I bought an aftermarket tacho because I don't have a stock one in my truck.
The car is a 1992 C1500 with L05 350 engine and 700R4.

Simple wiring to tacho:
Red to 12V ignition.
Black to ground.
Green to negative site of coil. (That would be White in my case)

Hooked it all up, and started the engine. All looks good. Also when reving...

But as soon as I put the vehicle in gear and drive, the tacho doesn't show more than about 3000-3200rpm... Just bounces between there...
There is no problems below 3000rpm... Maybe a little wobling tach-needle but not much...

The problem only shows when car is moving. Not if it stand stills in neutral and I rev the engine...

I've tried another tacho.
I've tried the "10K-resist-in-tacho-signal-wire" trick
I've tried hooking the ground directly to the battery (-)

Nothing solves the problem...

Anyone knows whats wrong?

 

Wow, that sounds weird.

 

Very weird...
Is it possible that the electronic speedsensor or wires are faulty, causing the rpm signal to fail at a certain speed or rpm ?
Wonder if that could be the problem since it only appears when the truck is moving...

 

Problem solved, if anyone ever need an answer on this issue....

The tach gets 5V pulse from the coil... For some reason, the signal from the coil gets below 5v when the truck is moving... Even lower as the truck gets at higher speed...
Thats why the tach didn't show more than 3000-3200 rpm when speeding...

I solved the problem by simply put a 330 ohm resistor between the signal wire, and a switched 12V wire... Now it's working flawless.

EDIT: I was a little worried about the pull-up going the wrong direction and into the ignition coil... So I soldered in a diode on the signal wire... After that I didn't even need a pull-up resistor or anything else.. Simply just the diode in the signal wire.. That's it :-)