The aux fandoes not switch on when the ac is functioning but eventuallycomes on when the engine temperature reaches 85-90C. Why, why, why..?

 

paul,

I can't swear for sure that your 350SDL a/c is the same as my 300CD, but if it is:

The auxiliary fan is controlled by two factors: a thermostatic switch and a hi/lo pressure switch. Some early cars only came with a hi pressure switch that you probably should replace with the hi/lo pressure switch. Although with the hi pressure switch, the fact that your fan wasn't coming on could be used as a clue that it's time to check for leaks in the system and top off the refridgerant. The thermostatic switch is designed to switch on at 80 celsius, if I remember correctly.

 

I believethe aux fan iscontrolled by a thermo-sensor. Mine does the same.

 

The auxillary fan runs independently. Having the air conditioning on is coincidental to its operation. It only comes on when the engine reaches a pre-set high temperature. For example, if you are driving at highway speeds it never comes on but sitting in slow traffic with all your accessories on, wipers, heated seats etc. it can even come on in cool weather as the engine load causes the engine to heat up to the pre-set temp.

 

gilgorm, that is not correct for the older cars. On the older cars there is a pressure switch on the a/c drier that won't turn on if the a/c system doesn't have sufficient pressure. As installed from the factory, it is a hi pressure switch. Many people change it out later with a hi/lo pressure switch so they still get fan if the refridgerant starts to run down.

 

I stand corrected. I believe my 87 190E had the feature I described (sold in 2002) but I have no recollection of my older cars. Over the years I had the 190E emission tested every two years as prescribed by law. It was always in January and the A/C was obviously off. The garage doing the test puts the hood up and positions a fan in front of the rad. They then run the car on rollers to heat the car and the catalytic converter to operating temp. They then do the test. It was during these tests that I watched the auxillary fan turn on and off to keep the engine at its proper temp.
Sorry about the mixup.