Quote:
Originally Posted by Rick Parker
What you are experiencing is an uptick in line voltage from the voltage regulator when the lights are turned on.
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A good regulator is supposed to hold its output voltage constant as long as the input voltage remains above a certain design threshold limit (whatever that may happen to be for the particular part in question, but typically 1.2V for an electronic regulator, IIRC) for a rated current load . If what Rick suggests is the actual cause, and it may well be (but I would have guessed a voltage drop rather than an uptick), then a couple of things:
1) You should see the temperature fluctuate within a few seconds by turning the lights on and off as the regulator falls out of regulation then comes back online.
2) You should be concerned that the alternator output voltage could be sagging under the load... thus causing the regulator to fall out of regulation.
3) Some gauges read higher when their input voltage drops. This could have been caused by the alternator not keeping up with demand, and the battery, acting as the backup, began to lose its charge thus causing the system's overall voltage to sag and the gauge reading to climb over time.
Test it by cycling the lighting load on and off with a 50% duty cycle and a period of 30 seconds to see what results and how fast the temp reading changes. If it moves quickly, then get the alternator and regulator checked out. I'd be surprised if it was a regulation problem if the temp moves slowly...
Good luck!