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Mercedes Cooling Fan temperature settings [message #352803] Mon, 16 March 2020 14:10 Go to previous message
captjack is currently offline  captjack   United States
Messages: 271
Registered: February 2010
Location: Sebastopol, California
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Senior Member
Since the previous post has now extended to 2 pages, I'm starting a new post with a focus on fan temperature control.

I've installed a simple PID temperature controller to turn on the fan and provide the option of varying the turn on and off temperature settings easily. The output of the controller is a pulse width modulated signal based on how far the temperature is from the set temperature, not just a hard "on" or "off" over a wide range. Based on my intuition and suggestions from the others in the previous posts, I set my unit with a target cold shutoff temperature of 160 degrees. I then set off on a trip to the GMC 49ers rally in Sacramento. The weather was hot and I was running the air conditioner.

Per the instructions that come with the kit, I had installed my temperature probe on the passenger side midway between the top and bottom the the radiator and alongside of the temperature switch provided in the kit which, according to the kit, is set to turn on at 210 and off at 195.

What I found was, that with the AC running, when the temperature at my probe rose to 130 degrees, the water temperature reported from the engine output to the radiator was over 220 degrees. I'm not sure what it would have been if my setting of 160 degrees was reached as I turned on the fan manually which quickly brought the temperature at the probe's location down to 100 degrees and the engine water outlet temp to 195.

What this says to me is that the temperature switch in the kit is inappropriate for this location as it would never turn on....well until after bad things happened. At its current setting it would probably best be installed where the water enters the radiator.

For my drive home, I set the target shutoff of the controller at 115 thinking I could see how the fan behaved when being switched on and off at a rate inversely proportional to the temperature over 115. Unfortunately we were having rain and it was cold, so no AC, and the radiator output never reached 115. So more testing needed.

I did come to the conclusion that the fan should be sent a pulse with varied "on" signal until the temperature at the probe drops to around 120. Also, based on how the temperature at the probe varied with the AC on and off, I can see why modern cars with electric fans are designed to have them turn them on at some level when the AC is running.


Jack Christensen - K6ROW, '76 Glenbrook/Clasco - "The Silver Bullet", Sebastopol, CA

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