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Old 04-19-2002, 06:33 PM
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Join Date: Apr 2000
Posts: 459
Interchangeable fan temp switch & electric fan updating

I have see a lot of questions on this board from people asking under what circumstances and temperatures the electric fans on Mercedes turn on, and I have a question:

Does anyone know whether a lower temperature sensore could be swapped in? Seems like most switches are designed trip the fans at 110 degrees, which seems quite high and makes everyone nervous seeing their car reach that temperature. My Saab had a switch that turned on first at 90 and 88 degrees thereafter. Everyone on the Saab webpages knows to order a certain VW part number and you get a fan switch that is exactly the same size (so it fits in the Saab radiator), but turns on at all times at 82 degrees (which coincides with the thermostat). Saabs are notorious for overheating because of this odd arrangement with the factory fan temp switch, and a simple swap fixes the problem. Seems like the same problem we have with Mercedes, so does anyone know whether a lower temp switch from another car would fit in place of the factory switch?

Secondly, why didn't Mercedes design the fan to be on whenever the a/c is on? Seems like you need it on for best cooling with 134a. Again, on Saabs, you hit the a/c and both fans come on and stay on until you turn the a/c off - and I'm pretty sure most other modern cars are set up the same way. Mercedes are very well engineered cars, but with my plans to convert to 134a, I'm thinking it wouldn't be bad to update the cooling system to more modern standards!

Lastly, the time delay on the Saab outfit would be nice to continue cooling the car even after you have shut it down. Think about how hot the engine gets in the summer after you shut it down.

Any ideas? Has anyone outfitted a w123 with aftermarket electric fans? I'm thinking about swaping in an entire Saab outfit for optimum summer cooling and a/c efficiency. Would there be any drawback to this? The only thing I can think of is additional load and stress on the alternator.

'84 300D
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Old 04-20-2002, 10:00 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The main drawback would be the alternator. The 55 or 65 amp alternators on those cars were really poor. Their output at 800rpm is abou 20 amps. Its probably linear to max output at around 2000 rpm.

That car with normal aux fan, A/C and inside blower can take 30-40 amps. In trafic this don't cut it even without the headlights. The car can not stay up with wipers, A/C, aux fan, and headlights never mind the rear window defogger. Luckily the A/C is cooled off enough by rain that the Aux fan usually shuts down or you would soon be dead soon with the wipers.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 04-20-2002, 12:23 PM
Senior Member
Join Date: Jul 2001
Location: San Antone
Posts: 408
Steve's comments about the alternator low output are confirmed by my own experience checking output with a volt-ohm meter with various accessories on or off and a computer printed test of the alternator on my '77 300D. At an idle your lucky to get 20 amps, 10-15 amps is more realistic. At 1,200-1,400 rpm output is about 20 amps, at 1,800 rpm alternator output is 40 amps, at 2,000 rpm output is 45 amps, at 3,000 rpm (about 60 mph) output is 60 amps, and then output increases marginally to 65 amps perhaps a bit more with higher engine rpm's. When I have to idle for extended periods with accessories running I'll shift to neutral or park and increase engine speed a bit to say 900-1,000 rpm to speed up the alt. and get more output and decrease the drain on the battery. When I have a lot of accessories running and increase engine rpm's a bit from idle I can hear the aux. cooling fan pick-up speed as more amps are available to it.

If you're going to change sensors to have the aux fan come on at lower temps. and/or install other fans then you need to consider the alternator's capabilities and compensate accordingly (at extended idle time increase speed a bit, run the AC/heater blower fan at lower speeds, turn the stereo volume down esp if you have sub-woofers, charge the battery once a week to bring it back to full charge, etc.). If you replace or add fan(s), then try to find fan(s) with lower amperage requirements. Also, if you have a high power stereo system consider using capacitors to store power for those times when the music puts higher loads on the alternator (typically sub-woofer and low bass sound waves).

Good Luck!
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
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Old 04-21-2002, 08:29 AM
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Join Date: Apr 2000
Posts: 459
This is very helfpul. Thanks guys.

'84 300D
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