I have to say, having owned four '86 and later 240s and still in contact with three of them, you can strongly suspect the temp compensator board. Even with the small chance that your car's problem is not the tcb, I would still recommend you bypass that board. Save yourself the stress and hassle of dealing with it later on one day.

Soldering the wire jumper works fine, but if you're solder-challenged you can do this:

Get a 2.5 inch length of 16 gauge "zip cord" lamp cord, just one side of the "zip" is all you need. I don't think 18 gauge is thick enough. Butt-cut the ends, no exposed conductor.
Poke a thin hole up each end of the wire with a thin nail or whatever you have handy.
Press ends of wire onto pins 1 + 3 as shown in the photo instructions.

If you use needle nose pliers to press the wire on, you don't need to remove the gauge from the circuit board.

Assembled cluster holds the wire in place; there's nowhere for it to go.
Did this on my daily driver 2.5 years and 100K miles ago.
Also done on our other two cars.

Beware the red and white wire behind your cluster. It's for a tach but the 240s all have the wire even if no tach. Mark it and don't connect to cluster when reinstalling unless you have a tach.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.