The Ford Barn

drolston · 11-12-2020, 07:09 PM

The little AutoMeter tach in my coupe started acting up a few months ago. Like it would read accurately above 1500 rpm, below that it was erratic and often sit at zero. Found nothing wrong with connections and decided to ignore it for the time being.

Then I had issues with missing at moderate to low rpm. The problem turned out to be two of the spiral wound spark plug wires had gone open circuit at their connectors to the crab distributor. When I fixed those connections, the missing cleared up, and surprisingly so did the tachometer problems. I have no theory as to why, other than the bad spark plug connections may have been causing very high voltage spikes out of the coil for those cylinders - perhaps enough to interfere with the tack pickup wire.

11-12-2020, 07:09 PM	#14
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,811	Re: Crazy Tachometer Behaviour The little AutoMeter tach in my coupe started acting up a few months ago. Like it would read accurately above 1500 rpm, below that it was erratic and often sit at zero. Found nothing wrong with connections and decided to ignore it for the time being. Then I had issues with missing at moderate to low rpm. The problem turned out to be two of the spiral wound spark plug wires had gone open circuit at their connectors to the crab distributor. When I fixed those connections, the missing cleared up, and surprisingly so did the tachometer problems. I have no theory as to why, other than the bad spark plug connections may have been causing very high voltage spikes out of the coil for those cylinders - perhaps enough to interfere with the tack pickup wire.