Can A Two-Pin Coolant Temperature Switch Handle Full Auxiliary Fan Power Through It? - PeachParts Mercedes-Benz Forum

Mxfrank · #1 05-07-2018, 07:45 PM

The switch absolutely cannot handle the aux fan directly. I don't have a spec for your car, but most Mercedes aux fans draw somewhere between 15A-25A. Far too much for any thermoswitch.

The good news (I'm going to hate myself for getting involved here) is that there's really no need for an additional relay. Wiring the new switch in parallel with the pressure switch shouldn't result in backfeeding current. Best practice is for the pressure switch in the A/C system and the new coolant thermoswitch to control GROUND, not power. Of course, if this is a shade tree hack, all bets are off. Maybe I should back off and ask to see a schematic of your circuit.

Squiggle Dog · #2 05-07-2018, 07:58 PM

Quote:

Originally Posted by Mxfrank

The switch absolutely cannot handle the aux fan directly. I don't have a spec for your car, but most Mercedes aux fans draw somewhere between 15A-25A. Far too much for any thermoswitch.

The good news (I'm going to hate myself for getting involved here) is that there's really no need for an additional relay. Wiring the new switch in parallel with the pressure switch shouldn't result in backfeeding current. Best practice is for the pressure switch in the A/C system and the new coolant thermoswitch to control GROUND, not power. Of course, if this is a shade tree hack, all bets are off. Maybe I should back off and ask to see a schematic of your circuit.

I am using a 16" auxiliary fan from a second generation W126. I think at this point I'm ditching the idea of using the coolant temperature switch to directly power the auxiliary fan. I had my doubts and it's good to have confirmation.

Maybe it's worth mentioning that I tore apart my main wire harness so I could completely rewire it to convert it from automatic climate control to manual, so I am figuring out how to graft it all together, plus I want the auxiliary fan to come on with the compressor and/or coolant temperature switch instead of the way it is was designed where the auxiliary fan was controlled by the refrigerant temperature switch and the coolant temperature switch.

I'm trying to find a happy medium between the 1980 W116 300SD with ACCII wiring diagram and the 1974 W116 450SE with manual climate control wiring diagram.

Let me wrap my head around the new switch being wired in parallel with the pressure switch not causing it to back power the compressor. I just need to think about it so I understand it.

The pressure switch is wired directly into the compressor power wire and takes the full power to the compressor through it. It's like an inline switch and doesn't go to a relay. So, I think that only a diode or relay would prevent blackflow from the auxiliary fan inadvertently powering it on. You may be correct; I'm just trying to understand it, but I might if I think about it for a while.

marco5 · #3 05-07-2018, 08:02 PM

IMO - You won't really gain a cooling benefit from the aux fan running full bore and you may wear it out quick with it running when the compressor is on. It's meant to power on when the drier hits a certain temp, and when that temp is hit, in order to protect the system it will lower the pressure by turning on either low or high speed. Maybe keep the temp. switch OEM setup? Just a thought but they designed it this way for a reason!

Squiggle Dog · #4 05-07-2018, 08:11 PM

Quote:

Originally Posted by marco5

IMO - You won't really gain a cooling benefit from the aux fan running full bore and you may wear it out quick with it running when the compressor is on. It's meant to power on when the drier hits a certain temp, and when that temp is hit, in order to protect the system it will lower the pressure by turning on either low or high speed. Maybe keep the temp. switch OEM setup? Just a thought but they designed it this way for a reason!

From what I've read, it seems running the fan all the time the compressor is running is the way to go as the refrigerant temperature switch is seen as unreliable and by the time it turns on the auxiliary fan, it's already too late and has to "catch up".

They way it was designed may be the best way, but there is a possibility it's not. The early W116s didn't even have pressure switches, but over the years designs change based on new findings. It's been claimed that a big improvement in air conditioning performance has been made by wiring the auxiliary fan so it runs with the compressor.

Whatever it is, I want the most efficient system that I can practically make. So, I'm just trying to read all I can to come up with the best solution.

ROLLGUY · #5 05-07-2018, 08:21 PM

Quote:

Originally Posted by Squiggle Dog

From what I've read, it seems running the fan all the time the compressor is running is the way to go as the refrigerant temperature switch is seen as unreliable and by the time it turns on the auxiliary fan, it's already too late and has to "catch up".

This is absolutely the case, and even many newer cars have this feature.