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

rmmagow · #1 03-22-2011, 07:19 PM

My tach was fine on the drive into work. After work, started the car and no tach action at all, totally dead. All other gague functions are fine. I've been reading some posts about how the 85 is different than the earlier W123's and need some help with trouble shooting. Should I just replace the bellhousing pick-up and see what happens OVP testing? My A/C situation is unclear, I don't think it was working anyway. Car's new to me and I've not yet gone through it.

Thanks

toomany MBZ · #2 03-22-2011, 08:56 PM

The 84's and later take the signal from the transmission, not the front crank.

You may be able to locate a silver relay with a red box on top, near the glove box. This is the OVP relay and that red top holds a fuse.

If you can find the relay, unplug it and inspect the fuse.

That's it, in the pic.

frankengrant · #3 03-22-2011, 10:29 PM

I just had this issue with my car, a new crank position sensor fixed it.

rmmagow · #4 03-23-2011, 05:32 AM

That part is located behind the passenger side kick panel underneath near the glovebox? The crank sensor is a sensor on the bellhousing near the oil filter housing?
If I read other posts correctly, I need this OVP thing to work properly before tackling any A/C related issues right?

Thanks for the replies. I'll post back with results.

JHZR2 · #5 03-23-2011, 07:45 AM

Check everything as is discussed throughout the forum. Check the klima relay for the AC, tach is part of that circuit somehow that I dont exactly remember.

There is info around such that you can bypass (jumper) the klima, and everything is installed in an 85 so you can retrofit the pre-85 standard tach amp, tachometer, etc. and get it working if you want to have a working tach. I had to run one wire on my 85, it was easy...

johninva · #6 03-23-2011, 01:30 PM

the egr computer is located behind a panel in the passenger foot well - big black box looking thing - it is necessary for the tach to work on the 85. Seems like it is common for moisture to get in somewhere - leaky windshield gasket maybe, and run down the computer cable and ruin the contacts in the connector and/or the whole computer itself.

rmmagow · #7 03-23-2011, 03:32 PM

So, is this thing in the pass footwell the kilma?? Do I just remove the cover underneath the glovebox or also the side cover? Never heard of this kilma thing until yesterday. Kinda thought this old beast would have something like this in use. My 83 was a little simpler I guess.

toomany MBZ · #8 03-23-2011, 04:10 PM

Yes, your 83 is simpler. But the 85 has a better rear end gear for mileage.

johninva · #9 03-23-2011, 04:49 PM

no, the klima is under the hood in front of the brake booster, the egr computer is behind the trim colored door in the passengers foot well - check the connector for corrosion, check also the fuse on the OVP relay

VW1300 · #10 03-28-2011, 05:11 PM

My '85 is doing the same thing. The OVP fuse is OK. Is the sensor on the transmission easy to get to, or is it inside? If it's outside, where should I be looking for it?

thanks,

charmalu · #11 03-28-2011, 08:57 PM

The sensor is on the outside of the transmission. If you look down behind the oilfilter housing, you will see it on the edge of the adaptor plate. It is hel in with one small hex bolt.

On the firewall to the left of the Monovalve, there is a large round plug that is snapped into a plastic bracket. that that cable comes from the back side of the IP. It is from the rack position sensor. (using this for reference). there should be another cable from the tach sensor that has a flat plug right in front of it. follow the cable from the flat plug, and it will lead you to the sensor on the adaptor plate.

Both of these cables go to the EGR controller behind the pass kick panel.

I just looked this up from Fastlane. What happened to AllPartsexpress

Is is not there. we just have one choice now

Charlie

VW1300 · #12 03-28-2011, 10:09 PM

Great description! I found the cables. But it looks like the cable from the sensor goes up into the instrument cluster, as does the cable coming over from the flat connector. I got the impression that the cable with the flat connector that you were talking about goes down to the transmission. Perhaps they do via the instrument cluster? The cable that goes into the transmission has a hex nut, but the cable you pictured does not.

How would I know if my sensor is bad, or if it's that darn computer? Or do I just roll the dice and replace the sensor in hopes of it being the former?

thanks,

charmalu · #13 03-29-2011, 09:58 AM

I`ll have to out in the morning and retrace where the cable goes. Iam sure they do not go to the instrument cluster.

The picture I posted, the left side of the sensor end is where the hex bolt fastenes it to the adaptor plate.

The male plug on the other end is part of the flat plug I mentioned. Following the female end of the plug, Iam sure it goes over to the Pass. side. I`ll check.

Mine has always worked, so not sure how to check it.

Every once in a while an 84 or 85 in in the Jy. I probably should pull one just incase. probably 5 or $6.

Charlie

panZZer · #14 03-29-2011, 02:48 PM

I got a spare 85 computer here in N Texas

VW1300 · #15 03-30-2011, 02:49 PM

Quote:

Originally Posted by charmalu

I`ll have to out in the morning and retrace where the cable goes. Iam sure they do not go to the instrument cluster.

The picture I posted, the left side of the sensor end is where the hex bolt fastenes it to the adaptor plate.

The male plug on the other end is part of the flat plug I mentioned. Following the female end of the plug, Iam sure it goes over to the Pass. side. I`ll check.

Charlie

I went out in the light of day and looked again - yes, you were right. It does a fake-out 90 degree bend up by the bundle of cables going into the back of the instrument cluster, which is what threw me off.

Next question - is replacement of said sensor as straightforward as it looks? I'm thinking I'll roll the dice and replace it if there's no way to tell if that's the problem vs the computer. Although I'm guessing it should be carrying some kind of signal...if I had an oscilloscope, perhaps I could see it? If it's not the cable, then I'll move onto the computer I guess.

thanks,