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Tachometer Question

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MGJeff

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Hi. Just joining up here. I've been around MG's for awhile, currently restoring a 1950 TD, 1959 MGA in storage, and 1965 MGB restored about 20 yrs ago but pretty much a good weather car and looks/runs great! Anyway, let me start with a question that is driving me batty! I think I've read all the tach articles and diagrams and I'm pretty sure my problem is that I need to replace the capacitor labeled C2 in most of the diagrams. It is a .25uF and I've seen that it can be replaced with a .22uF or .27uF on some of the diagrams. My question is this: there are so many different kinds of capacitors out there (electrolytic, ceramic, polyester, polarized, non-polarized, big, little, wide ranges of voltage ratings, etc. that I don't know what to buy! Will any .22uF or .25uF or .27uF capacitor do? I found a .22uF polyester film 50WVDC capacitor at Radio Shack and installed it. Tach is working perfectly but only registering about half the RPM's that it should. Does this represent a problem with the capacitor or do I now just need to fiddle with the variable resistor to calibrate. I even asked Nisonger and Gaugeguys who both would only respond "send it in and we'll fix it for $160 - $225 (OK, I understand they are in business to make money). Anyway, enough whining! lol If anyone is familiar with this and can advise me exactly what capacitor I need (or if the one I already installed is fine) and if I should expect it to need calibrating after replacing .25uF with .22uF I will really appreciate it.
I look forward to the forum, and hope I can be of service to others as well
--Jeff
 
Jeff - The .22uF capacitor should work fine, but you will need to recalibrate the tach. The easiest way to do this is to hook up the tach out of hte case in the car and then hook up a diagnostic tach to the coil and place it where you can see it while holding the car's tach. Run the engne up to 3000 RPM on the diagnostic tach and then adjust the pot in the car's tach to match the reading. The car's tach is not real linier, so set it as close as possible at 3000RPM and live with the small inacuracy above and below that point.
Cheers,
 
Hi Jeff. I'm no electronics guy. I don't know if you've seen this, but it sounds like you have:-

https://www.classictiger.com/techtips/motach.html

I bought an 'A with a 'B tach installed. It was about as inaccurate as yours seems to be. I've fiddled a bit with the pot, but it still isn't right. Once the snows melt, I'll fiddle with it some more, probably adjusting it to agree with an old dwell tach I've got. I'm not too concerned if it's super accurate. I never go anywhere near the redline, so as long as it's close in the 3000-3500rpm range where I cruise, I'll be happy.

Other than that, I'm afraid all I can do is to wish you Good Luck!

It might help if you let us know which car this is for (I assume the 'B) and the P/N marked on the face.
 
I would think the .22uF would be fine, too. I recently calibrated mine, out of the car ... you can see some details here (about 2/3 of the way down, lots of photos to scroll through I'm afraid):

https://pcbunn.cacr.caltech.edu/jjb/mgbgt/

If you have access to an audio signal generator, that has a square wave output, you can set it to 100Hz and feed it to the tach and adjust the tach's trimpot for a reading of 3000rpm (assuming a 4cyl engine).

I had some fun doing mine, but the I'm a bit of a nerd when it comes to electronic gizmos :smile:
 
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