Tachometer Calibration

[jjbunn]

I'm calibrating my tachometer, and also seeing if I can cure it of some "stickiness" above about 5000rpm.

Here's my setup:

I've hooked up the 12V bench supply, and a Tek sweep signal generator with +8V square waves (50% duty cycle). I can set the generator frequency to any frequency quite accurately, and I can also sweep the frequency between the tach's limits, to give it a bit of a work out

Is my math correct here:
For each revolution of a four cylinder engine there are two ignition sparks. So if the engine is rotating at 3000 rpm then that's a spark rate of 2x3000 = 6000 sparks per minute, which is 100 sparks a second.

So, if I set my signal generator to 100Hz, the tach should read 3000rpm. It does, more or less.

There is a 47kOhm pot for calibration ... varying this pot gives quite a wide adjustment, so I want to be sure.

I was surprised to see an IC in the tach:

(I expected discrete transistors).

What could the "stickiness" above 5000rpm be caused by? (And do I really care, since I'm unlikely to want to redline the engine.) Perhaps some oil is needed on the bearings. Anyone have experience with this?

 

[David_DuBois]

Julian - I just used a function generator when I did the same sort of set up, but I was using the same math. The later tachs apparently are much more "state of the art" than the older ones, our 66 MGB's tach not only has discrete transistors, they are the old point junction type of transistor, configured in a singls shot circuit that is not very linier at all. I wound up calibrating the tach to read right on at 3000 rpm and called it good (3000 rpm with O/D equals approx. 66 mph). As for the sticking, it could be anything, but I would try a very small amount of clock oil on the bearings to start with. If it still sticks, then watching it under good light and possible some magnification may reveal what is happening.
Good luck,

 

[dklawson]

I don't like using oil on these movements. I flush them with contact cleaner and CAREFULLY blow them dry to remove any dust. Over years, even thin oil can thicken and make the movement sticky.

Not having a signal generator and/or scope I use a different mechanical approach. I use one of my table-top lathes that has a DC drive on it to turn a target wheel at a controlled speed. I use an NPN (sinking) photo sensor to look at the target wheel and connect its output up to the tach input. By varying the lathe speed I control the pulse rate into the tach. Of course... this requires a digital speed readout on the lathe (and a calculator) for any of this to be useful.

Your setup has many advantages including that you should be able to increase and decrease the pulse width to more or less mimic dwell. I would certainly prefer your equipment if I had access to it. You've got a great setup!

 

[tony barnhill]

Julian - contact John Hubbard in Huntsville, AL...he has a laptop computer with a little box attached to it that he uses to calibrate tachs....from time to time, he drops by & calibrates all the tachs I have for sale.