931 - problem turbo

[Alan Herod]

No cause for alarm here -- I have not become a FEWC proselyte (but some of my friends are), and one of them has a problem with his turbo (81).. How does the pressure sensor on the intake prevent the car from idling with a turbo failure? How can it sense boost at idle? Can this be by-passed for testing?

He has radial play in the fan on the exhaust side, but has not pulled the intake to check the inlet side. The last time the car was running, a very loud high pitched rapid spool-up sound, spool-down sound and engine shut-down occurred. It will not start now. This occurred at a stop at or slightly above idle (i.e. No Load).

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Alan
83 SC

[drew1]

Alan,

Don't park too close to the 931 or your car will start wanting antifreeze.

The pressure sensor on the intake shuts down the motor (through fuel pump relay on 81 I think) if the motor is running too much boost. It can fail open, or like too much boost. If this is the problem, you can ground the wire to the sensor & the motor should start & run.

With these weird noises when it quit, could a big vacuum line have broken and made kind of a whistling noise. You might want to check this, the motor might be pulling enough vacuum through that flapper thing (fuel distributor) to give it any gas.

Does this motor have just a rotor button or does it have a pickup in the distributor ,too? The rotor buttons can go bad (a resistor must be under the black epoxy), but this usually just causes sputtering till it quits. On the late one, no guts distributor, theres a little Seimens box under the dash that reads flywheel sensor , intake air sensor & boost to fire the coil. This could do funny things with the timing to speed up or slow down the motor through big timing changes when it fails. This part is NLA in US but euro part is available in Germany for $700-1000 I've heard.

This box sends a signal to a unit on the driver's fenderwell before firing the coil. I think this just quits on failure, but the may be timing fluctuation since the bad gas plug for timing retard goes to this module.

The earlier model with pick up in distributor, fires through a CD box. An MSD box can be used in its place.

Wierd noises might if cam jumps timing.

I would see if motor is firing while trying to crank. If so look for above. Ignition switches can go on these cars, but it will usually crank up then die.

If firing check to see if plugs look like its getting gas.

good luck,
drew1

[This message has been edited by drew1 (edited 08-28-2001).]

[Alan Herod]

Drew, thanks for the pointers. I have not actually seen the car a friend relayed the question and hopefully he will respond directly here, because I don't know the answer to the questions you posed. Apparently he is in the middle of restoring this car to its past glory when this problem occured.

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Alan
83 SC

[Scislowicz]

Drew, Alan, Scott, thanks for the help!

I'm Jan, that intrepid 931 junkie. That noise heard on failure sounded mechanical, not pneumatic. I think that there is plenty of gas being delivered to the engine because it is being rapidly pumped out of the fuel distributer when the engine is cranked. However, there is no ignition. There is a lot of play in the exhaust side of the turbo fan, (which is now covered with black soot from CO2 adjustments) but it turns freely. Oil was lost on startup, but the high pressure inlet line seems to be secure, and as I said, the turbo shaft spins fine.

The timing belt is ok, and has very little milage on it. It is clean and tight.

Thanks for the pointers!
Jan

[drew1]

Jan,

No Fire?

I'm going to say that with an 81 you have the DIC ignition. Firing is done by a sensor picking up a signal from teeth on the flywheel, air temp sensor in intake, boost or vacuum pressure from intake, all being processed by a Seimens DIC unit. The Seimens unit sends a signal to a solid state module which fires your coil. The coil fire travels through distbutor cap, rotor button, back to the cap terminal through spark plug wire to the plug. The figures I give are from memory, have manual I will dig out & check tomorrow to verify.

You can do pretty good trouble shooting with a Radio Shack Digital Voltmeter, jumper wire with male spade on one end & ring terminal on the other, old American spark plug wire, & one of those spark testers that you can get for about $4 or 5.

1. Let's try the easy stuff first. You can use a gap type tester with a plug wire or an induction tester. Anyway hook the tester off the coil wire (may have to substitute American plug wire with boots off with gap tester) while somebody cranks. If fire here & nome at plugs, then you have a distributor problem.

2. I think that boost sensor works fuel instesd of ignition but let's check. Unplug the wire on it and plug spade end of jumper into wire. put the ring terminal under a bolt to ground it. Have somebody crank & see if you get fire on coil wire. If this solves the problem, you do you can run the car with the jumper & no overboost protection.

3. With the ign switch in on position, check terminal 15 for approx 12 volts. Have the motor cranked & see if voltage is still on this terminal. If either one is no go ign switch is probably bad.

4. Unhook all wires from coil. Ohm accross the little wire terminals - approx 1.5 K Ohms. Ohm accross terminal 1 & big dist wire terminal - approx 8 Ohms. If not close coil is bad. I have double checked coils by unhooking primary wires, running 12V to hot primary side, with an old Chevy condensor from other primary to ground & another wire in that terminal held on ground. When the wire is lifted from ground , a good coil will fire.

5. If our 1st test showed a distributor problem, take the cap off. Ohm across the rotor - 1 K Ohm? Have somebody crank & see if rotor turns. If no turn then since your timing belt is good, you have a gear problem or broke cam. I think the gears are not keyed but interference fit on the cam & dist shaft (If loose they won't turn the distributor.)

6. If we haven't fixed it yet the hard part comes. When you unplug the flywheel sensor from the Seimens unit, you'll see 3 pins in the plug. One is the sheild & others are sensor wires. I think you should Ohm about 4 K Ohms accross the sensor wire pins.

7. I'm not sure what kind of magnatude we have on the signal from the Seimens unit & fender well unit, but its small. I had to get an Actron Module tester to check these. This requires a wiring diagram also to know which pins to pulse or read.

drew1

[This message has been edited by drew1 (edited 08-28-2001).]