Measuring Tach Signal M116
 

Working on a 1982 380SL with a stubborn high idle (1200 in Park). The air valve and idle control module are known good. The problem is that the control module is only sending 1 volt to the valve.

Yes, the throttle position switch is working.

So I am going to check all of the inputs to the control module at the control module's plug. Simple enough except for one: how do I measure the "tach" (rpm) signal that comes from the ignition module?

TIA

Just to close this out
 

I measured this with a dwell meter off pin 1 of the diagnostic socket. It was reading around 25 degrees at idle. The dwell increased as rpm went up. Occasionally it would drop below 20 and the idle relay seemed to start working and brought the idle down. Since the ignition module controls dwell, I replaced it, and the idle speed is where it should be.

My theory is that if the idle relay sees a tach signal above a certain rpm, it quits trying to control the idle.

I did some more research on this. Per Steve Brotherton, the TD/tach signal comes from the distributor as a crimped sine wave AC signal. The signal is used and converted to a DC square wave engine speed signal by the ignition control module and is used by various other units including the tach, fuel pump relay, and the idle speed relay.

The 107 CD gives the spec for TD as 5-23 degrees of dwell at starting speed, measured at pin 1 on the diagnostic socket. I thought it odd that the TD spec would be given this way, but because this is a DC on/off signal, it can be measured in dwell or duty cycle and probably has to do with the Mercedes hand-held tester used with these cars.

Anyway, the car had an aftermarket Hueco ignition module installed with no thermal paste. I replaced it with a Programa rebuilt Mercedes unit ($90 with shipping), and got a reading of 12 degrees cranking. The tach stopped jumping around and the idle relay got its act together and started sending 3.5 volts to the valve, and the idle went down to 650.

The other lesson learned is that you can pull the idle relay out of its socket and test all the inputs while the engine is running. Depending on the year, they are power and ground, coolant temperature switch, AC compressor engaged, throttle position switch, and of course, the TD/tach signal.

My M116 idled at 1200 in park too until I replaced every single air boot and seal on the engine. Now its around 900 in park, 550-600 in drive.