New guy here. I'm trying to figure this out for a friend. I've never had a Mercedes, but have always liked them.Is there a simple solution to a tach that only works sporadically? Is this an electrical unit, or is it mechanical? I did a little looking around prior to posting, and found some tachometer questions, but not many answers. Thanks in advance for any help.
Bert

 

I am afraid I have bad news forya.
Tachometer, cruise control, AC and automatic transmission was in those years manufactured only for American market and didn't went much farther, than experimental stage.
That means, that 90 % of those models have a problem with them.
Your tachometer is electric. On drivers side inner fender you'll find the black, rounded module for it. Take it off, clean the connector with contact cleaner and put it back.
If that doesn't work, consider, that the tachometer is only optional.

 

HI Bert,
Here is a general run down on how the tach works.

There is a small round pickup coil mounted on the engine down by the timing marks. The pickup has a gray wire that runs over to the drivers side fender just ahead of the shock tower. There you will find a round thingy (tach amp). This amp is a common problem with the tach jumping all over the place. There is a web site that tells how to force the pins of the tach amp down into there connectors but for the life of me I can't find it this morning. What I did to mine was to unscrew the amp. Grab the lower part with one hand and the upper part with the other hand asnd pull until it comes apart. Place a piece of foam or cardboard (small piece) between the cap and the amp (upper part and lower part) and then push it back together. As you screw the amp on the foam/cardboard will push the amp down firmly into the socket. This will fix a lot of amps. If it doesn't fix yours then you may need to replace the amp or there is a problem somewhere else. The tachs are not designed very well and it will never be much more that "ify" at best.

If you have problems with your cruise let me know. I can tell you how to repair yours or I can sell you the repaired one off of my parts car.

I hope this helps.

Tony T

 

Double check this, but from what I've been told, the better engine pickup coil was mounted on 84 and newer models.
83 and older was feeding from alternator, or something.

 

My 82 300Dt and a 83SD that I worked on had the pickup coil down by the timing marks. I've never seen one that used the alternator for the tach but I've not seen all the Mercedes cars built either ------ so my standard disclaimer applies " I've been wrong before and very well could be this time".

Tony T

 

Thanks for the replies! It turns out the cruise control is a little buggy, too. I found this out this morning, and opined that the two problems were somehow related. Maybe un-related other than sharing a poor design! LOL I'll print your replies, and relay them to my friend, Sam.
Thanks again. Bert

 

I had a similar problem w/ my 85 300sd. As I recall there was a small wire going to the pos. battery terminal that was off. It had fallen behind the battery. It powered the tach, and cruse or AC circuit. I reconnected it and everything worked.

 

1985 190e 16v tachometer failure was caused by a shorted capacitor burning out a resistor on the tach board. I guessed at the value and replaced the cap and the resistor with a 10 ohm. works great. Pryorto the repair i checked for 12v supply to the tach and a tach pulses(signal)

 

had same problem with my 83 300sd. replaced round tach module. works fine.

 

I have had the same problem in the past and after trying the fix of putting something in the cap of the tach amp to push it down and having that work for about a week I unscrewed the amp from the base and then took apart the base and crimped all the female parts of the connectors just a little so they would make good contact. So far this fix has lasted for six months with no problem.