'93 w124 M104 Aux fan ground location.
 

Hey fellas I am having issues on my aux fan on the wifes '93 300te m104. I have a bad ground (w9) on the system, since when I pull the a/c high presure switch get nothing and also pull the coolant high temp sensor, nothing as well. When I jump a ground to each side of the switches, I get both a high and low fan speed like I should. I looked at the wiring diagram, it says W9 is the ground location, and its behind the left headlamp, but there is no ground point there. Than I looked through the same manual and it says w9 is in the front lower left engine compartment behind the Receiver Drier. But per the wiring, w9 shares with the left headlamp, so while the headlamps where on, I disconnected w9, left headlamp stays on!

I'm just trying to find the ground, since I suspect it corroded, hopefully allowing me to run the air con! Thanks guys in advance!

-----dirtyhands!

Pulling the connector off the high pressure sw will not get you an aux fan...you have to jumper the sw.
And if you have aux fan by grounding the sw, then you do not have a bad fan motor ground ..if you did , it would not run , regardless of the sensor.
The sensors are on the relays primary side . Not on the load side.
To test the fans, bring 12v pos from bat over to each side of the pre-resistor, one at a time.. the side with R15 in Series is low speed , the other is high speed . That will test fan motors integrity and ground , along with R15 speed resistor. If yes, then problem is upstream of R15.
Suspect for no fan w/ac is low refrigerant b/c you claim fan activation with high side sw grounding..........

Where is the preresistor located at? Is it to the right , behind the right headlight?

No, it is the left.

Remember , all R/L orientation on cars is expressed from the Operator/Driver position.
That eliminates any errors one may have [ some people look at the car from the front]..and others use Driver side /Passenger side ..again, no good b/c there are both RHD and LHD Benzs.
Operators Position/Direction of Travel...simple, and never wrong.

ok, if you are talking from the driver side looking forward then it is on the left. Is it the thing that looks like ceramic with two wires on it?

<ok, if you are talking from the driver side looking forward then it is on the left. >


Yes ..that is what "Operators Position/Direction of Travel" means.
..R15 is the Ceramic resistor ..referred to as Aux Fan low speed pre-resistor.
If you jumper 12 V to include the R, you get low fan, if you jumper to the other side, you are by-passing the R and you then get High fan b/c by-pass gives you full Bat V.
This is just a mid-circuit test point we use to make a rapid diagnosis on lack of aux fan complaints........it is very common for these terminals to burn off b/c of the heat of R15..this test does not require key ON or any other chassis controls , as it is a direct battery jumper test, the same as if you had the fans on a Bench and were testing them with Direct 12v....which is why I posted if the other poster had a fan w/jumper , he knows he does not have ground problem on fans as he suspects b/c they would not come ON with this test as you are not bringing neg power over from battery..you are picking that side of the completed ciruit up from the chassis ground..so that ground diagnosis is included in the test results. If he gets No fans with the bat pos jumper after R15, THEN the suspect can be a bad ground... [ or bad motor].......... If one gets High fan , No low fan with the jumper test, you know the R15 is open...........Simple

Thankyou, I dont think I have ever heard my fan come on, I am going to give that a shot.

Very Good ..
Be aware that these mentioned test are for his 124 chassis...you do not mention a chassis, so don't do these test if you have a different chassis.

It is quite common for a fan fuse to blow and the owner never noticed it b/c the aux fan just was not called for...they do not run all the time ..the high will only have a demand when the coolant temp reaches 105C and the low only gets a demand when the a/c high side pressure gets dangerously high ... That is why they are AUX FANS, They come into play when a taxing condition presents itself. A quick test in your case is to simple turn key ON and pull the 2 wire connector off the blue sensor on the termo housing...that will default the ACC Panel to High Fan, verifying that circuit as OK.
Low fan circuit test is to jumper the High pressure sw pigtail wires at the AC reciever drier.
The part that confuses DIYers is the fact that these fan circuits are completely independent of one another, with the exception that they share the same fan motors ..otherwise, they have their own relays, sensors , wiring, fuse , etc and have to be diagnosed as seperate circuits, b/c they are .

Thanks Arthur! You saved me before, and your truely great at what you do, thanks for the help! I will check it out tomorrow! I have replaced the resistor, relay, and both aux fans! I hope its something simple! I will keep you posted!
---Dirtyhands

Very Good,
Just do those simple test and see where you stand with the results of them.

I have a 1986 300E, so I am good to go. Thanks.

Sorry, I didnt mean to steal your post.

Nah don't worry, we are we both have a common goal of having operational aux fans! ;-)

<I have a 1986 300E, so I am good to go. Thanks>

Coolant temp sensor test does not apply to your year. My test were written for his year...['93 HMF/SFI-124]

Allways state that info, b/c your sensor system is completely different. You do not have thermistor style CTS.

Your explanations have probably helped a whole generation of MB DIYers by now. Thanks. By installing Jim F's Cool Harness on the thermo sensor, I guess all I'm doing is putting a second identical thermistor in series with the existing CTS, and that's enough added resistance to lower the temp threshhold for fan activation. Is that about it?