[Mgs] MGB electric tach

[Paul Hunt]

That is a late internal inductive pickup so probably negative earth.  It 
has male and female bullets for two white wires, the wrong way round the 
tach won't work and the right way it should but original the gender of 
the wires was correct for the internals.  The spade is for 12v.  They 
were used on the MGB until the 1973 model year so could be either 
positive or negative earth, usually printed on the dial,  but positive 
could have been converted to negative by a PO.

 From 73 voltage triggered tachs were used with a single white/black 
wire from the coil -ve to a single bullet on the back of the case with 
the spade for the 12v supply, earth again either a spade on the case or 
a black wire under one of the fixings.  All negative earth of course.

The tach 12v feed comes from the voltage stabiliser as that is just a 
convenient point for picking up the 12v supply rather than going all the 
way back to the fusebox.  It is a green wire i.e. fused ignition that 
feeds the stabiliser as well as the tach, power is not passing through 
the stabiliser to the tach.

PaulH.

On 23/10/2024 20:25, dave northrup wrote:
>
> Working on an MGA with a swapped in early 5 main MGB engine.
>
> Trying to figure out how to set up an electric tach.  With the car 
> came an MGA tach with the internals swapped for an MGB tach but it 
> doesn’t work.  I purchased an MGB tach, and am trying to test it 
> before I attempt to swap the internals for the non working one.
>
> Here is a picture of the tach; there are 2 white leads, which 
> according to the wiring diagram, one goes to the coil and one to the 
> ignition.  Which is which?  Or should I just try them?  I know that it 
> goes power to tach to coil.  We had the car running with the old tach 
> but no movement of the needle before we found out that the gage was bad.
>
> Also, I am assuming the extra spade in the tach goes to power?  At 
> least the ground spade is obvious as its part of the case.
>
> And, while I’m at it, the MGB wiring diagram shows one lead to the 
> gage voltage stabilizer.  I’m thinking that can go directly to the 
> fuse box to power?
>
>
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