Tachometer woes

[lotusbc]

My Elan S4 came with a non working tach. I had it rebuilt this winter and, now installed, it is still not registering any rpm. I have looked through a number of threads and am getting a little confused. I have a 'modified wiring diagram' by Peter Wrisley and it indicates that -ve coil goes to distributor and +ve coil goes to tach which seems to contradict what I am reading on other posts and opposite of my connections. The tach is grounded and there is 12v to coil, there is a small movement when ign is switched on. Can the leads at the coil be safely reversed? Is it possible that a coil fault can affect the tach?, no problems with starting and running.

[vxah]

The standard tachometer is not actually physically connected to the coil. It has its own feed and ground which judging by the fact it flicks when the ignition is switched on, I would suspect is ok? The feed to the ignition coil goes inside the tach and a couple of turns around a ferrite core pickup then, to the coil + terminal.
The tach will sense the pulses on the coil feed wire and convert it into the rpm reading.
If you have electronic ignition fed by the same wire to the coil the tachometer often won’t work because of the strange loading and pulses on the wire.

[HCA]

It would be helpful to know:

You say you had the tacho rebuilt? Is it rebuilt as an RVC or is it still an RVI do you know?

Do you have electronic ignition of any type fitted?

[lotusbc]

It is still an RVI tach as original. Yes I understand how the circuit works. I am hesitant to just switch the coil pos and neg connections to 'see what happens'.

[HCA]

And do you have an electronic ignition? I labour the question because the RVI will not detect the correct signal when electronic modules are fitted to ignition.

I am not going to say it is ok to wire a coil backwards - in theory it should not be done, and could ruin something.

[lotusbc]

Ignition is points, a Mallory dual point distributor fitted by the PO. With further reading, I gather that a negative ground system should have the -ve to the points. It is looking like the PO had it backwards, another in a list of errors!

[Craven]

There is a difference in the internal connection made to the + and – terminals of a coil but no damage will happen if you reverse the connections. Neg terminal to distributor is correct for a Neg earth car.
I will be very surprised if this is your problem, check on Tach dial for Neg or Positive earth operation and suitable for your car. You know that a +12v supply is required separate to the pulse connection.

[lotusbc]

Yes the tach is neg earth, correct and believed original to the car. Coil wires have been switched to +ve to tach and -ve to points. No discernible change, reading under 500 rpm at all engine speeds. I took voltage readings at the coil. Tach side ,+ve, is fairly steady at over 13.5 and points side -ve jumpy all over from as low as 9 to as much as 18. The engine starts and runs fine. Do I have a coil problem? What is a recommended coil? Fitted is a Mallory dual points distributor, otherwise all as original.

[Craven]

Mallory dual point’s distributor is a popular mod on MGB’s. MGB uses the same Smiths Tacho have a look on their web site and see if the extra dwell time affects theirs.

[holywood3645]

I have one that was totally rebuilt in the UK by speed cables. It was never installed. LMK if your interested.
James

[HCA]

Ignition is points, a Mallory dual point distributor fitted by the PO. !

You can see the two sets of points inside the distributer, yes? Many Mallory dual pointers on MGBs were converted to the Pertronix pointless module that was developed for the Mallory.