Hi: I have a 300 D 2.5. it is a new car for me but I notice that Climbing a Hill it goes a little High with the AC on. It almost touch a 100 C with a few seconds at 70 MPH. I pull over and notice that the 2 electrical fans was not working and also on a traffic jam I have to turn off the Ac because I found the 2 electricals fans are not working. What should I look for, And where is the sensors. Thanks for any procedure you can tell me to checks the fans. Fan clutch looks pretty OK it is Brand New.

FErnando Q

it should be one of those with 2 prong plug sitting on top next to valve cover.
U can try using a paper clip to short the two terminal together to see if the fan comes on.

Usually there is a 2 prong connector near right inner fender ( as standing infront of car) u can trace the wire to the fan, unplug her and run 12Volt see if the fan comes on. Good idea to fuse your power wire, atleast if it shorts it wont produce a big spark. 20 amp may do or resettable type so u wont blow one.
Is polarity sensitive, or wil reverse the fan LOL.

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O.K.
The auxiliary electric fans are activated two different ways:
1.Engine OverTemp. (I believe this sensor is screwed into the housing at the
top of the head where the top radiator hose attaches))
2.Refrigerant OverTemp. ( #292 in the Pictogram)(Screwed into the Reciever/
Dryer)

#295 in the Pictogram is the Series Resistor for the Fans (White Ceramic holder behind the Driver's Headlight.)
12 Volts Positive to the bolt on the rear gives you low speed fans.
12 volts Positive to the bolt on the front gives you high speed fans.
[That will tell you if the fans and the resistor are working]
There are also Two relays in the relay section of the fuse box(Behind the fuses)
AND a couple of fuses

The Engine Overheat sensor MAY not even activate until engine temps reach
105c,depending on which color sensor is used.

EDIT: This is the only source I know of for ordering the FSM on 2 CDs for
the W124 chassis:
http://www.mercedesmanuals.com/Mercedes_Benz_W_124_Manual_p/w124.htm

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'84 300SD sold
124.128

If your auxiliary fan is not coming on under the thermal loads you're describing, there's a failure in one or more of the following: (1) the pressure sensor attached to the receiver/drier, or (2) the series resistor, or (3) the coolant temperature sensor on the valve cover, or (4) the fan motors, or (5) the wiring connecting any of the foregoing. As Compress Ignite says, the low-speed fan is triggered by a signal from the pressure sensor (when the compressor reaches something like 15-20 bars), while the hi-speed fan is actuated by the temp sensor (when it reaches about 105C, regardless of what the A/C is doing at that instant).

As for testing the temp sensor, yes there is a test available using a multimeter. You should get the service CD for complete instructions and specs. But because you state that the aux fans were not operating when you were running A/C going up a hill and raising temps to near 100C, it sounds like the pressure sensor is not turning the fans on to low speed as it should. Remember, low fans should come on whenever A/C is engaged and compressor pressure reaches ~20 bars, even if the motor isn't overheating. The CTS (temp sensor) isn't going to do anything until at least 105C, by which time the low fans should already have been triggered by A/C pressure.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Simple test procedure:
Test 1:
Key ON
Disconnect the 2 wire connector from the blu sensor..that will default High Fan.

Test2:

At reciever/drier, thereis a sensor with a 2 wire pigtail..slide the insulation sleeves back at the end of pigtail and jumper those 2 wires ..that will get you Low Fan.

Test3:

Bring a jumper wire over to the R15 resistor [ behind driver headlamp] from battery pos. Jumper to the single wire on R15 will get you Low , jumper to the 2 wire side of R15 will get you High..If NO, then the fan is bad [ or wiring to fan]

If NO on first 2 test test, you have a fuse/relay / or reistor problem...If YES , then you have a low refrigerant problem.
Post results of these 3 test.
The first 2 test verify sensor circuits , the last test verifies the fan motor/s.

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A Dalton

I disconect te Blue Sensor that is on the Head near the Radiator Hose & one of the Fans is Working strong the Other Still not working I leave the Switch Disconected so the fan is working as soon I put the key switch ON. I will try to figure out the Other test And let you klnow I will put Photos so you guys can help me Thansk A lot

FQ

Compress Ignite:

I call Mercedes for the srvice manual and they told me they do not produce those manuals or Not have it Any more. A question Would you make a Copy for Me, I'll pay you if is OK Thansk

FQ

Tap the front hub with a rubber mallet on the non-working fan and spin it to make sure it is not bound. Key ON..............

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A Dalton

AD,

Thank you.

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'84 300SD sold
124.128

You're Welcome.........

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A Dalton

Sorry for the Late I had a Comp Problem... I did The Test and Tthis Are the results:

I take out the blue sensor and fans were High

2: I jump sir Conditioning Sensor and I get low Fans

I believe is the Air Conditioning Sensor or What Can it BE?

FErnando Q

S'view,
Yeah, I don't know what kinda silly game MBUSA is playing with the FSM CDs.
I've sent you a link to another source.

You're Two for Three on Mr. Dalton's test sequence.

Both of your different "Sensors" wiring for the Auxiliary Fan System are working properly. (Tests #1 and #2, Which you've completed.)


You've proven the Low Speed Fan circuit works by jumpering S32 the sensor.

only two things would keep the Low Speed Fans from activating, NOW:
1. Low Level of Refrigerant in the A/C system.
['Not enough Refrigerant = s No Pressure/Temperature to "trip" S32]
2. Bad S32.

How did the nonworking fan react to a diagnostic "whack"?

Picture of S32 below.
Test it by turning on A/C with the engine ON,
UNPLUG THE SYSTEM'S WIRING FROM THE S32 CONNECTORS.
OTHERWISE THE SYSTEMS VOLTAGE/AMPERAGE WILL BLOW THE MM!
Connect leads of multi-meter to the two S32 connectors.
With MM turned to "Continuity" the S32 should show a Closed
Circuit.

When the A/C is On How well does it "Cool" ?

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'84 300SD sold
124.128

uyuytru

well this is what I did:

1: I take out the Connectors on the S32 and I put a voltimeter on the S32, when I turn the AC On the Continuity reading moves. I turn the AC off and no reading at all.

2: The air Conditioning it is cooling pretty Ok, But i do not know the pressure, I will going tomorrow to check the pressure. What should be the correct pressure on the system? Thanks

looking for answer, will find one, Thanks


FErnando Q

Looks to me that could be the pressure on the system, What do you think??