I have this phenomenon on two 420Gs.
After at least an hour, more like two, the rev counter will add 500-800 rpm on the instrument.
Anybody else experienced this?
yes…it is known to be caused by the aging of several components in the circuit.
I think the circuit board needs to be replaced with a new circuit board, and I believe they are available, and that Mike Eck probably would know
They can be adjusted but it wont cure the temp problem, as that is caused by component failure.
If you Google “Repair Smiths Tacho” there is an article that describes the situation
The impulse tachometer that Jaguar used had a very simple circuit that
worked surprisingly well, as long as everything was perfect. It doesn’t
respond well to changes in the ignition system or any drift in its component
values. Technology has come a long way since it was designed, and that’s
why Tony’s suggestion to change the circuit board is what all instrument
rebuilders do when they need to repair a tach. Many of those instrument
rebuilders buy those replacement boards from me. The original circuit board
in the impulse tach appears to have been put in place first and then the
tach built around it, so it is very difficult to remove, in addition to
being wrapped around the fragile meter movement. I suggest you send your
tach to someone who is familiar with that model. Contact me off-list for
more information.
Mike Eck
New Jersey, USA
www.jaguarclock.com
'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2
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peder:
I have this phenomenon on two 420Gs.
After at least an hour, more like two, the rev counter will add 500-800 rpm
on the instrument.
Anybody else experienced this?
yes.it is known to be caused by the aging of several components in the
circuit.
I think the circuit board needs to be replaced with a new circuit board, and
I believe they are available, and that Mike Eck probably would know
They can be adjusted but it wont cure the temp problem, as that is caused by
component failure.
If you Google “Repair Smiths Tacho” there is an article that describes the
situation
Thanks for the explanation. Now that the rev counter is out (for Mike Eck to repair), the engine won´t start. It has always been a one, sometimes, two push starting car. I suspect that with the wiring to the distributor detached from the rev counter, something is preventing the engine from starting. Full battery, starter working as normal, plenty of fuel, but no spark…!
Hey Peder. I’m not familiar with the 420G tach but suspect you need to jumper the terminals in the now empty female plug left after detatching the tach - to complete the ignition circuit.
If the tach circuit is the same as those in early XJ6 cars, then Nick is correct. Absent the tach or a jumper the car will fire whilst cranking but die on the “run” circuit. Paul
the rev counter is in SERIES with coil/dizzy…car wont start or even fire (on a 12V system) unless you bridge the tacho spades
mentioned in FSM, and shown in circuit diagram
Thought so, and thanks for confirming. So what do I do then?
2 green and one white cable, i think, ending in a black female socket. A cable with 2 spades between ???
The two greens. A paper clip might work.
Thanks Robert.
2 twin spades into one another gave the right distance to reach the 2 green female contacts, and the car started!
Down in the swamp country we would have used a live .22LR bullet. Lots of stories about how this works out once they get a bit warm.