My tach is not working. I pulled the tach drive off, which is no easy task, and it appears that all of the parts inside their head are present. So I assume it’s either the tac drive unit itself or the tachometer itself. I checked the continuity between the wires in the engine compartment to the back of the tack and that seems to be continuity there so I don’t believe it to be the wires. I was wondering if there’s any way to Check the tac drive itself to see if it is working. If I can rule that out then I assume that the tachometer is bad
You shoulda checked it before removing it. The tach-generator is a tiny alternator, that outputs an AC voltage, which should be 1V for every 100 RPM, so ~6V at idle, and 50V at 5000 RPM.
Put it back on the engine, start the engine, and see what you read with an AC voltmeter on the output terminals. If you have something close to 1V/100 RPM, then it’s ok. Most will read lower as they age (which is dangerous…).
You can briefly apply 12V across the two terminals on the back of the tach gauge, and it should jump up. If it works at all, it is almost certainly fine.
Regards,
Ray L.
Or if the unit is still off the engine drive it with a drill.
Thanks so much guys for your help. I put the voltage to the tack and it works so I put the tac drive back on the engine connected it up and now the tachometer works. The only thing I can think of is that the tachometer needed a little boost and/or I didn’t have the tac drive fully engaged into the back of the Cam. Either way it’s working now but who knows for how long. So now my next adventure it is getting the oil pressure gauge to work. Put 12 V to that and the gauge worked . I have a new sending unit so I don’t think it’s that. I have continuity from sending unit to the gauge… what else could it be?
Without question the most common failure with these is that the bushing the shaft rotates in gets nudged backward so that the shaft disengages either intermittently or fully rom the drive dog in the end of the cam. The bush should protrude a bit from the generator casting and the shaft needs to fully engage with the dog.
If it has been pushed backwards it needs to be driven forward and pinned in place so it doesn’t move again.
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Exactly what I had to do earlier this year, following Erica’s advice
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So, what I need to do is drive the bushing further out from the housing…correct?
How do I do this on a fully-assembled tach generator? Do I have to open the unit up and drive the bushing out from the inside?
More importantly, how do I secure the repositioned bushing in place?
Many thanks,
Alan
N.J.
Yup the bushing should not be flush. I’m confused though. Where is your shaft drive? Has it pushed all the way inside? That shouldn’t have been possible. It is supposed to have a circlip holding it out past the end of the bushing. Did yours fall off inside the engine? Not a good thing because it would fall straight into your oil pan. Hopefully you just removed it.
Assuming so then you’ll have to carefully tap back the three (or is it four?) stake marks on the back cover. Maybe run a sharp razor around the circumference in case there is built up oil varnish. Then press or gently tap back on the shaft. The cover will pop off, and the spinny thing will pop out.
Then press the bushing forward. Do a test with the armature and make sure it can still rotate with the clip on it. Obviously if it goes too far forward it can bottom out inside the housing and won’t turn. You might also want to confirm the correct protrusion distance using a probe into the drive dog in the cam. On mine it sticks out maybe 1/8"? Haven’t seen it for years.
I pinned mine with a tiny grub screw, drilled/tapped, and epoxied in place. You pin it along the side of the bushing. You obviously don’t drill into the center of it. People have also used segments of small drill bits to pin it. Drill small hole, break off a chunk of bit and epoxy in place. Whatever you choose, make sure it’s permanent. Your valves won’t like a tiny piece of metal vibrating loose and falling inside.
Heh…I have the identical granite counter 
Erica:
Thank you so much for the comprehensive guidance.
I will remove my tach generator in the days ahead and make the necessary repairs…(the tach case shown in the photo is just a spare part).
You have excellent taste in countertops and, obviously, automobiles!
Many thanks,
Alan
N.J.
I have now removed my tach generator. Here’s another question: How do I remove the circlip that’s holding the shaft in place?? There’s no room to pry it off the shaft.
Thanks again!
Alan
It’s been driven so far back that you have no choice but to chuck up that tang in a vice a bang the housing back until the clip is revealed. Since you’re already banging it, you may possibly be able to get it far enough forward that the back cap doesn’t need to be removed and you can just pin it. I have to wonder how many hundreds of thousands/millions of these things they pushed out the door, all with this one fatal flaw.
All it needed was a pin…
You’re going to have to push the circlip from its open ends. Cover the process with a rag or something to keep it from flying into a black hole never to be seen again.
pauls
As suggested by Erica, I put the tach drive in a vise and gently banged the housing back with a hammer and piece of wood. The circlip is now fully exposed.
But there’s really no need to remove the circlip…I just need to pin the bushing in place (after a few trial fits into the cam drive dog).
Many thanks for your suggestions!
Alan
N.J.
Now that I have the tach generator shaft positioned correctly I’ve noticed a benefit…my tach is now reading accurately (as compared to Jaguar’s published speed-in-gears table).
Previously, the tach always read low (when it operated at all). This makes sense to me now. If the magnetic rotor wasn’t properly centered between the electric coils it wouldn’t have generated enough electrical current to drive the tach accurately.
So, check your tach generators to make sure the circlip on the shaft is fully exposed…this ensures your magnetic rotor is properly centered.
Alan
N.J.
Very cool. Did you ever figure out the optimum protrusion of the bushing? It may not be universal but might be a good data point for the case where people are trying to reset it.
In my case the optimal protrusion seems to be when the bottom of the circlip is about 1mm above the housing flange…see my photo above for a visual reference.
I don’t think it matters much if the protrusion is a little more or less…as long as the circlip is just past the point of being fully exposed I think you’ll be good.
Alan
I have mentioned this before and I have just gotten back to this. My 1967 S1 tach reads low. It reads about 3000 rpms at actual 4000 (or there about- short memory.) As I have mentioned before, it seems to be my tach generator being low, but I wish to confirm as I see some irregularities. I measured the tach generator output:
1000 rpms- 5.2 VAC
2000 rpms- 9.8 VAC
3000 rpms- 14.1 VAC
4000 rpms- 18.5 VAC
These voltages read about 0.5 VAC per engine rpm, but about 1 VAC per cam/tach generator rpm. I want to be sure the 1VAC per 100 rpm is indeed engine rpms.
The irregularity is if indeed the tach generator is suppose to output 1 VAC engine rpm, then at 3000 rpm with the tach generator reading 14.1 VAC, my tach should be reading 1400 rpms but it actually reads about 2500 rpms- meaning I have a tach generator that reads half low and a tachometer that reads two times high. It seems to me, based only on what I have, that it would make more sense the tach generator is suppose to output 1 VAC cam rpm and mine is basically okay, but my tach reads about 25% low across the range.
Any thoughts? Thanks,
Tom
You are absolutely right.
Its 10V per 1000 camshaft RPM.
I used a 115V/24VAC transformer on the tach.
24VAC showed exactly 4800RPM on the tach.
Works fine with the car’s tach generator.
Regards.
John G.
I believe 12 volts on the tach should read 2500 rpm and inside the cam is a plastic dog that disintegrates with age ,it captures the tach drive have a look for it , it’s available and not expensive
Cheers
Thank you, so my tach generator may be weak, but not by much. It seems the main issue is the tachometer itself. I have not disassembled the tach yet. Anyone know how it can be calibrated?
Tom