[Mgs] tachometer with electronic ignition

[PaulHunt73]

I suspect the problem is at the tach itself.  Wired as per that diagram 
where the electronic ignition picks up it's 12v supply from the coil +ve 
i.e. via the RVI tach is when you can get over-reading, it's always best 
to power them from the white at the fusebox where the RVI is concerned.  
Ironically it's also best to do that with RB cars even though they have 
the later RVC tach due to the lowered voltage at the coil +ve.

However first thing to confirm is that the ignition _is_ being powered 
via the RVI tach, and to do that remove either of the white wires from 
the back of the tach with the engine running and it should stop.  If it 
doesn't then the coil is being supplied from somewhere else and the tach 
is being bypassed.  I accept  it may not be easy to get at the tach but 
it will have to be done sometime.

If the engine does stop then reconnect it, and check you have 12v on the 
spade terminal on the back of the tach (green wire) and an earth going 
to the case, which is usually a black wire under one of the knurled 
fixing screws, with the ignition on.

If you have both of those then either the tach electronics or internal 
connections are faulty, or maybe the needle is stuck.  So before doing 
anything else, with the engine revving at about 2k or so give the glass 
a sharp rap with a knuckle.

PaulH.


On 24/06/2023 08:44, R. Martin Rogovein wrote:
>
> Hey Paul
>
> Haven't had the tach out to try and bench test it, that'll take a 
> little doing.
>
> but meanwhile everything is wired according to the attached diagram 
> (left side diagram).
>
> Engine runs smooth, but the tach currently shows no response at all 
> (needle doesn't even bounce on start).
>
>
> If i understand correctly, taking the red from the + of the coil to 
> the CSI (as shown) could cause interference with the old style tach, 
> and it might work (but read high) if i were to take that red to the 
> CSI from some other switched 12v and not through the tach?
>
>
>
>