Tach drive voltage output?

XK-Engine
1

I just bench tested my Mk2 tach drive and find that the a. c. voltage is 13.8 v at 1500 rpm. Does anyone know if this is within spec? Is a speed vs voltage curve available?

The drive I tested is a spare but I would like to use it on another Mk2 motor that I’m building.

Any help greatly appreciated.

Tom

2

Hi Tom,

At least it’s in the ballpark. It’s supposed to be 10V per 100 RPM, so 1500
RPM should give you 15V. After 50 years of sitting behind a hot engine the
magnets tend to lose some of their power, so the voltage gets lower, as
yours has. The problem is that the tachs respond to the voltage and are not
adjustable, so yours will always read low. I had a customer who discovered
that the hard way when he accidentally over-revved his engine and put a
connecting rod through the side of his engine block. While he was getting
his new engine rebuilt he sent the tach to me so I could modify it to read
the frequency of the signal, either from the original tach-gen or from the
ignition pulses. That way the tach was adjusted to read very accurately and
from then on he knew exactly what RPM his engine was turning at.

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

3

Hi Mike,
thanks for the info. I sort of guessed at the rpm because I used a
cordless drill that is rated at 1500 rpm; I didn’t actually measure the
speed. And I think you meant 1 v per 100 rpm, not 10. A typo I’m sure.

I think an easy way to calibrate a tach in the car is through the use of a
timing light that reads engine speed. Think I’ll check both my Mk 2 and
E-Type now that you have shared the horror story of the exploding engine.
Better safe than sorry!

Thanks again,
Tom

4

Of course you’re right, 1V per 100 RPMs, 10V per 1000 RPMs , 15V per 1500
RPMs.

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

. And I think you meant 1 v per 100 rpm, not 10. A typo I’m sure.

.

5

If I was going to do anything costing money, I would pay to have the tach converted to an accurate impulse type running from the coil. Then leave the AC generator and wires for appearance but loose and taped inside the dash. Since I had several S3 XJ tachs lying around I put the S3 guts inside the larger E-type instrument and got the result basically for free.

Pete

6

Thanks Pete, for the hints. There is a local shop who can do the conversion and they do good work. But your approach of using S3 XJ tach guts is intriguing. The local Pick Your Part yard always has a few XJs and a tach is only $13 so I’ll give that a shot.

Tom

PS I installed an app on my phone that turns it into a strobe tach thus allowing me to determine the speed of the drill that I used to spin up the tach generator. It is worse than I thought; 1640 rpm produces only 12.8 volts. Time to modernize!

7

My stuff goes low tech, not high.I have a
totally mechanical hand-held tach that has a rubber cone at the end of the shaft. When pressed against the rotating object the clockwork inside reads the rpm which is shown by a large needle sweeping across the scale. I bought it as a thing of beauty and ingenuity rather than to throw away my advanxe timing light. IK Brunel would be familiar with the mechanical tach I’n sure.

Pete

8

Mike, PLEASE contact me: I owe you money!!!

9

Hi Paul,

Thanks for your diligence. If you want to pay me something put a check for
$50 in the mail.

Thanks!

Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

10

I sent a personal email: let me know if it still works!

PW

Thanks for your diligence. If you want to pay me something put a check for

11

Hi Pete,
could you tell me more about how you went about converting the tach using
the S3 XJ tach guts? In particular how does the triggering work? What do
I need from the XJ besides the tach itself?

Thanks for any help you can provide.

Cheers,
Tom

12

The S3 tach is triggered by a wire from the coil. The wiring diagram is in Haynes or Jag manual.

To fit the mechanism I cut several square inches bearing the guts out of the back of the XJ tach and cut a smaller hole from rhe 5-inch E tach and used contact adhesive where the the two overlapped when the needles.were concentric. I fitted the 5" dial face direct once to the S3 tach chassis