OK, I switched out the tachometer and the cannister thingy on the wheel well with one that came from another 300SD that I know worked. On this 300SD the tach when first starting moves to about 900RPM, but when I give it some gas (diesel) it drops to zero and stays there.

I've checked voltage to the back of the instrument cluster and it gets 14.5 or so but never above 15.

With regular driving the tach just drops to zero and stays there.

What is left to try?

Anyone have any ideas?

Rev. Dr. G.

Without reading about what you've done before now I would just relate my experience...'83 SD, zero on tach ...no responses anywhere or anytime on the dash tach gauge, thought it was dead. Removed the CAP of the canister thing ( Tach AMP ) but left the rest of unit bolted to wheel well, prodded the unit out of the cap, noted the silicone which faces upwards when mounted, inserted a cigarette butt into the cap, replaced the unit in the cap so that the butt pushed down on the silicon layer, re-installed the cap and unit onto the plug part still bolted to wheel well, started car and bingo! Tach. The wires under the silicon get loose and something needs to push them a tad in the right direction....PS, I think I'd try this to the original and then the repl...unless you've already done all this in which case I'll just go back to reading old posts on torbochargers...

__________________
1983 300SD... 269,000 miles, nearly 2,500 on my B-100, Faded Grey, Ugly in an elegant sort of way...Duh-Benz


If any of this has been a blasphemy to you, then good, because it's been a blast for me to...A.Whitney Brown

Car-1983 300 S D--

symptoms--car/tach would operate properly with heater blower motor on. With heater or air con on/blower running, tach would not register at idle and would jump on the band wagon at about 2000 rpm when driving. Then at about 3500 rpm in hi gear, would take a dump and go zero again.

WTF cure-- all this stopped when the mono valve for the heater was replaced - torn seal problem. Tach now operates properly at all speeds, air con or heater on or off. So this is the ?WTF? cure I should suppose.

Regards

Run-em
1983 300 S D -aks- SPARKY THE DIESEL

I've read most (if not all) of the previous tachometer threads.

I've replaced the emitter and the tachometer with ones that I know worked in a another 300SD.

The 300SD has no monovalve, just an auxilliary pump which I've also replaced.

There seems to be no connection with the heater fan motor--or at least no correlation between the tachometer and the fan speed.

I DO have voltage to the tachometer, just not the correct amount I assume.

Only thing I can see left is the wiring (although I get voltage) or the pickup thingy on the flywheel. Has anyone had to replace that or had it not function? It should be just an on/off switch magnetically driven that send signals to the emitter.

Rev. Dr. G. Church of the Subgenius

You are certain the tach amp worked on the other car (like did you see it work)....or did someone just tell you that it had at some point?

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Both the amplifier and the instrument cluster worked fine. It was in a 1979 300SD that I put into a 1980 300SD in order to locate the problem with the 1980.

At this point I'm inclined to think it is the sensor on the flywheel but I don't have a clue nor a scope to test to see if it's putting our any signal. I know it's supposed to emit a "square wave" pattern on a scope.

I hate to do it without knowing for sure but I guess I'll have to buy a flywheel sensor to see if that's the problem.

With the voltage fairly regular at the tachometer (14 v or so) it would seem to indicate there is little if any variation coming from the sensor--bearing in mind that the tachometer works at idle but then drops to zero with any acceleration.

Any ideas?

Rev. Dr. G.

have you tried wiggling wires with it running and someone watching the tach?

as old as these are it could be nothing more than a flaky connection or faulty wire.

I have a scope but that does you no good as you are nowhere near....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Thanks for the ideas Dr. BH. I'll try the wire jiggling thing. I noticed today when driving with the turn signal blinking it seemed to affect the tach causing it to vary a little. I seem to remember a post somewhere that indicated a faulty ground....?

In any event, I think I'm going to remove the flywheel sensor from the other 300SD and try it on the one that doesn't work to see if that is the issue. The weather will have to get better than today, however with wind at 30mph and temps in the low 30s.

Rev. Dr. G. Church of the Sub-Genius
"It doesn't tale a nuclear physicist"

It turned out to be the reference sensor on the flywheel was faulty. Replaced that and it works as it should.

__________________
Rev. Dr. G. Church of the SubGenius
It doesn't take a genius