borrow a laser thermometer, click on GP see if all have same temp. Do it when engine is cold or else u cannot see the temp difference.

My old SD had the prob, and I checked it, 2 out of 5 didnt make any temp increase, also you can use voltmeter to check the individual GP.
My 79 SD has individual wiring, it turned out one GP's wiring has no power going to it, I attach a wire from another GP to the dead one.
If worry one wire supplying 2 GP, one can run a relay there.

My last 240D I wire my own relay and hold her for 15 20 secs.
Put 2 off-road lamp relay in it, one can carry limited current.
Good Luck

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