My 1979 MB200 had a large clock but no tacho in the instrument panel.
On buying the car, I converted the contact point ignition with Bosch Universal electronic contactless ignition and it has been working well.
Lately I bought a combination tacho/clock that was fitted to later model MB123.
The wiring harness had no provision for tacho. So I had a wire pulled in all the way from the coil negative to the TD terminal of the tacho.
The problem is the lowest reading I get at idle is about 1500 rpm but I am very sure the engine is only doing about 700 to 750 rpm.
At running speeds, the tacho seems to show steady and correct reading.
What could be the problem?

 

the way you wire it is okay. the new tachometer must be fitted for an 8 cylinder engine, the reason why you get the double reading.

[sm=smiley23.gif] am just guessing, so i could be wrong too.

 

But I have checked that.
The tacho carries inscriptions on the metal casing to specify it is for 4
cylinder (written in German)

 

hmnnnn....

you need an oscilloscope to see the reading pattern on how many pulses that bosch contactless ignition unit gives. the problem must be from there?

put it on a hermann, bear, GT1, or other tester and see. then you will find out.

 

Thanks.
I'll try it out over this weekend.