Trying to understand my cooling system: '84 300SD - PeachParts Mercedes-Benz Forum

braverichard · #1 11-18-2004, 11:44 AM

I'm trying to learn more about this car's cooling system components and what they do. I read the stuff in the service CDs but the information wasn't sufficient. I have attached four pictures to this post that show the items I'll be asking about.

From what I can see, the car has two sensors for the coolant temperature. Picture A shows what I now know is the sensor that tells the temperature gauge what to show right? That sensor is located right between the glow plugs, next to the injection pump.

Picture B however shows the second sensor, right after the thermostat housing (going from radiator to the engine). What is its purpose?

I can also see that the car has two cooling fans. Picture C shows what looks to me like a clutch driven fan located between the radiator and the engine. This is the fan that is supposed to exchange the heat in the engine coolant with the atmosphere via the radiator. Am I correct or not? In my engine, it seems like each time I look under the hood this fan is spinning.

Picture D however shows a second fan, located between the radiator (or condenser) and the grill. I can see an electrical connection, so it isn't clutch driven. On my car, I've only seen / heard this fan when I was doing a cooling system flush some months ago. It came on briefly as I was pouring water into the reservoir and draining it right away through the radiator drain plug and the engine block coolant drain plug, both of which were open. I since haven't even seen or heard it on. I know it works because I jump started it and it came on full blast. What is its purpose? To come on when the engine gets extra hot?

Thank you all for your help. Hopefully some ignorantos like me may also learn from my questions. :p

SD Blue · #2 11-18-2004, 12:06 PM

Sensor A is for the engine temp guage

Sensor B is for the temp sensing of the glow plugs.

The "inside" fan, controlled by the clutch, is for normal engine cooling.

The "outside" fan comes on either when coolant temp. reaches 100*C or a pressure switch on the air conditioning system.

BTW, do you know of any independent foreign parts stores, that handle MB parts, in the Kansas City area? The last time I needed a part, while visiting my brother in Overland Park, all I heard was "only the dealer". Even his mechanic told me this.

braverichard · #3 11-18-2004, 12:14 PM

Quote:

Originally Posted by SD Blue

Sensor A is for the engine temp guage

Sensor B is for the temp sensing of the glow plugs.

Does the amount of heating done by the glow plugs vary with engine temperature? Do they heat for a longer time when the engine is colder?

Quote:

Originally Posted by SD Blue

The "inside" fan, controlled by the clutch, is for normal engine cooling.

So is it normal for it to be always on like in my car?

Quote:

Originally Posted by SD Blue

BTW, do you know of any independent foreign parts stores, that handle MB parts, in the Kansas City area? The last time I needed a part, while visiting my brother in Overland Park, all I heard was "only the dealer". Even his mechanic told me this.

Oh yes. In the Greater Kansas City Area, there are two places from which one can purchase M-B parts... actually three. Two are dealerships, of which one, Jay Wolfe, just opened in July. Then the third one is Star Motors, an independent European (originally M-B) auto shop owned and operated by Emery. Emery is the true master of Mercedes-Benzes in the Kansas City Area. No one at any autoshop has more experience on these cars than him in the KC area. He has been working on them for over 40 years! In fact, he is so good that Aristocrat Motors, the original M-B dealership here for years, sends cars they can't repair to him to complete the repairs!! And here's the best part: he can get any M-B parts faster and cheaper than the dealers can get them (don't ask me how)and charges just $75/hour of service!!

SD Blue · #4 11-18-2004, 12:32 PM

The clutch for the cooling fan is fluid driven so the fan will "free wheel" but "engage" when the temp. comes up.

As for the glow plug system. I am not completely sure of the "correct" operation since I am not sure my system is operating correctly. (Haven't had it in conditions that were cold enough.) The wiring diagram is what I am referring to.

Thanks for the parts info. I believe Aristocrat Motors was the only one his mechanic knew of.

braverichard · #5 11-18-2004, 01:04 PM

I just found a section of the Service CDs in which it is stated, "the duration of the preglowing depends on temperature of engine and on ambient temperature." I guess my answer is right there!

Indeed, Aristocrat is the original M-B dealer for the Kansas City Area. Right now they continue to be the only dealer for Porsche, Jaguar, Aston Martin and Land Rover.

Thanks for all your help.

Pete Burton · #6 11-18-2004, 01:27 PM

Quote:

Sensor B is for the temp sensing of the glow plugs.

I agree with all the other info except this. The glow plug relay uses some funky internal electronics to measure proper glow time including an electrolytic capacitor which can change its value over time and even not being used for a long time(although that condition is temporary) The sensor you refer to as B is used by the climate control to delay application of higher blower fan speeds until the coolant temp is something like 52C. This prevents cold air being blown out of your heater on winter mornings until the coolant temp has risen somewhat.

#7 11-18-2004, 01:30 PM

That sensor on the water neck isn't for the glow plugs from what we dicussed over the violet wire and the GP Relay thread...
Someone believed that it was for the heating/cooling to tell the fan (the inside one) when to not be on due to cold water in the system.

He verified this when he hooked his glow plug circuit (we have been playing around a bit) to this sensor and it was dependednt on how the heater controls we set.

Anyway the glow plugs are ambient sensed. We have been designing a circuit so that the glow plugs are water temperature dependet (do a search for that thread "violet wire")

The GP's heat more if the engine is cold but poster starter glowing is only achievable by the newer relay OR our handy dandy circuit that I came up with.

Just thought I would set things straight..

braverichard · #8 12-05-2004, 10:44 AM

Ha! Very interesting info. The more I learn about this car the more sophisticated it comes across... ultra impressive for a car engineered from 1971 - 1978!! My 1999 Toyota Avalon has a climate control system that doesn't quite work that efficiently. I guess the 300SD truly is engineered like no other car in the world .

Whether that's a good or bad thing is another issue of course.