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.

Gamma2.2,

You mentioned the problem had become more frequent recently and then, this past weekend, it didn't fail at all. How's the weather where you are? Have you noted any difference in humidity or temperature between periods of high failure and no failure? If so, you may have a poor ground. Check the ground point for the power stage in particular.

A corroded connection might be sensitive to temperature and humidity too. The only way to find corrosion problems is to inspect the wires and connectors hand-over-hand looking for poor connections and breaks.

I think it is much more likely you have an intermittent open on the power side. Since it is so very intermittent, get a cheap panel indicator light (an "idiot" light) and solder some long wires to it. On the other end put a ring terminal on one wire so you can attach it to a solid ground point. Remove some insulation from the end of the other wire and solder the strands to make a probe. Now you back-probe connectors. Once you have a good probe connection, tape the wire to the bundle to hold in place. Put the indicator on the dash board so you can see it while you drive.

You can see in real time if a point changes state. Say you are probing the 12V supply to the power stage and the light was glowing brightly and all of a sudden it goes dark when the problem occurs. You know to look at the the power side, maybe move the probe to the fuse. If the problem happened but the test light showed no interruption of 12V to the power stage, you know to look somewhere else.

If power is the problem, I would suspect the fuse block.

I haven't done it but I think you could probe the output from the ECU that controls the power stage too. The light should glow dimly because there are going to be short duration pulses there. It may not work though. The test bulb may load the circuit.


--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL

So to start, in summary, look at that black injector/RSR relay especially for loose wires at the connector. Consider replacing it, it is on all 700 and late 240s (as the fan relay) in the salvage yards. Grab a power stage too, in case that mysterious device is failing, but usually that is a heating up issue. That one might be dmodel specific, there are a few part numbers on them. Look for bad connections, tighten things. Finally, that Hall thing.

The "systematic way" to troubleshoot intermittant problems is to replace stuff one at a time and see what happens. That's the tough thing about parts that don't stay broken. Sounds like you have a clasic hall affect problem. If the tach doesn't bounce slightly when cranking then that's usually it. Or the power stage. Was the replacement distributor new or use? A used one could also have developed a leaky oil seal. If the tach does bounce then maybe the injector relay has a cracked solder joint.

1) Intermittants wouldn't be so difficult if there was a reasonable, systematic, way to find them.

2) Ferget yer local supplier and get ye hence the local pick-n-pull.

3} The commonest problem is the plastic connector body on the distributor.

Could an '88 have a bad wiring harness?

Next time, while it's running, shake the engine wiring harness especially under the manifold and where it "snakes" up the rear of the firewall. If it dies you've found the problem.

Possibly a very early '88 that had the biodegradable wiring?

Bad wiring is unlikely on an 88, though not absolutely impossible. The production date on mine is 9/87, and it has the late type (durable insulation) harness, with no evidence it has ever been replaced.

The symptoms you are describing sound like bad solder joints in the radio suppression relay. I've had several of these fail, and they sometimes behave exactly like what you are experiencing. In well over 150K miles in two different 740s, I've never had any type of ignition component failure--not that it couldn't happen, but I would check that relay first. It is usually possible to repair the relay by resoldering it, or you can buy a new one (about $40) if you are not inclined to try.

When I first got my 88, I discovered that the black plastic connector block on the distributor was crumbling, (a common problem) and dangling loose with the wires exposed. However, the car ran just fine, never missed a beat, even at 70 MPH for an hour or more, in heavy rain. It is a good idea to check the condition of this connector---if it is left to flop around for too long, the wires will fray. It is possible to replace just the connector block, if you can get the part (I took one off a car in a junkyard) as long as the three little wires to the Hall effect sensor are intact. The distributor has to come off to do it, so it's convenient to replace the distributor O rings at the same time, stopping or preventing an oil leak.

Thanks for the suggestions, everyone. But now the darn car hasn't acted up all weekend! It's baiting me, I just know it. But I want my daughter to start driving this car when she gets her license, and I figure then it will die again & she'll be calling me to come get her...

The other Brickster was right with the difficulty of fixing things that don't stay
broken.

One place that you might look, while everything working, is the fuse/relay block.
I found that the relay socket pins get loose and can have bad crimps.
You might get lucky and wiggle some relays/wires/fuses and zero in on any number of gremlins.

Good luck, Bill