mpg indicator under the tach not working

Where is the sensor for this vacuum gauge? My batteries in the SI board haven't been replaced for about 10 years, I looked at them about 2 years ago, the acid is coming out of them because they are so old and I think one of the connectors is broken. It was working fine until about two months ago, it started working periodically, whenever it wants to kick in. What's going on?

I don't know how to fi x your gauge but I do know that the mpg gauges are notoriously wrong.

The MPG indicator doesn't work on vacuum. The ECU works it out from the speedo pulses, engine RPM and fuelling.
The SI batteries can fail short-circuit and kill the guzzle gauge.
Removing them and cleaning the gunge off the board may allow the guzzle gauge to work but i'm not sure if the SI will work correctly without them.
Replacement NiCad Cells are available from numerous sources, including model shops! (Tag-ended cells for making up custom battery packs!!)