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I took a voltage measurement on my starter solenoid this morning. I was curious how much drop I'm getting thorugh the ignition switch (new) and the stock wiring. Measuring at the IMI starter between the battery cable (disconnected at the starter) and the solenoid terminal, I have a 3 volt drop when cold. This decreases to about 2.8 volts with the starter in the on position after several seconds, due to the solenoid winding resistance increasing as it heats up. So, the solenoid voltage can get as low as 9 volts due to wiring losses. I'm putting on the relay as this will get me nearly full battery voltage and takes the stress off the switch.
RM is right, the stock wiring is undersized and voltages vary with load changes. I've got my engine stuff all running on a separate new wiring harness with relays controlling everything directly off the battery. My VDO gauges still use the stock wiring for the 12v though. I notice the voltmeter jumps around with brake light and backup lights. I haven't noticed any variation on the temp but it could be happening, I'll check that. When on the workbench I didn't see too much effect on the temp gauge by varying the +12v a few volts (using a lab supply). But it is worth another look.
On my alternator, I moved the sense line off the alternator's +12v output terminal by running a new wire over to the terminal strip that feeds the rest of the car. This regulates the voltage a little closer to the loads.
I'll post back later after checking the VDO temp gauge with different lights and accessories turned on.
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