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marknanh
06-01-2006, 01:09 AM
I am getting low RPM readings on my VDO tach for my 1988 Tristar, so I decided to remove the gage and do a benchtop test. I connected a 12V power supply to the + and gnd contacts on the back of the gage and used a function generator to create 12V square wave pulses to emulate the signal from the coil, sending the signal to the proper contact on the back of the gage.

My basic understanding is that for every revolution of the engine crankshaft, there are four ignition events (pulses) that are sensed by the tachometer. So, for example, if I set the frequency generator to provide pulses at 133 Hz, the tachometer will determine the crank shaft rotation to be 133/4 = 33 Hz or 33 Hz * 60 sec/min = 2000 RPM.

Interestingly, in this scenario, the tach actually reads 1000 RPM not 2000 RPM. Any ideas why the factor of 2 difference? For clarity, the potentiometer on the back of the gage is set correctly to 8 cylinders.

Thanks for your input.

jimmer2880
06-01-2006, 06:58 AM
huh?:confused:
..........

jimmer2880
06-01-2006, 07:00 AM
I am getting low RPM readings on my VDO tach for my 1988 Tristar, so I decided to remove the gage and do a benchtop test. I connected a 12V power supply to the + and gnd contacts on the back of the gage and used a function generator to create 12V square wave pulses to emulate the signal from the coil, sending the signal to the proper contact on the back of the gage.

My basic understanding is that for every revolution of the engine crankshaft, there are four ignition events (pulses) that are sensed by the tachometer. So, for example, if I set the frequency generator to provide pulses at 133 Hz, the tachometer will determine the crank shaft rotation to be 133/4 = 33 Hz or 33 Hz * 60 sec/min = 2000 RPM.

Interestingly, in this scenario, the tach actually reads 1000 RPM not 2000 RPM. Any ideas why the factor of 2 difference? For clarity, the potentiometer on the back of the gage is set correctly to 8 cylinders.

Thanks for your input.

I can help in a little bit of this though... for every 360 degrees of crank shaft rotation, there are actually only 2 strokes (1 down, and one up). So - your engine would only fire every other 360 degree rotation of the crank.

Man I hope I'm not wrong on that :o

JLeuck64
06-02-2006, 12:00 AM
I follow your math and believe you are correct.

In the past I have used another known good tachometer to verify proper operation of one that is suspect. Maybe you could borrow a Tach/Dwell meter from a friend and then hook it up to the boat's wiring to check.

Regards,

6ballsisall
06-02-2006, 12:04 AM
Is the generator you used to test an 8 cylinder generator? That could be your answer right there if not......

marknanh
06-02-2006, 12:43 AM
Thanks for your help. I discovered this morning that the little cal pot on the back of the tach was stuck. Simply sticking a flathead screwdriver into the pot made the tach jump by a factor of 2. Everything else checked out so the problem is solved