My tach stopped working, so after checking all the connections and finding them good, I took it apart.
The circuit in mine is quite different from that described in this useful link from Mark Olson: http://www.classictiger.com/techtips/motach.html
For example, mine only has one transistor, and the toroidal transformer has two windings apart from the turns of the wire from the coil.
I suspected the 2uF electrolytic capacitor (these are prone to failure when very old), circled in yellow in the following photo:
Sure enough, it was a direct short when measured on the multimeter. I snipped it out of circuit and soldered in a 3.3uF tantalum (the value is almost certainly not critical)
in its place.
Now my tach works fine again 🙂 I wish all the jobs on the Jag were this simple.
PS Gordon Tompkins, who has a Series 1 XJ6, has a similar tachometer, and contacted me about its repair. Here is a photo of Gordon’s unit: notice the different orientation of the 2.2uF capacitor:
Fettling a '69 Jaguar E-Type 2+2 
Is there the vague possibility that you have a wiring diagram for this .. I have just replaced the capacitor but havent yet tried it in the car
Hi Barry,
I don’t but check the link in the post – he may have one. I have since sold the car, so no way to make a schematic now 😦
If you have a signal generator, you can test the tach on the bench …
Julian
Hi Julian, although you don’t own the Jag any more, perhaps you can help me out with the connections? In your picture I can see 3 wires coming out, one green (+ feed I presume), and two whites wires. I have the same wiring, where one of the white wires is marked red, and the other one black. Is it reasonable to suppose that the white wire with red mark is connected to the positive connection on the ignition coil? Looking forward to your help,
Karel Kuijpers
Yes, that sounds reasonable – the white wires are just used as a primary winding on a transformer if I recall correctly, so it may not even matter (it’s the rectified DC on the secondary that moves the needle). Yes, the green is battery +.
Hi, I’m looking for some insight if possible. My tach (Series 2) was reading high and acting erratic so I thought I’d try renewing the 2.5 uF capacitor (circled in yellow above) but after renewing it with another 2.5 uF capacitor it stopped working completely. I’ll send it off to a pro if needed but I was hoping to fix it myself. Many thanks, Adam
Hi Adam,
Did you get the capacitor in the correct way round? If you used an electrolytic or tantalum, they have a polarity, so check that. Also make sure you don’t have a dry solder joint, that the leads are making electrical contact with the circuit board traces.
Julian
Hi Julian,
Just seeing your reply here. I did install it the same way the original was installed. I cleaned and tinned the connections well so it should be a good joint. Maybe the issue is something else. I installed the internals from a XJ6 tach and it works well (if 300 rpm low at idle, 500 rpm high at redline, and accurate around 3500 rpm).
Kind regards, Adam