I posted a problem in December with my '94 940 n/a, 210k miles, having an intermittent missing problem while driving. At that time, the distributor cap (Bosch), rotor (Bosch), plugs and wires (Bougicourd) were all new. Since then I replaced the distributor cap again and the power stage. I replaced the distributor cap because I could see spark at a particular angle. Am I missing something that should be on the cap? I didn't think I should see spark in there. I also put a timing light on all of the plug wires. The problem does not seem to be limited to any particular speed or engine load. When the timing light was on, I noticed the miss appearing in park while idling. At some point or other, all of the wires had at least one miss according to the timing light. So I figured it was a spark distribution problem.

I seem to be down to either the coil or the RPM sensor. The question I have is which one. Here's an interesting twist - when I bought the power stage, I ordered an off brand (Huco brand or something like that). The off brand product actually made the problem worse. I exchanged it for a Bosch and the problem seemed to go away for awhile. I just had the car on a 1,500+ mile trip and it is now missing more frequently.

It feels like more than one cylinder is missing because the car usually jerks a little as if it loses power and regains power in a half a second - more so than I would expect to feel if only one cylinder was missing. Again, it's not limited to a particular speed or engine load. I just don't want to be left sitting somewhere along the road. I was told by fellow brickboarders that the fuel pump relay or radio suppression relay may also be the cause, but given my timing light test, I'm not so sure about those relays being the culprit. Any tips are appreciated.

Better late than never but problem solved ... strangely enough it was the a/c clutch and/or compressor in the process of failing. It eventually seized and burnt the belt. With a new compressor and clutch, it's as good as new!

Good to know! Thanks for posting the solution. I'll check into this for my 740T. New engine and it's still "missing". I never saw any missing sparks on the #1 cylinder, but I didn't check the others.

A new thing to check before I replace the ignition computer. I have replaced the rpm sensor, coil (from my 240), power stage, dizzy/cap/wires/plugs, and none of them made any difference.

Glad you found it and fixed it! How was evacuating the system and getting the AC working again? I have the older R-12 system and suspect it's charge may be low. Also, when driving on the freeway for a while, less cold air seemed to come from the vents - as if the blower motor was slowing down, the evaporator was icing over, or maybe there was an issue with the compressor. Turning the AC off, or the heat on for a few seconds would usually help - the heat helped more.
Did you notice any issues with your AC before replacing the clutch


Happy Bricking!

Sorry for the very late response. It's been so long now, but I don't remember any issues with my a/c, which is why I never suspected it. I have R134a refrigerant, already installed before I had to evacuate and recharge. I don't have any experience with R12. Recharge was very simple with a friend's vacuum pump. Much easier than I thought it would be.

Last June I replaced the compressor in my sister's van and again had no difficulty recharging the system.

If there is less air coming from the vents, make sure there isn't a blockage in the ducting. Just recently condensation was getting in my ducts and directly interfering with the blower, to the point that very little air came out. But there were symptoms - water on the floor and the sound of the blower splashing through the water. Unclogged the drain on the firewall and I'm back in business.

If the system freezes, the airflow won't stop, but it will get warmer. If it's freezing, then check the system pressure to be sure it's correct. Incorrect pressures can cause freezing.

Thank you to all who posted responses. I have through trial and error diagnosed the problem as the a/c compressor and/or clutch. Apparently the clutch engaging was drawing such a high amount of energy from the engine that it felt like missing. The a/c compressor is now not working and the missing has gone away. Magic, huh?

Anyway, I discovered that the clutch plate is actually bent or warped. The plat was then slipping a little and creating sparks against the pulley assembly. Finding just a clutch assembly has been difficult, so I am buying a compressor and clutch assembly for now. The a/c has been working fine. I did replace two pressostat switches (on the dryer/accumulator) in the past 2-3 years. Otherwise, it has been cooling consistently (approx. 42 degrees in FL heat - coming out of dash vent).

I took the clutch plate off to see if I could find any kind of a fixable probably, but now the clutch will not engage at all. My guess is the air gap is off now.

I wonder if anyone else has had a similar experience.

The A/C compressor on my 1996 940 locked up few months ago. Prior to the failure, the car and the A/C was running fine, and did not have any symptoms like yours. I was backing the car out of the drive way, and about 50 yards from my house, the compressor locked up, and the belt made a loud squeeling sound. I shut the A/C off right away, and the sound went off. Turning the A/C back on brought the sound back.

I replaced the compressor with a reman unit, and gave my old one as core. Works just as before.
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...and the bricks keep on rolling

I have exactly the same problem with my 96 940 with 65k on the clock. At idle it feels like the engine is missing (the engine seems to wobble around).
I know what you mean about seeing the spark through a gap in the distributor cap ! It shocked me a bit, but I really don't think it's anything to worry about (the design of the cap is at fault).
I have changed the fuel pump relay to no avail also plugs/leads/rotor arm /intake manifold gasket/ cleaned throttle body /cleaned injectors - other things I plan to do:

1. replace air mass meter (from a well running scrap car) to rule this out
2. as above with the rpm sensor
3. as above with the HT coil.
Then I plan to check the valve clearances, cylinder compressions. Once these have been done I will be at a loss to know where to go ......
If you manage to solve it pls let me know, I will do the same......


Cheers

Ted suggests the fuel pump relay which of course is always suspect in these cars. However you say you have seen spark failure when you had the timing light connected, so it's "likely" the problem lies in the ignition system. Now if you wanted to explore the fuel system for no other reason than to eliminate that possibility, it might not be time wasted. I believe your 940 will have a schrader valve on the injector rail to which you can attach a fuel pressure gauge. Run the hose out the back of the hood and tape the gauge to the windshield so that you can watch it while you drive. Observe what the pressure does when the engine stumbles. Pressure gauges aren't super expensive and having one in your tool box will prove invaluable as your car ages.

Anyway, I'm one of those that answers questions with more questions.....

When you replaced the plug wires, did you also replace the coil-to-distributor lead?

What ignition system does your car have - Bosch (with the regular looking coil), or Rex (with the big square coil on the LH strut tower)?

Have you read the fault codes to see if there is anything related to the RPM sensor?

Does the problem get worse as things warm up?

...just looking for clues.

it sounds like a classic symptom of a failing fuel pump relay.