Testing generator driven tacho

[abowie]

Can anyone tell me how to test the function and accuracy of the early tachometer? I gather that the generator produces about 1V AC per 100 rpm. Today I connected two different AC power packs to the generator, one producing 17.8v and the other 25.4v. The tach reads a lot higher than I expected; 2500 rpm at 17.8v and 4200 rpm at 25.4v.

Interpretations? Can the early tachometers be adjusted?

[christopher storey]

I presume you really meant that you connected power packs to the instrument ? Also, although I make clear that I am not an electronics expert , I think it may be relevant that the power packs presumably work ( like AC mains ) at a constant frequency , whereas the generator on the cam bank by its very nature will produce current at a frequency which varies directly with engine speed . Someone with rather more knowledge than me will doubtless be able to tell us the extent to which that affexts readings on the instrument

[Heuer]

The tachometer rarely fails or needs adjusting, it is the magnets in the generator that become weak which lowers the output voltage is where the trouble lies. Standard test is to put the generator in an electric drill with a know rpm and see what the tach reads. Repro generators are not good so a matter of finding a decent s/h one. Ray Livingston over on Jag-Lovers did a full description of how the S1 tach works:

"The S1 tach consists of a number of components: the tach-generator, the gauge itself, and inside the gauge a selenium bridge rectifier and a series inductor. The tach-generator outputs roughly 1V/100 RPM. Doing a little quick math, you'll see that's almost 60V at 6000 RPM. The gauge movement needs only a few volts to reach full-scale. The combination of the series inductor and the gauge resistance forms an AC voltage divider. Since the tach-generator is really a permanent magnet alternator, the frequency of it's output increases linearly with RPM. The impedance of the series inductor increases with increasing frequency, so the voltage drop across the inductor increases at higher RPM, limiting the voltage applied to the gauge movement to a safe level. The current load on the tach-gen is highest at max RPM. What tends to happen to these things is the magnets get weak from heat, vibration and age, which causes the voltage output to fall off and/or the ability to output enough current to provide the necessary max RPM output. The result is, quite consistently, when a S1 tach is off, it reads low. Sometimes *very* low. Mine was off by nearly 25%. Replacing the tach-gen may or may not really solve the problem, since the new ones seems to be essentially un-calibrated, so there is huge variation from unit to unit. My solution was to build a microprocessor board that uses the tach-gen only as a frequency source, not caring about the voltage or current levels, and drives the gauge as required to achieve about +-25 RPM accuracy at any RPM. Personally, I think your best bet is to consider having yours converted to a modern ignition-driven gauge, or slip in a S2 tach. I believe MoMa will do this, among others. Other than that, about all you can do is buy a new tach-gen, but be prepared to return/exchange it if you find it is off-calibration. Also, *never* trust the S1 mechanism at high RPM, unless you've recently verified its accuracy. I'm sure more than one E-type owner has blown an engine because of the lousy tachs."

Dave Rawle designed a board which does not rely on voltage but detects pulses from the generator. Unfortunately Dave is no longer making them but the circuit diagram and circuit board layout are all described here: viewtopic.php?t=292 if anyone wants to have a go.

[abowie]

generator on the cam bank by its very nature will produce current at a frequency which varies directly with engine speed .

After my initial post I found Ray's quote that David has reposted below.
I have the Rawle circuit diagram and photos. I have never etched a pcb board but the time may have come :-)
Both Mike Eck and Speedograph Richfield will convert the tach to run without the generator.