Tach Failed 1985 W123 300D - Page 2 - PeachParts Mercedes-Benz Forum

charmalu · #16 03-30-2011, 11:34 PM

I appologize not getting back to you.

Both of those cables go into a larger bundle above the monovalbe, then run along above the battery, down the right side, and then bend back under the battery into a larger harness into the cabin. I presume into the black box behind the passenger side kick pannel.

The cable is really simple to replace. Just that flat plug, and the one hex bolt at the adaptor plate. Its a 5 or 6mm. you can get to it from under the car.

Dang I saw an 84 300D in PNP yesterday, I should have pulled the cable off it. I might be going back next week and could pull it.

Charlie

VW1300 · #17 03-30-2011, 11:38 PM

I thought the '84s don't have the adaptor plate sensor - still use the sensor off the cam? If your PNP is anything like the one I used to frequent, just about any electronic or rubber part is nonfunctional/worn out.

OK, if it's that easy I'll go ahead and order a new sensor give it a try. Keep your fingers crossed.

thanks,

rmmagow · #18 04-02-2011, 12:22 PM

Found the OVP, fuse was fine, connections are clean. I'm going to find the Kilma now under the hood and check its connection. I also found the other EGR thing behind the door on the pass side kick panel. Guess I'll check the connections there too. Are there any other fuses to check? my regular fuse box looks like all are good but I know sometimes looks are a little decieving. Does the fact that the tach died once and completely indicate anything special? There was no preliminary odd-ball tach issues, alive in the morning, dead that afternoon.

rmmagow · #19 04-02-2011, 12:56 PM

Kilma, all I see is a black plastic box cover thing next to the brake booster, close to the left fender. I tried to remove the cover, met some resistance and ceased since I'm not wanting to break anything. I had the engine running and was just having a look-see as the car is new to me. Wiggle this, wiggle that and poof, tach is working!! Ran to front of the car and closed the hood before I screwed something up. If that black box next to the brake booster is in fact the kilma, I probably wiggled it and cleaned something by accident. So fixed, not sure why but I'm happy now. A/C 's next :-)

Jeremy5848 · #20 04-02-2011, 01:12 PM

Wiggling Klima should not have helped -- but whatever works! In the following diagram, wire "a" goes to the EGR computer and brings the tach signal to Klima. Perhaps there was a short-to-ground (due to missing insulation or a mis-positioned wire) that you "fixed" with your wiggling.

Jeremy

rmmagow · #21 04-02-2011, 03:57 PM

That might have been it. I was also working under the dash to check the OVP and while it was open, I replaced the side heat vent. There was a wire in that area that I was moving around to put in the dash vent. Maybe that was the wire. If so, my problem should crop up again, but I'm not going to sweat it now. Gads, this is an 85 diesel, a new Mercedes must just cause heart failure looking through wires and wiring diagrams.

VW1300 · #22 04-02-2011, 10:42 PM

UPDATE:

Replaced RPM sensor on bellhousing. Easy job, took half an hour including hunting down my tools and cleanup. Sensor is held on by a single allen head, pops right out. I did it from underneath - pulled front of car up on 2x6 boards - I'm skinny and was able to get right under there.

It's fixed! Tach works now. Thanks to all who replied, esp. Charmalu.

charmalu · #23 04-03-2011, 12:01 AM

Great, glad it was an easy fix, except for the $65. I think 84 is the first yr for this set up. If you come across one, it will have the little box on the back side of the IP, that is the rack position sensor same as your 85 and there is the EGR control box behind the pass side kick panel. not sure if all 84`s have this or not. may have started during the model year.

I have pulled a couple of the EGR boxes for a spare, along with a section of the cable and plug, just incase, for down the road. there is a thread where one member had a problem with water getting behind the panel and corroding the plug conection.

I was poking around in FastLane, and it looks like this RPM Sensor cable was used in the 190-D up to 89, and the 300D up to 93.

Charlie

rmmagow · #24 05-12-2011, 01:56 PM

Curious about the sensor wire. Suppose I pulled the bell housing wire, hooked up to a gauge set to milliamps and waved a magnet across the sensor pickup end, think there'd be a twitch on the gauge? Maybe voltage setting instead? Tach's still dead, seat-belt thing is still going nuts occasionally but I've not had time to play.