1985 745t

A week and a half back my brick decided to become possessed. Random stalls, wierd and rapid tachometer fluctuations, died while driving, etc. I pinned it down via tests of the ICU et al to (I think) either the coil or the hall sensor. Cash being tight and a trip being imminent I decided to swap in a new coil for $40 and hit the road.

Fast forward 2700 miles. The car made it 2/3 across the US to my destination without a SINGLE problem. Started and ran stronger with the new coil, started every time in the morning when I was driving out, ran strong and idled well with only a slight (and acceptable) variation at idle.

However, I go to start it yesterday morning for the second or third time since I've been here and the problem is back in full force. Car sometimes won't start at all (just cranks), car will stumble at slower speeds especially, tach flips and flops, etc.

So can I be sure this is the hall sensor? Its a pricey replacement (even if you decide to just drill out the sensor) and the car is old, so I'd like to be sure before I go spending more cash on it. Just to check, if you get a slight blip on the tach when you turn the key to start cranking BUT you don't get any blips on the tach WHILE its cranking does that mean the Hall Sensor is dead? Mine always gives me one little hop when I first turn the key but then just pins to the bottom of the tachometer.

FYI, jumping the fuel pump whirs it right to life, so its most definitely not a supply issue (yet ANOTHER of my developing problems...).

Thanks in advance for any help.

-rt

p.s. Is heavy duty pressure escaping from the fuel cap on really hot days normal? I'm in UT now, and through SD and out here I've been getting really big wooshes whenever I unscrew the cap as pressure releases from inside the tank (I've smelled teh gas vapors as it equalizes). Is it a sign of a faulty cap?

I recently replaced the crapped out Hall sender in my 87 745T with a Bosch distributor housing base from Import Parts Specailists out of Boise, ID. The housing is basically the distributor base with installed Hall sender. The cost $95 bucks. All you need do is salvage the pinned center shaft and washers from the old base and install them in the new base. Put a new seal on the new base and old shaft and install it in your Brick. I'd get a pin punch to remove the shaft pin if you go this route. That was the only dificult part of the repair. Very simple, no drilling or riveting and with a new cap and rotor you have a new distributor. Check them out at http://www.ips-parts.com/
Hope this is helpful.

Might be obvious, but I'd suggest a boneyard distributor replacement which should allow you to experiment with rebuilding yours at your leisure. Or you could do like I did; pull two in the boneyard for insurance. Cheaper by far than the sensor alone, much less a new distributor.

Forgot my PB'laster for that sticky distributor O-ring, but found plenty of transmission fluid handy in the PS reservoir.

Don't overlook those lousy ICU harness pins, if a 700 has the same going for it 240s have.

You are probably on the right track with your troubleshooting, but your car is old enough to have the problem described in the FAQ as "rotten battery cables" and can cause some weird problems. If you have not done anything with the battery cables you might want to take a real close look. Corrosion of the wires under the crimps can cause high resistance and lower voltage to the car's electronics. Next time it is acting up, watch the voltmeter.

This may not be your problem, but watch out for it because it can make you think it is a different problem.

R Duke

Replacing just the hall sensor is not that hard. All covered in the FAQs. If you go that route, replace the seals while you have the distro out. Fuel tank is working like it should. You are also a candidate for the engine wiring harness. Next time it doesn't start, ask someone to help crank it, try moving the wires to the hall sensor that go into the distro.

If the failure causes the tach to drop to nothing, it's the hall sensor. you need a new distributor!
The coil is the last piece in the system- a bad coil won't cause the tach to drop.
Chances are the hall sensor plug is all burnt up at the bottom of the dist. The wires inside it short out against the body of the distributor.
I had to put a new dist in our 88 745 at about 200K miles 2 years ago.

Not a hard job really. Expensive part though. Good luck!
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 230K, 88 744GLE- 220K, 82 245T-181K Also responsible for the care and feeding of: 88 745GLE, 231K, 87 244DL, 239K, 94 855GLT 189K