Well this is a new one...

I was driving this evening (about 40 degrees out) and my temp gauge passed the half mark which I have never seen it do outside of 100+ degree heat with the AC on.  I noticed that when I turned off my cabin heater fan, the temp gauge would drop down to where it normally sits almost immediately (a little below the half-way mark) but rose back up past the half mark when I turned the heater on again.

I had a long drive so I had a chance to try this over and over again and that is oddly exactly what's going on here.  I played with the temperature slider to see if max hot would change the degree of movement on the coolant temp gauge but it literally is an on/off issue.  How is that possible?

must be an electrical problem. 

Maybe your alternator is over or under charging? 

2 minutes ago, jonbill said:

must be an electrical problem. 

Maybe your alternator is over or under charging? 

I actually was thinking about that, because that happened a year and a half ago and I ended up changing the alternator which solved it.  The difference is this time my volt meter is reading  a normal charge rate compared to before which I think was around 15v.

I'll keep that in mind though next time it happens.

I had an overcharging alternator in my 240 a few years back. The glass fuses under the radio would get too hot to touch. Maybe you could figure out which fuse associates with the heater fan and give a touch next time? Could be as simple as a new fuse.

Ever figure this one out? I used to have the same issue when I had load on the electronics from the fans I was running, i switched to a OEM clutch fan because it scared me. 🙈

3 hours ago, heyitsrama said:

Ever figure this one out? I used to have the same issue when I had load on the electronics from the fans I was running, i switched to a OEM clutch fan because it scared me. 🙈

Yea - my alternator was overcharging the battery causing all sorts of weird electrical gremlins to pop up.  After changing it out for a “new” (remanufactured ZX unit which is what was already in there), it was good. 

3 hours ago, heyitsrama said:

I had load on the electronics from the fans I was running, i switched to a OEM clutch fan

Yeah, most people don't realise that a elec fan to replace a standard mechanical clutch fan needs a lot of juice!