Hello, i can't diagnose your car from here, but 240's have this problem, as others have said.

there is a 1" x2.5" circuit board that controls temp variations. what happens is the contacts that connect to the instrument cluster circuit board get a little corroded.

you probably won't have to buy the $40 board, just clean the pins or posts, but don't bend them.

what you have to do is take the instrument panel out and remove 6 little screws.

remove the black plastic cover and look at the front underneath the fuel and temp gauges. there is the temp board. pull it off and clean the tiny little pin contacts (1/4" long?) don't bend them.

also, look at the board where the pins stick in--4 tiny metal tubes. close the tubes very carefully with tiny plier after you have cleaned the contacts and put the board back on. the contacts between the tube and pin may be loose.

i used an eraser and very very fine sandpaper.

i have not temp problems, since--1.5 yrs ago!

hope this helps & get a bentley manual cheap on ebay, no need to pay more than $30 for new--some good ones <$20, also.

regards,
byron golden
bc1_2002
86 245, 1 owner, 240k+ mi