is the low speed auxillary fan always supposed to come on with ac - PeachParts Mercedes-Benz Forum

dennish · #1 07-03-2009, 07:27 PM

I've been reading the threads re aux fan, but it's still unclear to me if the low speed on aux fans are always supposed to come on with the ac on or is it variable depending on the pressure in the system? Does the pressure vary? My high speed comes on about 107 C, but the low speed hasn't been comming on with the ac on my 1992 190E 2.6. thanks

macdrone · #2 07-03-2009, 07:52 PM

Yes it varies with pressure, the red switch on the side of the drier controls it, jumper those wires together to test it low speed should come on. Thats what is supposed to help the A/C when in town driving and stopped.

Arthur Dalton · #3 07-03-2009, 08:34 PM

No.

As MacD has stated , it is a pressure sensed ciruit and it is there to check a "High Thermal Load" condition. That condition does not exist if the ambient temps are not taxing the systems high side pressure. But if a thermal load starts to tax the system, [ like in traffic or high ambient temp.], then the fans get triggered by the rising condenser pressure and that additional airflow takes that burden off the system..that is why it is an Aux Fan and that is it's sole function. Additional aux. airflow accross a high thermally loaded condenser.
The problem when diagnosing the fan circuit when ones jumpers the high side sw and that test gets a low fan, one can assumes the sw is bad...but the real diagnosis is more often a low refrigerant condition..where the a/c still cools , but the high side does not get to fan sw trigger pressure b/c of inadequate refrigerant level .

This lack of low fan w/ac can also effect the coolant temps b/c of passive heat exchange between the condenser and radiator. I see many coolant temp complaints that are misdiagnosed b/c the low fan cicuit is not working
and it is not considered in the diagnosis for temp over-heat.. But it should be.
If one sees a high fan triggered at 107 coolant temp w/ac use, first suspect is low fan check, right off the bat. It is the lack of a low a/c fan that causes the coolant temps to start rising, not a coolant system malfuntion.

And as previously stated , a simple jumper of the red sw by crossing the 2 pigtail wires will verify the low fan circuit, minus the sw/refrig level.
So, that is the first test.
If No fan w/jumper , the problem is the low fan relay/fuse/or dropping resistor.
If Yes w/jumper . the prob is sw or refrig level, but only under thermal load condition.
That chassis has R12 and an eyesight glass, so refrig level is easy to verify w/eyesight.

dennish · #4 07-03-2009, 11:32 PM

OK... that helps a lot!! thank you!!

TimFreeh · #5 07-04-2009, 09:07 AM

Quote:

Originally Posted by Arthur Dalton

...But if a thermal load starts to tax the system, [ like in traffic or high ambient temp.], then the fans get triggered by the rising condenser pressure and that additional airflow takes that burden off the system..that is why it is an Aux Fan and that is it's sole function.

100% correct but from my observations in the real world if you have a semi-full charge of refrigerant in your A/C system the condenser pressure will always cause the aux fans to engage if the car is in stop-and-go traffic at ambient temps > 65-75F.

They may not run 100% of the time but they will run around 30-100% of the time, again depending on ambient temperatures.

If its 85F outside and your start with the A/C on and let it idle in your driveway the aux fans should engage within a few minutes.

If they don't you have three possibilities 1) your fans are not working or 2) your refrigerant level is low or 3) your A/C isn't working.

Arthur Dalton · #6 07-04-2009, 11:00 AM

<100% correct but from my observations in the real world if you have a semi-full charge of refrigerant in your A/C system the condenser pressure will always cause the aux fans to engage if the car is in stop-and-go traffic at ambient temps > 65-75F.

Chassis dependant..some models have the 16 Bar sensor , some have the 20 bar...but regardless, low refrig will not get you a low fan cut-in as Designed .
And the first suspect on No Low fan after jumper verification is low refrigerant. That is the point that most are not aware of.

Don;t confuse High Thermal Load with just ambient temps.. high thermal can be cause by many other factor besides ambient temp...[engine coolant temps/dirty condenser/high cabin temp/ window shading/etc]
Ambient is just a part of a possible high load condition.....and the fan cut-in is not geared to ambient, it is geared to Thermal Load.

ps2cho · #7 07-04-2009, 11:07 AM

Does anybody have a link to adding your own relay like MB did on some models to turn the aux fan on at low speeds? I'd like to do it to my wagon.

david s poole · #8 07-04-2009, 12:47 PM

one thing missed in all this advice.mercedes had a problem with the wire going to one side of the dropping resistor for low speed fan operation.look at the resistor and wiggle the wires at each end.one side is likely to come off in your hand.repairing that wire connection fixes the problem.

Arthur Dalton · #9 07-04-2009, 02:28 PM

If jumper verifies fan circuit , then you do not have a R15 wiring condition...but you are correct in that common fault.

That is why we do the jumper test first.
..but you know that anyway

dennish · #10 07-04-2009, 03:05 PM

david s poole, where is the dropping resister for the low speed fan ? I'd like to check it

Arthur Dalton · #11 07-04-2009, 03:09 PM

Did you ever do the pigtail jumper test mentioned???????????/

DANSMB · #12 07-04-2009, 03:21 PM

Quote:

Originally Posted by Arthur Dalton

No.

As MacD has stated , it is a pressure sensed ciruit and it is there to check a "High Thermal Load" condition. That condition does not exist if the ambient temps are not taxing the systems high side pressure. But if a thermal load starts to tax the system, [ like in traffic or high ambient temp.], then the fans get triggered by the rising condenser pressure and that additional airflow takes that burden off the system..that is why it is an Aux Fan and that is it's sole function. Additional aux. airflow accross a high thermally loaded condenser.
The problem when diagnosing the fan circuit when ones jumpers the high side sw and that test gets a low fan, one can assumes the sw is bad...but the real diagnosis is more often a low refrigerant condition..where the a/c still cools , but the high side does not get to fan sw trigger pressure b/c of inadequate refrigerant level .

This lack of low fan w/ac can also effect the coolant temps b/c of passive heat exchange between the condenser and radiator. I see many coolant temp complaints that are misdiagnosed b/c the low fan cicuit is not working
and it is not considered in the diagnosis for temp over-heat.. But it should be.
If one sees a high fan triggered at 107 coolant temp w/ac use, first suspect is low fan check, right off the bat. It is the lack of a low a/c fan that causes the coolant temps to start rising, not a coolant system malfuntion.

And as previously stated , a simple jumper of the red sw by crossing the 2 pigtail wires will verify the low fan circuit, minus the sw/refrig level.
So, that is the first test.
If No fan w/jumper , the problem is the low fan relay/fuse/or dropping resistor.
If Yes w/jumper . the prob is sw or refrig level, but only under thermal load condition.
That chassis has R12 and an eyesight glass, so refrig level is easy to verify w/eyesight.

These two posts call this a pressure switch but I just pulled one from a car I am parting and it looks like a temperature switch not a pressure switch, the hole where it screws in is not drilled into the freon area and no charge is lost when removing it so it can't be pressure sensing.

Dan

macdrone · #13 07-04-2009, 03:26 PM

It doesnt go through? The red one are you sure? That would be awesome as I need to replace mine. I figured it went all the way through even if it was for thrmal load. If I wont lose my charge I will replace it post haste to get my low speed fans back and cold air with in town driving.

Arthur Dalton · #14 07-04-2009, 03:26 PM

Quote:

Originally Posted by DANSMB

These two posts call this a pressure switch but I just pulled one from a car I am parting and it looks like a temperature switch not a pressure switch, the hole where it screws in is not drilled into the freon area and no charge is lost when removing it so it can't be pressure sensing.

Dan

That is an early chassis car that used drier temp..we are not taking about the early cars here , we are talking the pressure sensed cars...and that is why the Factory specs for the switch are in BARS, not DEGREES

The hole goes thru to the sw and it read BARS pressure.

But the parts books never did change the name , so it can confuse those who are not aware of the change. I heard they also stayed with temp sw designation so as not to confuse with the other drier sw , which is called the pressure switch.

Take that sensor off and you will have an empty system in about 30 seconds......

DANSMB · #15 07-04-2009, 03:31 PM

The one I pulled was from a 1983 300D. I suspect the switch is bad in my wifes 1983 380SL as the fan doesn't come on unless I jump the wires, would this car have the pressure style switch where I will loose the freon?