The deal is, you want to remove the temp comp board and replace / bypass it with the jumper wire.

When working properly the board smooths out (removes) minor gauge fluctuations and so turns the gauge into an idiot light. No small swings - just huge ones when it's too late. More often it gives false reads which can be very alarming since it usually gives a false high reading.

I have such disdain for the t.c. board that I remove them soon after buying a 240 even if it seems to be working OK.

If that doesn't take care of the problem you can check the connection at the temp sender. It's under intake branch #1-2 and has a single wire to it (yellow??). We've had one sender go bad but you absolutely must get rid of that temp comp board.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.