Temp reading changes when I turn on the lights

This is usually related to the "quality" of the ground connection at the instrument panel (all the gauges) or the individual gauge itself. If it's only water temp gauge, the trouble could be downstream just at the ground connection for that gauge itself; if it affects all the gauges, the trouble could be somewhere in the upstream ground connection that get branched locally to all the gauges. (And none of these connections pass thru the fuse blocks so probably not related to the fuse blocks.)

I'm more calibrated in the units of "needle width":

1. If the needle motion is 1/2 a needle width or less, I'd call it "normal" and not worry too much about it;

2. If the needle motion is more than 1 needle width, I'd probably try to investigate/fix.

 

Put a meter on the battery voltage and note voltage with lights on/off. It is possible that the alternator isn't providing enough voltage with the lights on and that drop is causing a misreading on the temp gauge. Over 1500 rpm the battery should be no less than 13.6v even with lights on.

 

I have the same problem with my 328. I just thought it was normal behavior for Italian electronics?

 

all of my Italian cars have done this. my lotus does it too to a small extent also. i think its an aging car thing, and i have not bothered to investigate it, as i keep an eye on the gauges on long trips. ( i know what range they are usually in. )

unless its a cheap easy fix, i would not go messing around in the dash or with any of the wiring.

 

ps, bypassing the dimmer rheostat seemed to help in the ferraris, and made the gauge lights much brighter. i think i did a thread with pics on this a few years ago. the Sylvania bulbs really helped too.