Quote:
Originally Posted by Mxfrank
It's incorrect for any car to have 20-30 temperature swings in normal driving. Maybe sitting in traffic on a very hot day or climbing a mountain. But in normal driving, no. These are the only cars I know where such swings are routine, due to a very wacky design. The fans are intended to engage progressively, but even in perfect condition, don't keep temps anywhere near level. And with three separate mechanisms for fan control, the odds of component failure are high. I've heard a lot of excuses for why this should be, not one makes sense.
As a design goal, a cooling system engineer wants to hold temperatures as steady as possible to limit pressure fluctuation. For a cooling system, varying pressure is like flexing a piece of metal until it fatigues. Highly variable pressure will eventually promote component failure.
I use the temp switch to engage the aux fans just prior to the clutch fan engaging. They run quite a bit more than they normally would. But my temperatures stay in a tight range, rarely over 90, and never hit 100. This effectively eliminates the clutch fan, because air temps almost never get hot enough for it to engage. And the added benefit is that I have working A/C, even in heavy traffic.
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What year/model Mercedes diesel are you referring to that has a 20 to 30 deg swing? I have currently an 85 and had an 83 300D, and neither one have the swing you mentioned. They stay within 80 to 85 C on the temp gauge both summer and winter with the viscous fan.
I think I will be taking a different approach in controlling the AC aux fan. Rather than adding a temp switch to the T stat housing or the cyl head, I will build a circuit that interface with the coolant temp gauge sender (iirc it is located between glow plugs #2 and 3) to control the AC aux fan. I will make it adjustable so I can have the fan come on earlier or later. It will be less work in the end and provide better temperature control.