Hi All,
I am wondering what type of signal a magna tacho needs to work
I have fitted a TS Magna dash to a HJ Holden with a S/C V6 in it
they have a tacho wire that will run a standard tacho
Does any one know how to test a magna tacho
if i can test it then i can figure out a way of making it work.
Cheers

poss relevant bits from manual
TACHOMETER
The tachometer is calibrated to display the engine
speed in revolutions per minute (r.p.m.) by converting
the ignition pulses from the coil into a proportional
voltage which will be shown by a calibrated movement
of the tachometer needle.
NOTE: The tachometer is a negative earth type
If incorrectly connected, electronic components
could be damaged.
Calibration Testing
Connect an accurate test tachometer (preferably '. - '
digital) to the engine and compare readings. If speed
variations are greater than specified remove the unit
and have it repaired by an instrument repair specialist.
NOTE: Do not test tachometer on a distributor
test rig as damage will occur to the meter
movement.
NOTE: The speedometer and tachometer have
highly sensitive surfaces and if touched, even
with clean hands, a permanent imprint will be
left.
CAUTION: THE INSTRUMENT CLUSTER
MUST NOT BE ALLOWED TO FACE DOWN
AT ANY TIME. THE TACH0 MECHANISM
USED HAS A SILICONE DAMPENING
DEVICE WHICH WILL LEAK.

edit add

Tachometer
Tolerance (r.p.m.) @ 13.5V (25OC). . . . . . . . . . . . . . . . . . . . .
1000 . 3000 . 5000 , 6000
+/-100 , 150 . 250 . 300

Hi skull,
The tacho almost always uses the pulse signal from the ignition circuit.
Generally, if this signal is taken from the coil, the signal consists of the basic 12 volt signal due to the battery voltage PLUS a pulse component which drives the total voltage from the coil to around 80 to 150 volt.
The tacho circuit will use a signal attenuator to filter and fix this signal to make it such that the electronics isnt damaged.
In the Magna tacho circuit I have seen from both the Sigma and the TP magna, an integrated circuit type LM2917 is used. Later designs may use different devices.
You say you have a holden HJ, and if this uses a conventional ignition arrangement, the coil voltage would rise to a peak of around 300 volt. So the tacho may need a mod to protect it from over voltage.
If you supply more info about the Holden ignition I can make additional comment. I'm not sure whether you want to test the magna tacho or whether to ensure the drive signal from the holden is within the requirements.
hope this helps.
ps. my post crossed with ts370000 post