AIR CORE TACHOMETETER
Tachometers operate by counting pluses from a signal source and interpreting this count per second. Once the tachometer has this number, it will move the pointer to the appropriate position on the dial. The tachometer is a relatively simple gauge only requiring power, ground and a signal. The tachometer signal source can be one of several items.
-Negative side of the coil (high energy type) HEI coil or Pertronix
-CDI box (MSD output on the side of the box) This is a 12V square wave signal HEI with a hall effect tach output also use this type of signal.
-Tach output on a distributor (HEI or similar) This is usually a high energy or hall effect signal
-PCM pulse output (most commonly an open collector signal on GM PCMs) GM PCMs also output a 4 cylinder signal and a 10k-ohm pull up resistor will be required.
-Pulse sensor on crankshaft, camshaft or flywheel. Most commonly an AC sinewave signal
Know where you are picking up a signal before proceeding, this will aid greatly in troubleshooting.
Before proceeding check the following:
Turn on the key. Pointer may "jump" it may or may not be exactly on zero. If not:
Check for power on the IGN terminal
Ground on the GND signal
Signal on the SIG terminal
Turn on the lights, if the lights do not work, there is not a ground,.
Check that the signal selection switch is on the proper setting and the position is in the detent firmly selecting the cylinder.
Gas vehicle cylinder selection
#1 4 cylinder also used on GM LS engines
#2 6 cylinder
#3 8 cylinder
Issue:
Tach does not operate at all
Tach does not read properly
Tach reads slowly
Tachometer climbs with power on
Vehicle will not start with tachometer installed