Quote Originally Posted by Yendor View Post
Does the voltmeter also drop when you use other electrical items such as indicators or when headlights are turned on?

If so I would check the main earth that runs from the negative side of the battery to the body, is it clean and tight?, also I would check the main earth for the dash, from memory it connects in the panel around the inside fusebox.

Does the rear wiper move slowly or is jerky in it's movement? I would try lubing up the wiper mechanism as per Paul post.
The voltmeter only dropped below 8 volts (the minimum reading) when the rear wiper actually wiped, so it was an intermittent drop of voltage. The wiper just did a steady slow wipe of the window. It dipped of course when the glow plug was engaged, but when the engine started the volts were pretty consistent around the 13-14 volt mark.

I did what Ben-e-boy suggested and disconnected the motor and the voltage drop still occurred, which indicated that it was a wiring issue - not a motor problem. That's when I decided to follow the wiring from switch to wiper and realised the voltmeter was using it as its power source.

I checked the earth connection from the battery and dashboard, and lubed the wiper motor.

I've disconnected the voltmeter from the wiper wire, so it is not connected to anything at the moment, while waiting for advice of maybe a better 'ignition source' to connect to.