The 4-pin coolant temp sensor at the back of the cylinder head (driver side). That's actually 2 sensors enclosed in a single connector. Both sensors are identical thermistors, but one supplies signal to KE controller and the other to the EZL. Test diagonal pins (1-3 and 2-4) with ohm meter and compare to resistance-temp scale shown in FSM. Readings from both sets of pins should be identical, or nearly so, throughout the temp range. If not, there is a mismatch between what the EZL and KE controller are reading.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't