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Tachometer Repairs

When I acquired the MK 2 the tachometer [as well as the oil pressure gauge] was not working. It became fairly obvious that one of the problems was that the tachometer [tacho] generator had a broken electrical connecting lug. This tacho generator uses the same principle as a pushbike generator to create electrical voltage by rotating a permanent magnet inside a coil. This electrical voltage is then fed to the tachometer instrument, which in reality is a voltmeter. The principle is that the faster the tacho generator goes the more electrical voltage is produced which produces a correspondingly higher reading that we see as revolutions per minute [RPM]

This same tacho generator is also used in the MK9, S type, earlier E types and Daimler 250. Enquiries for a replacement tacho generator part no C14996 resulted in prices in excess of $300. Hmm –it was time to put the thinking cap on and see if repairs could be affected.

An inspection of the terminal cap of the tacho generator, which uses push on spade connectors revealed that the lug had been broken off flush with the surface of the plastic cap. By using my trusty dremel tool I was able to excavate around this broken lug to a depth of about 4 to 5 MM or 3/16 inch. I was then able to cut the male section of a spade terminal down to a suitable size and solder it to the old lug. The whole thing was then finished off by using a dam of masking tape to allow a suitable build up of epoxy resin over the excavated area and the base of the new lug. This all resulted in a now sound lug projecting from the terminal cap.

I had checked the wiring from the generator to the tacho instrument and all measured OK with about 2.5K or 2500 ohms resistance, which indicated a correct circuit impedance existed through the instrument. I replaced the tacho generator [with a new O ring fitted] hooked up the tacho instrument, started the car and achieved nothing. The b.......y instrument was U.S. ! [unserviceable]. I was also able to check that the generator was working as Ben Stafford had lent me an old battered tacho with a partly broken needle etc. but still serviceable as an indicator and I hooked it up and it worked.

Muttering and cursing I removed the tacho instrument [and a couple of squre inches of skin as well]. I cleaned off the workbench which is a miracle happening in it’s own right and laid out some clean cloth to allow me to dismantle the tacho. It does come apart very simply as the faceplate only needs to be twisted to align the spaces on the faceplate with the lugs on the case. The faceplate can then be lifted off. The tacho instrument itself is only secured in the case with two screws and the clock at the bottom can be left attached to the case.

Applying gentle pressure to the needle revealed that it would move but was rather bound up on the “jewels” or pivots. In my time as a radio technician and later on in aircraft engineering I had been exposed to and qualified to do instrument repairs. One facet of instruments that few people ever consider is that they do need lubrication. More particularly so as they get older and the original lubricants dry out. With an artists brush I applied some penetrating oil to each of the needle pivots and allowed it to take effect. After about an hour the needle was quite free and I applied some sewing machine oil to each pivot. The needle was now quite free and I reassembled the instrument. I made a temporary connection between the tacho generator and instrument, started the engine and wonders to behold it all worked.

Tearing off the usual square inch of skin in the process I refitted the tacho and reconnected the wiring. A quick check ensured that all was functioning and the beer fridge in the shed was raided to celebrate.