w116 300sd Aux Fan - PeachParts Mercedes-Benz Forum

xaliscomex · #1 06-18-2016, 03:44 PM

1979 300sd w116

The fan (Aux Fan) located in front of the condenser does not turn on.
Fuse looks OK

Temp gauge on my car moved up over 212 deg F and wondering if the Aux Fan not turning on is the cause.

What else can be checked

Any advise is greatly appreciated

Diseasel300 · #2 06-18-2016, 04:47 PM

The Aux fan doesn't normally come on until the temp is up over 100˚C, usually somewhere between 105-115 depending on the thermostat fitted.

The engine driven fan should be doing the bulk of the cooling. If the cooling system hasn't been serviced in a while, you probably need a flush and refill to get any sediment and contamination out.

Common causes of overheating (that's what you have here) is a stuck or partially stuck thermostat that isn't allowing coolant to flow properly and a fan clutch that isn't locking up to draw enough air through the radiator.

Big clue to the latter is if you're on the move, the engine doesn't really get that hot, but when you come to a stop or are in city traffic, the temp rises. If the thermostat is sticking or the cooling system is clogged up with scale and other debris, the temp will stay high whether you're moving on the highway or crawling in traffic.

xaliscomex · #3 06-18-2016, 05:04 PM

Repaired the radiator with a new core, replaced and tested thermostat using boiling water, replaced hoses, checked the fan clutch and its good.

All of this corrected the previous over heating symptoms.

California temperatures were high yesterday was in the 90's and expected to climb on Monday.

I was driving to Lancaster CA (Desert) and the drive up there was mostly a climb from 700 elevation to over 2000 elevation.

Once down hill and coasting on flat roads car temp gauge returned to the usual.

One other thing that I have noticed is that from time to time the temp gauge needle starts to climb when I shut the car off.

Again, the Aux fan will not turn on and I was clearly driving above the operating temp at 212 deg F. Shouldn't the 212 switch activate the Aux Fan?

Is there a second fuse that I don't know about that can be replaced?

Diseasel300 · #4 06-18-2016, 05:14 PM

Assuming the switch activates at 100˚C (212˚F) and has some slop in it's tolerances, it may simply not have been hot enough to switch the fan on.

You'll have a temperature switch somewhere on the engine that controls the Aux fan. You'll need a wiring diagram or service manual for your car to find out what controls what. If you can find the switch, you can test it with boiling water to see if it is actually switching like it should.