After reading Dave's excellent post (thanks Dave) I figured out that Euro Car Service in Seattle had wired my gauge to the EDS sensor. Once I rewired to the correct sensor and installed a new sensor the temperature gauge reads 83-85°C like it should. My temperatue gauge is fine. Now if I can just figure out why I have a 100-110°C Electric fan & AC protect switch (3 prong).


here are 4 total temp sensors on the 1986/87 OM603:

1) Dash gauge temp sensor (one wire) - This is located on the side of the head, below the injectors and intake manifold, behind the injection pump. Not easy to access.

2) Tranny shift softener sensor (one wire) - This is a round sensor above the thermostat housing. It triggers a solenoid that is supposed to soften tranny shifting when coolant temps are below 50°C. (I have this disabled on my cars, it just makes them flare when cold.)

3) EDS (Electronic Diesel System) sensor (one wire) - This is on the water oulet from the head, where the upper radiator hose connects. It tells the EDS computer what the engine temp is. AFAICT this is only used for EGR and ARV (emissions junk) operation.

4) Electric fan & AC protect switch (3 prong) - This is a dual-temp switch, it triggers the electric fans on high speed at 105°C, and will kill the A/C compressor as a safety when temps get abnormally high (115, 120, or 128°C depending on which engine & model you have). The 105/128 switch used on the 124 is expensive so I used the 105/120 switch from the SDL, which is cheaper, and I never liked the 128 rating anyway. Photo below.

5) The glow plug temp sensor is inside the relay on the 86/87 OM603 cars. Models with afterglow use an external sensor that replaces the 1-wire dash temp gauge senor in the head (#1, above) with a 2-wire sensor, one still goes to the dash gauge, but the other wire goes to the afterglow relay. (I upgraded my car to factory afterglow and have this 2-wire setup on my car. Photos of the parts required are at this link.)