For several months, my '88 740 Turbo (B230 FT w/ EZ117K ignition, manual trans) has had the engine cutting out, seemingly at random. Cold or hot, accelerating, coasting, or stopping--it can happen any time, although it seems most likely to happen at high revs (over 2500 rpm.) Usually it restarts within a second or two--if I am still rolling in gear, it usually restarts on its own. But sometimes it can die completely for several minutes. In the last few days the problem has become much more frequent, and when it starts acting up it will often cut out several times in a mile.

Clearly, the ignition primary circuit is cutting out, because the tachometer drops to zero even if the car is still moving in gear. (The warning lights all come on only if I push in the clutch or come to a stop. This would seem to indicate that the ignition switch is not my problem.)

I have not really checked whether the fuel supply stops when this happens, but clearly the spark is cutting off. I don't think the problem is a loose connection, because we have plenty of speed bumps & potholes, and I have seen no correlation between bumping the car and the engine cutting out.

I have read all I can find on this forum, including the great FAQ section, and on the Volvo Community UK forum. It seems the most common explanations for this problem are failure of the Hall effect sensor or the power stage. It seems to me that it could also be potentially caused by a problem in the ignition control unit, since the signal from the Hall sensor does not go directly to the power stage, but is mediated somehow by the ICU.

The intermittent nature of the problem has prevented me from isolating the source. When I checked the output of the Hall sensor (at terminal 24 of the ICU) it looked good--manually cranking the engine, it switches crisply between 0 and 4.5 Volts. With the engine running, I checked terminal 5 of the power stage, that gets the pulse signal from the ICU, and my voltmeter read 3.5 Volts. I don't have an oscilloscope. (Interestingly, when I cranked the engine manually & checked tml. 5, it would only get one pulse (4.5 V) from the ICU. Each time I shut off the ignition & turned it back on, it would then get one more pulse when I rotated the engine.) I also checked the resistance of both coil windings, and that was within specs.

When the engine dies, it is never out long enough for me to do these checks again.

My power stage was replaced about a year ago, and the whole distributor about 5 years ago, so it seems they really shouldn't be worn out yet. None of these parts is cheap--even the power stage costs 120 bucks from my local supplier. So I hate to just replace stuff at random. Can anyone tell me a systematic way to determine where the fault is?

--Paul Gillis
Falls Church, Va.