So today the 300D got up to the 120 degree C mark, very unusual. I found that the belt that goes around the water pump and the alt. was loose and replaced it, problem solved, now runs a about 80 degrees C(I added some Red Line Watter Wetter and it dropped the normal operating temp about 10 degrees).

My question is that when the engine was up near the 120 mark, the aux fan did not engage. It has worked in the past but this is the first time that the temp has really gotten over 100C and from my understanding the temp has to get to a certain degree to engage the fan. I know that there are some 'Ice Box' relays, on top of the drivers fender well, that may have some control on the fan, and have taken the relays and made sure they are getting power, which they are. I also check the aux fan connection under the front bumper and no power was getting there(with relays in of course).

From what I can determine the relay is the probable cause for the fan not working. I have also checked the fuse in the fuse box and it is ok, I replaced it just to make sure.

Am I missing something? Is there anything that I am missing that may cause the fan not to work?

__________________
1980 300D European
"The Red Fox"
200,000 miles

2003 Chevrolet Silverado

609 Certification

Have you verified that fan works when power is applied directly to it........just to eliminate that.....bearing do wear out and fans do die..and this is easy to eliminate...

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I have applied power directly to the fan, and yes it does spin as it should.

__________________
1980 300D European
"The Red Fox"
200,000 miles

2003 Chevrolet Silverado

609 Certification

OK, I thought you did, it just wasn't mentioned.......Like Leathermang says....likely a a relay issue.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Hey,
The bottom right relay as you look at the box should be your compressor clutch relay. remove that, jump the connectors on the temp sensor on filter dryer, then use relay you removed and start replacing other relays until the fan comes on.
Replace bad relay. I think it's the second from right on bottom next to compressor relay.

Danny

__________________
1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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This fan must activate on its own based on the cars operating temp then correct? Because mine comes on when its around 95C despite my completely non-functional A/C system (no drive belt, seized compressor, and compressor clutch cable disconnected)

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

A relay is typically a deal where a smaller current controls a larger one...saves on wire cost...and sometimes allows several things to control one thing... did you check to see if there is electricity from the control circuit... or just the ' main' which would be tapped if the relay engaged ?