Tachometer failure

[John Quilter]

On a recent short local journey my usually reliable tachometer failed to read on the return trip. There are two devices in this system, the tack head and the signal generator at the back of the intake cam. I had a spare generator so spun that up with a drill and it would light a small 12V bulb, proving it worked. Tried that with the one mounted on the engine and ran the engine, bulb did not light. Concluded my engine mounted generator was faulty or had become disconnected from the drive dog. (that had happened once before some 35 years ago!) Then using a spare NOS tachometer I tested that with a small AC model railroad transformer with varying AC voltage from 7-15. Proved that my spare tach would read, at least up to about 1400 RPM. Then the same test with the dash mounted tach so connected the transformer to the wires I disconnected from the engine mounted generator. My installed dash tach would not read anything. Then with my known good spare generator connected to the dash tach wires, spun up the generator with a drill, no reading on the dash tach. Final test was connecting my spare generator to my spare tach and spun the generator with my drill, and tach showed a reading. My conclusion is that BOTH the generator and dash instrument are faulty but I find that very strange that both components would fail at the same time. Currently awaiting Moss to sent me a replacement drive dog just in case I need it. May hire someone to do the generator and dash tach change, both look like a PITA jobs.

[cass3958]

Fiddly but not really a PITA.
For the Tacho getting up behind the dash to disconnect the wiring and connections is hard going as your wrists do not seem to be able to bend in the right orientation. I found it easier to take the dash top off. There are four 7/16 nuts on long studs. One in the glove box. two behind the drop down centre consul and then the forth which is the hard one to find, up the side of the steering column where on a LHD car is where your knee would be. For this one you need a long 7/16 socket on an extension bar. Once the top is off access can be gained from the top making the job so much easier.
For the generator I have not removed one in situ but from what I recall it is only three Allen headed bolts holding it in but they are between the firewall and the top of the cam cover. I think the cam cover needs to come off which in turns means the air filter needs to be removed. I find getting to the back of the engine to do any work is a PITA with the bonnet attached so I would take the bonnet off with the help of the Jaguar Churchill tool "wife" which then gives you all the access so you see what you are doing instead of lying across the wing with your chest on the cam cover, back of your head hitting the underside of the bonnet and the wing mirror precariously set between your legs.

Below is a picture of the Generator on the back of the engine showing the three Allen headed bolts and their position.

Tacho Generator.JPG (264.69 KiB) Viewed 1669 times

[John Quilter]

OK, Thanks for that tip! I did get all four dash top mounting nuts off and removed the dash top. Tach accessibility is better but still not great. I sure hope I can get all those nuts and washers back in place later. It still may be necessary to remove the top of the column nacelle to get the tach out. I await my generator drive dog part from Moss.

[John Quilter]

Tach fixed. The tip that cass3958 provided to remove the hood (bonnet) was a big help, also taking the battery and tray out also improves access although it is still awkward. But with the help of a fellow British car club member we were able to fit the spare generator and connect it up at which point the tach now reads correctly again. It is a bit if a challenge getting the drive slot and the dog on the generator lined up correctly. The drive dog was in good shape do I did not mess with that and now have a Moss supplied spare one. I did not need to remove the air filter housing. I did take the top of the dash off just in case, but that was not really necessary but allowed me to fix a wonky connection at the central map light. I note that the old tach generator has no lumpy resistance when turning the drive dog. Must have lost its magnetism after 35+ years Thanks for the remove and refit suggestions but we still did not match the published .75 hour factory labor time guide time! I believe these generators put out one volt per 100 engine RPM of AC power. Anyone confirm that?