MPG Gauge

Can anyone describe to me how this system works?.  I know that it works off inlet vacuum but how is the gauge only activated in 5th gear?

The reverse light switch also kicks in the gauge when 5th gear is selected. I'm sure that's why there are two 'plungers' on the gearbox side of the switch.

Thanx mate.  So there is a plunger for reverse and 5th gear then?  How does this switch allow the vacuum to act on the gauge when if 5th .gear only.  Does .it switch a valve or something?

Oh where is Wellsy when you need him  

Does anyone know how this system works then?

It works when the switch on the gearbox tells it its in 5th and there is vacuum from the inlet manifold, its simply a vacuum gauge, big vac accelerating hard and low MPG, low vac cruising and high MPG. There is no relay or any fancy electronics, unlike the change up indictor (LED under the gauge).

If its not working try connecting the wires at the gear box switch end (poke a piece of wire in the loom connector to fool it its been switched by the gearbox). If it still doesn?t work check the vacuum pipe.

Its about the only function that works reliably on my cluster!!!