So I recently bought a 2005 s2000 with just under 42,000 miles, shipped from IL.

I've already changed the oil and oil filter, drained/filled the coolant with the same Honda blue stuff, and installed a new OEM thermostat(was this needed, probably not, but I did anyway..)

Minus a couple cosmetics I'm having 2 hopefully little issues.

Doing little test drives, maybe a mile or so worth of driving around the area(car not registered yet so can't go too far) I've had my temperature gauge stay steady at 7 or 8 bars, then slowly go up to 1 bar from Red, then seconds later it drops at a somewhat quick consistent rate back to 8 bars and stays there. I let it idle for almost 10 minutes and everything seems normal. Temp gauge stays at 8 bars, idle is smooth, no signs of any overheating.

And while I was idling afterwards any time the radiator fan came on(maybe for 30 seconds each time) the dome lights would dim for a second

Both test drives, same distance, before and after doing coolant/thermostat, the temperature gauge did it's little jump up near H for a few seconds then drop back to normal temps with no sign of the engine overheating, idle is normal, etc. I didn't notice the dome lights dimming until tonight when it was dark(wanted to see how driving was with crystal clear lights )

Anyone have any ideas what might be the problem? Does anyone else's dome lights dim for a second when the fan comes on?

Faulty engine coolant sensor? Short somewhere, maybe leading from the coolant sensor to the gauge?

I should probably mention this too, the radio that came with the car has a tach display, it does work but it's not accurate with the actual tach in the cluster. Maybe somehow it's related to the radio and how it's connected, just throwing ideas out there.

thanks for any help.


some notes if anyone is wondering:
-no drop in coolant level
-oil level is fine
-battery is a fairly new durlast gold battery
-coolant system was bled properly and heater works
-top radiator hose gets hot, bottom hose stays somewhat cool/warm after driving around for a few minutes