My 240 temp gauge shows the engine overheating after driving just a short way. It sometimes acts erratically (showing very cold sometimes), but mostly it just gets hot very fast. I changed the thermostat, and the coolant is fairly new. I've heard it may be the gauge, or the sending unit, and not really overheating. On other cars, I'd check this by opening the radiator, letting it warm up, and seeing if water was circulating after the thermostat opened. But with no radiator cap, that won't work this time!

Any thoughts on how I can see what's really happening here?

=Steve=

 

Steve, start it up and put your hand on the outside of the radiator in different places and see if it reall y hat. If it really that hot, you won't be able to hold your hand on the radiator. Did you purge the air from the system when you changed the thermostat?

 

No, I thought the manual said it would purge the air itself through the fill tank. How do I purge the air?

=Steve=

 

You don't need to purge it.It does it by itself.

See if you can find someone that has one of the laser heat guages and have them read the temp on the thermostat housing.

 

Of course I cannot be sure on your car but it sounds to me as if your 'temperature compensation board" is faulty. This board actually compensates for changes in temperature so you do not worry your pretty little head over small changes in temperature. It is an abomination pushed on us by car manufacturers and a bunch of them do it. Fortunately the fix is simple on a volvo and after your compensation board ectomy your temperature will read actual temperature! I had the same problem with my 240. To get to the compensation board you have to pull the instrument cluster and remove the seven screws that hold the instruments in on the back of the cluster. Now looking at the gages from the front and just under the temperature gage you will see a small electronic board. Pull gently and it will unplug revealing four pins. To get your gage to work properly you have to connect pin 1 with pin 3. To do this I cut off the backs of a 2 computer jumpers and stick them on one and three. Then fashion a small staple with a kink in it to plug into the back of these jumpers. You can get more info on the www.ipdusa.com site. I will try to post a link.
They sell the jumper for $15 and they have instructions for you to downlosd. Looking at the gages from the front the pins are 1,2,3,4 (4 is the one farthest to the right).

 

from here you can download the instructions

http://www.ipdusa.com/ProductsCat.as...668&RootID=629

 

Just what I was looking for. My daughters 89 244's temp gauge
fluctuates (sp?) alot,
even on steady state conditions. Think this might be the
compensator?

bill