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

Squiggle Dog · #1 05-07-2018, 06:47 PM

I plan on using one of these two-pin coolant temperature switches (part number 006 545 15 24, red color, M4X1.5X1 threads, on at 100C/212F, off at 95C/203F) to directly power the auxiliary fan from the fuse box:

The factory setup only uses this switch to click the auxiliary fan relay on and off and does not supply power directly to the auxiliary fan; once it clicks on the relay, the relay then draws power from the fuse box and uses this to energize the auxiliary fan power wire which powers the fan. Only small 0,75mm² wires are used at the coolant temperature switch because all they do is click on the relay when the coolant temperature switch turns on.

What I want to do is run power directly from the fuse box to the coolant temperature switch so that when the switch opens, it directs that power to the auxiliary fan power wire. Of course, I will use much larger wires such as 2,5mm² or 4mm². But, I do not know if the coolant temperature switch might short out or melt having the full power to the auxiliary fan running through it. This is what I want to know.

Granted, the switch will only have power flowing through it when the coolant temperature is nearly overheated, which would be almost never, and even then only for relatively short periods of time.

renaissanceman · #2 05-07-2018, 06:54 PM

I would imagine that would be WAAAAAAAY too much current for that little switch. What's wrong with having a relay?

Squiggle Dog · #3 05-07-2018, 07:08 PM

Quote:

Originally Posted by renaissanceman

I would imagine that would be WAAAAAAAY too much current for that little switch. What's wrong with having a relay?

I'm already using the auxiliary fan relay to power the auxiliary fan when the compressor clutch is engaged. If I hook the coolant temperature switch to this relay, then it could inadvertently power on the compressor.

I could probably make a second auxiliary fan relay, though...

marco5 · #4 05-07-2018, 07:22 PM

A separate switch for both the a/c compressor and aux fan in the dash would be super cool

Squiggle Dog · #5 05-07-2018, 07:22 PM

Yeah, it looks like it will work to use a second auxiliary fan relay for the coolant temperature function. One auxiliary fan relay to switch on the fan when the compressor is engaged, and a second one to switch on the fan when the coolant temperature reaches 100C/212F. By doing this, I won't risk overloading the coolant temperature switch and can use the original single pin style.

At the same time, both relays will energize the auxiliary fan from the #2 fuse which on the W116 is reserved solely for the auxiliary fan. With two relays, the relays will prevent the coolant temperature switch from inadvertently powering on the compressor when it turns on the auxiliary fan.

I removed the power window relay since I converted them to manual. This relay is identical to the one used for the auxiliary fan, so I can just use it as a second auxiliary fan relay instead.

renaissanceman · #6 05-07-2018, 07:28 PM

You may be able to just a diode on the compressor to keep it from being back powered?

Squiggle Dog · #7 05-07-2018, 07:37 PM

Quote:

Originally Posted by renaissanceman

You may be able to just a diode on the compressor to keep it from being back powered?

I could place a diode on the jumper wire that goes between the auxiliary fan relay and the auxiliary fan power wire to keep it from being back powered. Right now I'm sketching a diagram to see how it would look to use two auxiliary fan relays, though I could probably get away with one if I used a diode on that jumper wire.

Mxfrank · #8 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.

tjts1 · #9 05-07-2018, 07:56 PM

Use a relay.

Squiggle Dog · #10 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 · #11 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 · #12 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 · #13 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.

Diseasel300 · #14 05-07-2018, 08:39 PM

If you have a healthy clutched fan, it's going to do most of the heavy lifting for the A/C system anyway. The electric fan is there for assistance on hot days when more airflow is needed.

The reason your system has a pressure switch to control the fan is because you actually DO need head pressure for the A/C system to work properly. If you have the fan blasting away all the time, you have operating conditions (wet, cool days, early in the mornings, etc) where the head pressure drops to the point that the A/C just doesn't really do any work. Even modern cars use pressure switches, listen next time you hear the compressor kick on, there's a delay before the fan kicks in. That's the pressure switch doing its thing.

Also make sure you upgrade your alternator if you're running the fan on high speed. The 70A in the cars that fan was designed for won't keep up at idle if you're running the A/C. You need 90A+ unless you just hate your battery.

Squiggle Dog · #15 05-07-2018, 09:20 PM

Quote:

Originally Posted by Diseasel300

If you have a healthy clutched fan, it's going to do most of the heavy lifting for the A/C system anyway. The electric fan is there for assistance on hot days when more airflow is needed.

The reason your system has a pressure switch to control the fan is because you actually DO need head pressure for the A/C system to work properly. If you have the fan blasting away all the time, you have operating conditions (wet, cool days, early in the mornings, etc) where the head pressure drops to the point that the A/C just doesn't really do any work. Even modern cars use pressure switches, listen next time you hear the compressor kick on, there's a delay before the fan kicks in. That's the pressure switch doing its thing.

Also make sure you upgrade your alternator if you're running the fan on high speed. The 70A in the cars that fan was designed for won't keep up at idle if you're running the A/C. You need 90A+ unless you just hate your battery.

There are no shortage of hot days here--most days are 100F or hotter. But, I would like my car set up to work properly in all climates and not just extremely hot ones.

The air conditioning in my car has never worked the entire time I've owned it and now it's coming into the seventh summer I've been in the Phoenix, Arizona area.

I didn't realize that having the condenser be too cool could adversely effect the performance of the air conditioning system, but I will keep this in mind. I'm all for keeping the stock wiring if it's better. I just want the best, which ever way it is.

I upgraded the 55 amp alternator to an 80 amp one.