1992 400SE

The Engine Temperature Guage on my W140 instrument cluster stoped working.

I found 3 sensor mounted on top of the intake.

a single prong sensor.
a four prong which is the ECT sensor.
a two prong which is the CTC sensor.

I know that on my W124 the two prong CTC sensor controls the output onto the cluster, but have heard that the single prong sensor OR even the three prong sensor controls it on other models.

Can anyone shed some light on what sensor I should be investigating/replacing OR how to test it first.

Everthing else works fine on the cluster.

Thanks.