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Intermittent Start Problem

I have a 1988 780 (v6) with the famous intermittent start problem. I have changed both the fuel injector relay and the radio suppression relay with no effect. The car usually starts right up in the morning but once you drive it it has to sit awhile.

I checked the voltage between the negative lead of the distributor and ground while I cranked it today based on and old tip from Myles Cullen to test the ignition amplifier. I got a voltage, which dropped from 12 v to around 8 volts (the battery was getting low from cranking), but I didn't see any pulses at all. Does that mean the amplifier isn't triggering?

I do really know how the amplifier works, but presume that it is triggered by the crankshaft sensor.

I measured the resistance between the terminals of the crankshaft sensor on the bellhousing. Between the center and one side I got 586 ohms. Between the center and the other side I got nothing. Is the sensor bad?

What else should I check?

Thanks.

Reply to
smyrna45
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The amplifier is triggered by a control unit under the dash. It is a common intermittent failure point, and sometimes you can make the failure go away by just cleaning out the corrosion between it and the aluminum heat sink it's bolted to. Also the flywheel sensor is a common intermittent failure point, but I can't tell you if the resistance numbers you're seeing are good or bad.

Reply to
Mike F

Thanks. I was going by a message someone posted a while back that said that when their crankshaft sensor went bad they saw an open circuit between the center and one of the two side terminals. They bought a new one and got 160 ohms from center terminal to both side terminals.

Does anyone know what this device looks like inside? It looks like terminal 8 is grounded inside the ECU so the other two must just be magnetic pick-up coils. Whether they are supposed to have the same resistance I don't know.

I do have a spark by the way when I crank my car, and disconnecting a fuel tap reveals fuel pressure. I don't know if the existence of a spark says that the amplifier and cranskshaft sensor are good or not.

Reply to
smyrna45

Just a little follow up. On my 780 B280F wiring diagram Volvo shows that the red wire marked terminal 1 on the crankshaft sending unit goes to terminal 10 on the ECU. The terminal marked 2 on the sending unit goes to ECU terminal 23 (blue wire). The terminal marked 3 on the sending unit (thick black wire) goes to terminal 11 on the ECU, but the picture shows it grounded inside the ECU.

I am getting an open circuit between all terminals on the sending unit, except between terminals 1 and 2 (blue and red wires). I get an open circuit between the ground wire and the others.

Reply to
smyrna45

I've never really investigated the sensor closely, but I think that two terminals are for the winding that generates the pulses, and the third is grounded and shields the other 2, so it sounds like what you're getting is OK. The trouble with intermittent problems, the components are always going to check out OK.

Reply to
Mike F

My local volvo parts guy was kind enough to loan me a used, but known good crankshaft sensor and I measured the resistance on it as well. It gave me the same 500 or so ohms between two of the terminals and an open circuit between the other one and either. I did inspect the part and you are correct one of the terminals is simply connected to the shield around the cables. That is also the one that goes to ground inside the ECU. It looks like the sending unit is good at least in a static test. I may check it when its warm to see if there is a difference.

I think I have narrowed my current hard starting condition down to a failed pre-pump fuel pump. When I replace that, it may solve the problem. Thanks.

Reply to
smyrna45

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