Coolant Temp Sensor
 

I have a question about the coolant temp sensors on my 81 300D. I noticed that there are two of them, one above the thermostat and another between the 2 & 3 cylinders glow plugs. Which one is for the gauge on the dash?

I noticed on the wiring diagram for the glow plug system that there is a temperature sensor for the glow plug system. Could the one between the glow plugs be for that purpose?

Which fan?

I thought that the aux fan was controlled by the HVAC system, and that it only came on when the A/C compressor is running, to blow air across the condenser?

Not all models are the same. Are your thoughts specific to a 1981 300D?


That's not exactly how it works, even when it does work that way.

The aux fan has only ever come on when I'm running AC. To Tango's point, I'm sure the mechanism is very similar across all model years, though mechanical parts would be updated as needed over time.

I am sure that they are not. 100% sure.

Looking at the wiring diagram for my 81 300D, it appears that the aux fan for the condenser is controlled (via a relay) by the temperature switch on the receiver dryer. So that throws the idea of the temperature sensor on the t-stat controlling the aux fan out.

I wish my wiring diagram would show the location of the coolant temp sensor for the gauge. I think i'll just ground the wire of one or the other sending units to see which one moves the gauge, but if one is for the gauge, what is the other one for?

That's a great idea. Should take about 4 minutes, and you will know for sure how your vehicle is wired. Which is a lot better than listening to a bunch of folks who just cannot comprehend that one year model might be different from another. The results should also enable you to recognize the correct answer when told.

Just keep you fingers out of the aux fan when you ground the switch in the thermostat housing!!!

Temp sending unit is in the head. The one in the T-stat housing is a cold eng lockout switch. It restricts blower operation in the heat position until eng reaches 33 deg

I'm not going to give you the satisfaction and argue with you. Just know that the way you relate to people in general on here isn't appreciated. And I'm 100% sure about that.

And you almost had me convinced that all MB models are the same.

I looked at the EPC and that looks correct.

Thank you all for your assistance. I put the wire from the sending unit on the side of the head between the glow plugs, to ground and sure enough the gauge was pegged. So that part of the mystery is solved.

So the other one is the cold engine lock out switch, that restricts blower operation when cold? I see that on my airflow diagram that I have here. Part two is solved.

Thank you all for helping me satisfy my curiosity. Now I know what sensor does what, and why.

One last question... What is this "EPC" that was mentioned by gatorblue92?

EPC = electronic parts catalog. Look for the thread "done the right thing" and it'll point you to MB's online catalog with blow-up drawings.

Ah, I see. So I have been using the "EPC" wiring diagrams all along, without knowing it. Thanks again for your help.