Rev counter stopped working 1961 Mk9

From one day to another it quit.
Should I begin looking at the generator behind the intake camshaft or the instrument on the dash board?
Peder

If you have the camshaft generator type, I would pull the generator and see if the armature has pulled back thus disengaging it with the camshaft drive dog. That happened to my 3.8S decades ago but I was able to fix the generator with a small pin that prevented the armature from retracting. Been working OK for 40 years since. To test the dash instrument itself, I used an AC model railroad transformer with a rheostat and connected it to the wires leading to the generator. Was not able to get full swing of the instrument needle but got enough to prove the instrument was OK.

As John said, it is most likely the generator, but you can easily test it without removing it. Connect an AC voltmeter to the two terminals of the tach-gen and you should read about 10 Volts per 1000 engine RPMs. If there is voltage there then connect your 12V battery to the two wires that connect to the meter, which should indicate around 2500 RPM. These tests should narrow down what you should do next.

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Grest,
Will bring the Volt reading instrument from my summer house to Stockholm, in 2,5 weeks, and check and report back.
(I did try to tap it with a long metal bar, but no change…)

If the generator is faulty, is it repairable?
I have one on the shelves. Don’t know if it works, or for what model it is. They might be different for various cars that used them in the 1960s

Search the forums for “Tach-generator”, especially the E-Types list. Much has been written on this subject.

I did a bit of reading, and liked the idea of adding a wire from the coil to one of the wires that attach to the generator. Is it that simple? Then why did Jaguar bother with the generator…?Or did I miss something?
On my 420G cars the rev counter is fed by a wire from the coil, I think.

No, the tach on your car needs the generator to work. Tachs can be modified to trigger from the coil signal, and most other cars were that way originally, but you know that Jaguar never did anything simply. I recommend the ignition trigger modification, since that way you eliminate several failure modes, including weak magnets in the generator that will cause the tach to read low.

Yes the generator is repairable.
I had on 2 different cars that the magnet was broken of from the shaft.
You can remove the backplate, and get the shaft and magnet out and glued together again.
I used metal epoxy, and it still works after 20 years.

I found a new one in my spares dept at my summer house.
But how do I remove the current one. I can open and remove the top Allen screw, but the other two….there just isn’t space to inset the angled Allen key.
Engine out job😱

You could possibly cut down an Allen wrench to allow it to fit, but when it came time to re-install the screw how would you get it started? I am in a similar dilemma because I want to renew the copper washers on the cam oil feed pipe, but there is no room behind the engine to unscrew the bolts. Perhaps remove the engine mounts and lower the engine a couple of inches? Or shift it forward a bit?

I saw oil around the right banjo bolt as well, and thought of tightening, which can be done with an open spanner.
My Allen screws have vertical ”threads” on the outside, so a pair of pliers might work.
Did I read that wires from the distributor, via tge generators’ terminals might work???
I tested with the Volt meter, and got some readings, but it was difficult to hold the pins on tge termibals while increasing the idle revs from 600 to 1000. I think it read around 3 Volts. Far from the 10 at 1000

Any thoughts on the above?



Impossible to change the rev counter with engine in place!!! :scream:

A common misconception:
The generator provides 10V per 1000 CAMSHAFT RPMS, so per 2000 ENGINE RPMS.
So if you rev the engine to 1000 rpms, you should read 5V AC.
John G.

Hi John, I would be interested in learning where you found that information. All I know is what I read, and on page P.45 of the Jaguar MK2 Service Manual it states, "as a rough guide it can be assumed that there is one volt output per 100 engine RPM".

Hi Mike,

I don’t have my MK2 manual with me right now.

I do have the E-Type manual that states : 'as a rough guide it can be assumed that there is one volt output per 100 RPM”.

They are talking about the generator RPM, as does the entire paragraph.

I have tested a few Jag tachometers over the years, using a small home 24VAC transformer, and the results are always a reading of 4800 RPM or close to it.

Update: if I start the engine the rev counter remains dead, but when it has warmed up a bit, I press the gas pedal and the tacho gets going. It then continues to work correctly for the rest of the day, incl various stops/engine off.
I can live with this.

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Looks like it is sticking. Sometimes mine sticks, a tap on the rev counter and it gets going again.

I wonder if the problem is in the gauge rather than the generator.