I am having trouble with the tachometer related systems on my w126. When I put the key in the ignition and turn it without starting the engine the tachometer needle pins at full rpms.

I have done some tracing and some disconnecting to see if I can eliminate what I assume is a short or if it's not then maybe a broken tachometer. I have pulled the klima, the OVP relay, the fuel pump relay, the rear speed sensor plug, the "trash can" connections, the cruise control connections, temperature sensor connections and numerous other connections to see if I can isolate the short with the key on.

The only thing that makes the needle drop with the key on, is removing the #6 fuse, which is the fuse for the tachometer. From that I deduce that the short must be on that circuit.... ?? right?

Some of the things that I believe rely on the tach signal don't work either. I am tracing wires related to #6 fuse circuitry and disconnecting them to see if the tach needle drops, but I also assume it could just be a broken tachometer. Anyone know if tachs pin when they fail

I would really appreciate any insight from anyone here if it seems I am missing something either obvious or obscure....

My radio antenna has stopped working recently, but the radio does work... I don't think they are related, but maybe they are.

thanks in advance for any help...

I ordered a new/used tachometer and it's fixed. strangely enough now my AC compressor works too. The only thing I did prior to installing the new working tachometer was I disconnected the 2 multiple ground bolts behind the instrument panel and I wire brushed the ring connectors and the ground surface and reconnected them.

Pinned tach = tach went bad

Replacement = !!!!!

I hope someone else benefits from this post in the same way I have benefited from reading other peoples information here...