Ignition problem (B230K)

Hi all!

I recently bought a 740 GL -87 with 400.000 km on the clock, but still is in pretty good shape. Amazing cars... However, the ignition system is not quite what it should be. Brief story: The car runs fine for a while, then suddenly start misfiring. The tachometer drops to zero and the engine looses all power, after maybe a second the engine fires again (tachometer needle jumping up), then looses it all over again. This can last anything between one minute (and the rest of the ride is fine) to several minutes and at last stopping completly and not being able to restart.

Reading the 700/900 FAQ, some different Volvo forums and Usenet, I've learned a bit about this fault. I'm quite certain that there are no fuel problems, since the misfirings happens so "fast" - there is no middle ground, there either is spark or there is not. Also, I think that the problem is on the low tension side because of the tachometer dropping instantly when it happens. (I've checked plugs, plug wires, coil, rotor and dist cap - all seem OK. Measured coil resistance just in case, also OK.)

Suspecting the Hall Effect sensor, I dismantled the distributor yesterday. The connections seem fine, pulling the connector when idling does not affect engine (same thing with coil connections). When measuring the voltage at the sensor as described in the 700/900 FAQ the voltage that should be below 0.7 V is 1.9-2.0 V in my car. All other voltages are correct. (I measured this at the sensor and not at the ECU, but I don't think that would matter?)

The strange thing is, after mounting the distributor again the car ran fine for about 5 minutes - then started misfiring. Would a bad Hall sensor really cause this type of intermittancy? It seems more like there is a loose connection somewhere, but I'm not sure where to look. I'll have a look at the ignition power module tonight to see what the connections are like there. I don't think that the module itself is bad, since the misfire has appeared right from cold start a couple of times and it doesn't matter whether I'm accelerating or not. (Spark is either good or non-existant, it seems.)

I'd really hate to spend ~1200 SEK (£90) on a used distributor from the breakers if there is just a faulty connection somewhere. According to the FAQ, replacing just the sensor doesn't look like a job for the fainted hearted, either. :-)

Sorry for the long post, any help is appreciated...

/Andreas