Alternator driven tachometer, how does it work?

downeast450 · #1 (**permalink**) 08-26-2009

I mistakenly (ignorantly) purchased a new tachometer that is designed to run from the alternator of a diesel engine. I thought I had found a "regular" tach with an RPM range that would work for my Atomic 4. Live and learn.
I currently monitor the engine rpms with a portable tach and dwell meter I built from a Heath Kit 40 years ago. It has a selector switch for 4, 6 or 8 cyl engines that "interprets" the pulses from the coil appropriately and gives a very accurate rpm reading. I want to install a tach permanently and was hoping to find a large dial with a 4K range that would be easily read in the cockpit. Unfortunately, for me, I was ignorant of the diesel tachometer and did not bother to read the box! I would like to be able to accurately adjust the engine's rpms within 100rpm+/-.

Now that I have this new gauge I am wondering how the diesel alternator creates an accurate signal for the diesel engine. Does anyone who is willing to explain it, know?

Thanks,

George

flatracker · #2 (**permalink**) 08-26-2009

The tachs for diesels that run off the alternator, count the pulses from the alternator. You have to set them for the number of poles the alternator has. Also, if you change pulley size, then they are off. I am not extremely familiar with this set up other than that. Can't you just return the one you bought, and then go to an auto supply and purchase a good tach for an auto. Four cylinder, four strokes are tachs that are easy to come by. They all read ignition pulses, and are easy to hook up. If you manage to hood up the tach you have, if the pulleys are not exactly the same ration as what it was intended for, it might read, but will not be anywhere close to accurate.

downeast450 · #3 (**permalink**) 08-26-2009

Thank you for the reply.

I figure it has to be a pulse generator that gets multiplied and set (there is an attenuator dial with 4 setting ranges indicated) by reading the pulley rpm and adjusting for the ma. It is generating its own pulse not reading the pulse generated at the coil. I assume it wouldn't be possible to calibrate it using my T&D because it isn't designed to "interpret" a pulse that originates outside it. Maybe if I could change that I could use it?

I have found a smaller 2 1/8", 4K that Moyer Marine sells and I will be satisfied with that. The diesel tach is 3 3/8 with a 3500 rpm range, Perfect ! Ooops!
George