I am having an intermittant problem with my '89 760 TI. --- LH 2.2

Saturday I was driving on the interstate and was about 25 minutes into my 30 minute trip. Driving along at 65 mph and the tach started dropping to 0 rpm. It would recover and show normal rpm, 2500 to 3000. I was loosing power but it never died and I barely lost any speed. I was near my destination and exited the interstate expecting it to die on the ramp, but never had any more problems, incluing the return trip home.

Tonight, local trip (40 mph surface street) about 12 minutes from home and the same thng happned. I got to my destination without any further problem and returned home without any additional symptoms.

Four or five weeks ago. Driving along about 1/2 mile from home and car looses power, tach jumps around as above and it stalls. I tried to restart, but it just would not run. Fuel relay clicking when I turned the key on, so I pulled the AMM connector. It restarted right away and I drove home in limp mode. Re-seated the AMM connector a couple of times and restarted and it ran fine till Saturday.

Question: Am I being confused into thinking it is a problem with the AMM or AMM connector? Could this really be an early warning of a hall sensor failure in the distributor?

I claned both connectors with electrical contct cleaner and applied a small amount of silicone dialectric grease to the connectors.

Thanks in advance for your help.

Dan

I had a similar issue with my 89 745T but my problem was cured by the replacement of the ignition power stage (little black thing on driver's side fender towards front).

Hello.

"car looses power, tach jumps around as above and it stalls. I tried to restart, but it just would not run."

This is a classic symptom of a bad hall effect sensor.
The computer uses this to calculate the engine timing.
When the hall sensor is sending bad signals to the computer (tach jumping around) the vehicle will lurch and loose power.
When this signal completely fails (tach reads zero RPMs) the computer assumes that the engine is not running and turns off both the spark/ignition system and the fuel pump. Thus the engine will turn over but not run.

You may wish to troubleshoot the sensor with some of the tests that can be found on this site.
Replacing the distributor is the best way to fix this problem.
Replacing just the sensor is perilous to the frail distributor wall and if you break it, you will have to purchase a new distributor anyway and you will have a rather expensive knick-knack.

Thanks to you and Sammy for your helpful answers. I had a hall sensor fail on '87 740, however I never experienced any symptoms when driving the car. My daughter parked the car and it would not start the next time she went to start it. I would much rather trace a complete failure than an intermittant failure any time. You can never be sure you are on the right path if you can't recreate the problem.

It was this problem on the '87 740 that lead me to the brickboard. Based on the info in the FAQs, I traced it to the hall sensor and rebuilt the distributor. I drilled out the old sensor and at the advice of my mechanic, I used JB weld for mounting the replacement sensor instead of setting the rivets.

Thanks again for the feedback.

Dan

Dan, I am having the same exact problem with my '86 740. The car always starts and idles fine but after about 5 minutes on the expressway it starts missing, not all the time but it's getting to be more and more of the time.

I reasoned that it was an ignition problem because the tack jumps around independent of the engine speed and the tach is electronic, driven off the ignition system. I replaced the distributor with a junkyard one and the problem remained. Still thinking it had to be the hall sensor, I replaced the hall sensor and the problem still persists. I have replaced the ignition module twice, the last time about a year ago. Both times the module failed the car would not start at all.

I have replaced the coil, plugs, wires, cap & rotor and hall sensor so far. I am beginning to wonder about what role the knock sensor plays with the ignition and if it might be sending a false signal to the ICU. I'm planning to swap out the ICU and knock sensor next and after that I can't imagine what is left.

Don't you just love intermittent problems?

You did not replace the AMM, right? I am having a simular problem with my 92 745 - and was just set to get an AMM.

Think I will wait just a spell b4 I order such an expensive part.

Andrew in AL
--
'94 940T (160K) , '92 745T (200K+)

No. I didn't replace the AMM, but after some of the comments here and thinking about the problem I don't think this problem is related to the AMM. The prior occurrance related to the AMM just muddied the waters I believe.

I think the problem is more related to the ignition system than air/fuel mixture. I have not had any additional symptoms since I cleande the hall sensor connector. I can't say for sure that was the problem, but like I said intermittant problems are a real treat to try to solve.

Happy holidays.

Dan

Dan, how is your car doing?

I spent a couple of hours yesterday tracing and checking every wire in the ignition system. I cleaned every connection and ground and even ran ran an addional ground wire to the power stage but the car still jerks.

Today I plan to start swapping out components and will post my results.

-John

Had a problem with an intermittent ignition miss while driving on the highway. After swapping out every other ignition component (I have another donor car) and checking every wire and ground to the ignition system to no avail, the only component left was the power stage. The car had an aftermarket (German HUCO) power stage in it.

I bought a new OEM Bosch and that solved the problem. What a relief!

I send my sympathy your way, but fear I may be entering into a similar nightmare. I have already replaced the cap, rotor, and wires. I did clean the connectors and doped them with silicone dialectric grease.

Thanks for the tip on the ignition module power stage. I will check it out and also the connections to the coil.

Have a nice Holiday.

Dan

Hi,
Hall sensor is your problem.
good luck
Sammy