1994 Volvo 940 overheating when I turn heater off
 

welcome everyones expert opinions. Im all about reliability is why I bought this Car as my next daily driver project.I purchased it on Saturday afternoon and took off on a 17 hour. Trip that evening
everything was perfect including the fuel mileage.but there had to be something and here ut3 is.
Was very cold when i left so heater was cranked. Temp sat at 1/2 way point. I checked coolant when I showed max. Anyhow when i got to warm i turned the heat off the temp needle started heading towards the RED. turned fan back on and cranked heat went back down to half. Completed my journey sometimes with Windows down and heat cranked. My thoughts. Thermostat or electric fan i have done no further investigating. Thank you all in advance. And im very impressed with this car

If you are driving down the road, then the air blast across the radiator will push sufficient air across the radiator to keep the car cool without the fan.

However, if you stop and the fan doesn't work, then it will overheat. Interesting that running the heater keeps it cool.

Did it get hot while driving or in traffic?

With the interior heater fan cranked up it stayed at half on the guage fan turned down or off it heated right up .going down the highway in traffic whatever.I cannot confirm.the electric rad fan was running or not running havent got that far yet

The heater core is typically not enough to cool a car that will overheat. So, I'm wondering if you have good coolant flow through the radiator. Going down the road there should be plenty of air coming across.

Check the radiator and air conditioning condenser for blockage (leaves, debris, etc.) that could be blocking airflow through the radiator.

I engine compartment was very clean recent steamed or shampooed prior to me purchasing the car. Everything looked clear and unobstructed. I noticed the rubber flaps on the one side of the rad. It says on the rad cap to use Volvo coolant im not sure if that whats in there i doubt it theres green coolant currently in the reservoirhttps://cimg3.ibsrv.net/gimg/volvofo...ce10b42858.jpgWhen i ended my 17 hour drive there was a coolant patch on the driveway the size of a basket ball i took the pic from underneith looking up

The vehicle appears to be very rusty underneath. That is a red flag.

Update**** can somebody give me some real advice
 

I have discovered the electric fan has not been cutting in. Its the sensor that is on the top left or passanger side of the rad. I unplugged the 2 leads and confirmed the fan works sensor is gone. O wired up a toggle switch for inside the car and ordered new sensor. Still nothing has changed going down thw highway 110km/h. The temp guage is sitting almost to the red but on red. Once again hi i turn heat on full blast it the guage goes back to sitting half way. Something in the climate control modul is not disappating the geat properly. Comments please

Like I said earlier, the fact that you drive down the road will force more air across the radiator than any fan can draw. You can replace the sensor if you want, but unless the car is above the set point, the fan won't run anyway. The AC system should call for the fan when the system pressure builds to need the fan.

Are you sure that the temperature sensor is valid?

Thanks again for your reply as im all ears at this point. I cannot confirm that the temperature guage is working proper or not . do you know of an easy spot i can install an analogue guage.
is there a possibility a alternator on the way out would cause this to happen. My fuel also has inaccurate readings. But your 100% correct. Theres probably nothing wrong with the fan sensor.
the other night when i pulled over on the highway to have a look and see if anything was melting under the hood. It was to my dis belief that the coolant reservoir was boiling hot wasnt even hot to the touch. Upper and lower rad hoses were firm with pressure

It sounds like you have a blown head gasket. That would explain the boiling in the reservoir yet the coolant is not that hot.