1984 300D 200K miles

I'm trying to diagnose a problem with my cooling system and it would be beneficial to measure the coolant temperature that the thermostat is seeing.

The CCU sending unit is in the perfect location to do that.

So I am in need of the specs for that sending unit. What ohms= what coolant temp?

Anyone have that?

Thanks,
Phil

I found the sensor section on STARtek but there is no ohms specifications.

Anyone?

Phil

What CCU sending unit?

If you are referring to the "sensor" in the thermostat housing, it's just an on/off switch. It either provides continuity to ground or it doesn't.

The OP has another thread about trying to troubleshoot a non-opening t-stat, so I presume he's trying to find a different way of measuring/verifying coolant temps seen by various sensors.

I'm not sure the CCU on the W123 has the info you're looking for. The climate control uses a cold engine lockout, but I don't think it has other coolant temp inputs. This page has some info on inputs to the CCU and their values: Middle-aged Mercedes Electrical Repairs

I think the info regarding coolant temps and the resistance of various senders exist, but I don't recall where I've seen it. I thought there was a thread at one time where a M-B tech explained that they used the 100 C hash mark to calibrate the sender by providing a specific resistance value to the temp gauge inputs.

The glow plug relay section in the manual has some values associated with coolant temp as measured by that sensor

0 C = 8200 ohm
25 C = 2440 ohm
80 C = 290 ohm

These values are more/less consistent with the same ones for the test for the CA EGR temp sensor (which may be the same one?).

20 C = 2.2-2.8 kohm
80 C = 290-364 ohm
100 C = 140-222 ohm