried to turn the car over.

 

you won't see 'power' on a sensor like that crank position sensor, its a very small signal differential pulse from a coil that has to be amplified to be 'seen'. the signal wires should NOT have been ground if the ICU was disconnected (but if it was connected, there likely is some input impedance to the signal amplificaiton circuitry in the ICU, which could show up as ground to your test light).

I think I'd unplug C156 AND unplug the ICU (its behind/under the glovebox), then ohm the various wires on both sides of C156 and see where the short is. the signal wires on the actual sensor should have some non-zero resistance between them (quite possibly rather low, like 50 ohms or something, I don't know), but should NOT be connected to ground. with both C156 and the ICU unplugged, all three wires on C156 that go to the ICU should be open circuit relative to each other. this should tell you if the short is on the sensor side of the harness, or the c156-ICU side. I'd probably just bypass the bad piece of wire with some shielded twisted pair automotive grade wiring rather than try and replace the whole harness. I dunno.

 

very well then ill give what you recommended a try and see how it goes. this would be a lot easier with a multi meter but the shop owner just refuses to by one.

 

Engine Sensors suggests just replacing the sensor if its even a little suspect.

ah, that also says center to either side is about 160 ohms, so if the center is the ground, I guess the coil is center tapped to ground, which means a simple test light might see that as a short.