Originally Posted by jay_bob

If I am not mistaken the W123 cluster relies on a metal strap to provide ground to the right pod from the left pod. The left pod gets its ground through the round connector.

I have to admit I have only seen a 240 D cluster that only has the big clock and no tach.

However I have worked extensively on a 124, and the 124 has a similar setup. The 2 pin plug is power and tach signal and the single lead is the unswitched power to the clock. Both instruments rely on that strap to get ground from the left pod.

I had an 83 240D that had the EGR controller and it had a pickup on. the damper. There is one pin on the damper that triggers the sensor. So the output would be one pulse per revolution. Is this how the 617 is set up?

The later 603/606 engines have a sensor on the ring gear at the bell housing. The gear has 144 teeth so you get 144 pulses per revolution.

And of course the gas engines of this era get their signal from the ignition coil. So that is (number of cylinders/2) pulses per revolution.