300SD tachometer puzzle
 

1980 300SD w116 model.

I've tried 3 different tachs and 2 different sending units on this one. All of those parts functioned in other cars. The tachometer registers at idle (not accurate, though), maybe 200 to 300 rpm. Higher than idle it drops to zero.

At idle, turning a turn signal on causes the tach to jump up and down.

I get volts at the back of the instrument cluster: 12 to 14 or so, but tach dosen't register above idle.

Everything else works on the cluster.

Anyone run across this on before?

Any ideas?

Have you looked at the tach sensor on the flywheel? I believe this holds true on the '80 -

Try grounding the instrument cluster to the body, it sounds like you may have a bad ground because the turn signals are affecting the tach.

Grounding cluster?
 

Pardon the question, but you mean just run a wire from the metal back of the cluster to bare metal on the body?

more or less. I think Craig is on the right track with the grounding suggestion. Next time my cluster is out that *#@ tach is getting a serious ground wire soldered to it's a*&!!:D

GDEMOSS01....Just curious, inquiring minds want to know and all that jazz, does your cruise control unit work?

Yes, my 240D instruments were acting strange, so I just ran a wire from one of the mounting screws on the instrument cluster (connected to the metal frame of the tach/clock or speedo) to one of the ground points under the dash (a bolt with a bunch of brown wires connected to it). In my case the problem went away, I must have had a broken ground wire someplace.

cruise control, tach, etc.
 

When I got the car all this did not work: climate control (heater only worked on defrost), A/C, door locks, cruise control, tachometer, door handles, cruise control, sunroof.

Replaced door locks with keyless entry. Replaced climate control with manual system that controls heat, A/C, and fans. Will replace cruise next month or so with aftermarket, probably will do same with cruise control, door handles work now. Likely will get diesel tach if I can't get it working, Also replace speed cable, shocks.

25+ year old mechanical systems do not last long and are also outrageously priced. At least I can replace these things if they go wrong because I installed them to begin with. Ozark engineering.

ground on tach made no difference 1980 300SD
 

Adding an additional ground made no difference at all. It does get somewhere between 14 to 15 volts from the wires that plug in at the back depending upn the speed of the engine.

One wire into the back is green. Anyone know if that comes from the fuse box?

i don't know if this applies to your chassis, as i do have a 124. I traced the path of the tach wires to the flywheel sensor, between those two the wires pass through the ELR DIEZEL black box and the KLIMA. I isolated the problem with the ELR box, bypassed it and now the tach works, hasn't jumped or acted funny so far... i have a recent post about it