If your tach isn’t working, join the club!
There are perhaps 10 different reasons why it quit. I’ll review many of these possible faults in this post.
Tachometers are often non-functional when Bugeyes arrive in our building. The mechanical tachs on the early cars are particularly problematic, because they rely on a number of parts and any one of them can fail. (We sell an electronic tach that’s more reliable and might help you get good information in your cockpit). Our electric unit is also very useful if you have an alternator with no mechanical tach drive. But if you insist on using that original mechanical assembly, this post will help you make it all work.
Above you can see one of our generator/alternators with the tach driven off the back of the unit, just like the original Bugeye and Mark II Sprites. As the generator or alternator spins, it spins the inner cable which transmits the rotational energy to the gauge. If anything gets in the way of the smooth transmission of energy, the tach will stick, jump, or otherwise fail to work.
Above is the tach gearbox (or gear reduction unit) and it has to be well lubricated and smooth. This screws onto the back of the generator. Once assembled, check to ensure that you can spin the generator pulley by hand, and that the inner cable for the tachometer spins smoothly and freely.
Many of the old original gauges are gummy and need to be rebuilt so it is important to also remove your tach and spin it on the bench to make sure it isn’t bound-up, because it can snap your inner cable if the tach is binding. (If your gauge needs a rebuild, send it in to us for that service-call if you want more info).
Anyway, assuming that your gauge is free and spins easily and that the cable is working properly, the next challenge is cable routing. You’ll notice in the photo above that the Tach cable is meant to go over the starter solenoid, in front of the battery, behind the heater box and then into the hole in the firewall directly forward of the tachometer (which should be on the left hole in your dashboard when looking at the dashboard from inside the car). If it isn’t routed properly (as shown in the red car photo below, where the cable goes through the firewall on the passenger side) you’re asking the cable to do the impossible… which is to rotate freely with a kink in it… so routing of the cable is also critical.
But wait, there’s more! Once all this is resolved you can then see the video above in which the tach on Thor is operating with an annoying ticky squeal. The inner cable was slightly too long and putting some preload on the cable inside the tach drive… so all we did was shave a millimeter off the end and once that was done the cable could spin freely without the tick! Like magic! (see below)
All these components have to be correct otherwise your tach won’t work.
And that is probably why later cars came with electronic not mechanical tachometers!