HELP! Tach dead, weird throttle response and shifting... WTF?

[Fastech]

HELP! Tach dead, weird throttle response and shifting... WTF?
Hi all,

Hopefully someone can help me diagnose a problem. This morning when I started up to go to work, I noticed very weird and delayed throttle response from my 1992 500E. I had to push the accelerator down much farther than usual, maybe 40-50% of the way, to get any response. I looked down and saw the tach was inoperative. The car was shifting very strangely, perhaps a limp-home mode? And as I got on the freeway, I found the cruise control was also inoperative. The speedometer and other gauges all are working fine. The A/C clutch stopped engaging at the same time, so no air. And later in the day (when I drove to lunch), the ASR light came on!

I had removed the air cleaner assembly last night to replace the temp sensor for the aux. fans, but it drove fine for the 30 miles (and several restarts) I drove afterwards last night. I wondered if I could have disturbed something under there, but don't know why it would have been a delayed reaction before the problems started.

The car has supposedly had the engine harness replacement done, but someone suggested possibly the throttle body harness? I didn't realize it was seperate. Still, would a fault there cause the tach and A/C problems? I doubt they would have all failed simultaneously if they were not connected, but who knows...

Does anyone have any ideas? I do not know where to begin looking. I would much rather try and diagnose this myself if possible, rather than taking it to our local dealer, who sucks...

Thanks,
Brian Flanagan

[peterhardie]

Your tach reads off the crank sensor, if I am not mistaken, along with lots of other engine controls. I would check there first. Mine went bad on my 400, produced erratic symptoms like yours. Either something chewed through it like mine, or it got old.

[Fastech]

OK will check. I noticed tonite that the check engine light not only is not lit, it does not light up when you first turn the key on either, when all the other dash lights come on. I know it worked last week, it was on for a bit. Strange things are afoot at the Circle K...

[JFawcett]

Dumb question, but are all your fuses good? Mercedes likes to tie strange systems on one bus.

[crhenkel]

The very first thing I would do is pul all the fuses and give them a good cleaning with either fine sandpaper, a fine wire brush, etc and reinstall them to be sure that they are all good and making proper contact. Often a MB fuse will look good but not be making actual contact from corrosion etc. Do it to make sure. Then go from there. many people have chased gremlins only to find a fuse was the issue. Check them all with a meter before reinstalling them to make sure they are good also. Good luck

[Fastech]

Fuses all look good. I went thru and gave them all a little wiggle, as I used to have to do with my old BMW E28 535is, just to be sure, but no change. No corrosion is evident, but I will go thru and clean contact points just in case.

My current theory is possibly a bad crankshaft position sensor. I will be looking into it as soon as I can get back to the shop; unfortunately, my Mother had a bad fall and I have been busy taking care of her, so may be tomorrow or the next day before I can test the theory.

Anyone know what a new CPS costs, by the way???

Thanks!
Brian

[Fastech]

OK, CPS wasn't it... My friend who is helping me (BMW Certified tech, but pretty intuitive with most cars) has studied the wiring diagrams on Alldata and been over everything he can think of and believes it may be a failure of the DME/computer. Anyone have any problems with theirs? Is this typical? And lastly, soed anyone know for certain if the 500SL unit is the same?

Thanks,
Brian