1980 242gt

I just did the dreaded heater core on my car. Once reassembled I start the car and the tach will goto 1700 rpm cold start raises engine rpm. the tach goes to 2100 rpm and stays there. no matter the engine speed tach stays the same.

I tried to reset the plug on the tach cluster. no change. any ideas where to start.

 

per the 1980 240/260 wiring diagrams, the tach has 3 connections, + goes to the + rail on the instrument panel, which is powered by fuse 13, - goes to ground, and '1' is a black wire to pin 1 of the ignition coil (coil pin 1 also has a white wire to the ignition power module)

 

thank you thats the start in need

 

in checking voltage the black single wire that i plugged into the #1 spade on the tach has a voltage of approx 5.5-7 volts.

the other two wires on the tach come from the cluster board. one is the positive lead from the cluster board to the + tach spade. the other is from the board negative to the tach negative.

is the #1 tach spade supposed to be positive or negative polarity

 

the #1 pin is wired to the low side of the coil. it 'floats' high via the coil when the ignition is on, and is grounded when the power module sees pulses from the distributor, then when that ground is released, the coil sparks, and hte tach counts a pulse.


battery + == ignition switch == coil pin 15 ---coil resistance--- coil pin 1 == power module === ground

the power module is like an electronic switch to ground.

when the engine is running, you should see a few volts of AC on that pin 1, at the RPM rate of the motor (actually, with 4 cylinders, this is at 2X the rpm rate... so 700 rpm == 1400 sparks per minute == 23 sparks/second). if your meter has a "Hz" or frequency mode, and you multiply the reading by 30, you should get the RPM.