240 Tachometer Not Functioning Properly

Hello,

I have a '92 240 (M47 II) which came with the big clock. I have purchased a gauge cluster from an '83 240 (M46) which has a big tachometer (and an 85 mph speedo, if it matters). The tachometer is the older style with only one horizontal male connection at its bottom (and of course, the three plugs for the small clock at the top).

Upon replacing the clock with the tachometer (and plugging the red/white wire into the tachometer), and reinstalling the cluster, the tachometer did not behave as expected.

When the car was started, the tachometer would go to 1000 rpm, and stay there. If the car wasn't turned off for a while, the reading gradually decreased (after about 10 minutes of driving, it was at 500). I tried unplugging the red/white wire from the tach (keeping it in the cluster) and it acted the same way. Of course, I reattached the wire to the tach afterwards.

I came back to the car a few hours later, and the tachometer only went to 750 upon starting the car. Two starts afterwards, the needle simply stopped moving when the car was started.

So, it appears the tachometer has never actually displayed the engine RPMs after being installed in this cluster.

Any ideas of what could be causing this?

I would start with removing the cluster and carefully checking the pins where the tach makes it's power and ground connections. The pins in the tach can spread and lose contact. The pins in the cluster can get dirty and fail to make a good connection.

Also I understand the tachs can fail from bad capacitors internally.