aux fan 300E - PeachParts Mercedes-Benz Forum

csnow · #1 05-22-2003, 02:43 PM

1986 300E

My aux fan is not coming on @ 100+ degrees (AC not working right now, so should not be a factor).
Used to kick in @ 92 degrees (cool harness), but does not come on now at all (with or without cool harness).

Jumped the terminals at the temp sensor (zero resistance)- Fan came on fine.

Blindly assumed it *MUST* be the temp sensor itself (yes, I have a resistance meter...), so I pre-emptively replaced it.... and no good!

My new theory is that whatever component measures the resistance value must be faulty.

Now which component would that be? Is this functionality part of a multi-function relay?

Other ideas?

csnow · #2 05-23-2003, 11:06 AM

Btt
Anyone?

Duke2.6 · #3 05-23-2003, 12:09 PM

Maybe this will help. My '88 201/2.6 has a themostatic clutch on the engine driven fan and two auxillary electric fans at the front of the radiator. I believe your car has a similar setup and algorithms, but I'm not sure.

The themostatic fan clutch on the engine driven fan engages at about 100C based on the temperature of the exit air out of the radiator, and will cause "fan roar" when starting out from a stop in traffic and accelerating to about 3000 in first gear, so once the engine gets up to at least 100C increase the revs and feel/listen for increased engine driven fan effort. The two electric fans engage high speed at about 105C.

I believe the electric fans will also engage at low speed based on A/C demand, but I'm not sure of the algorithm.

Let your car idle with the A/C off and observe fan operation. Let it heat up to 110C if necessary. Once you get your A/C fixed, you can run the same test with the A/C on.

Duke

Arthur Dalton · #4 05-23-2003, 01:47 PM

On cars with the Cool Harness mod, you can not test the default system for fan because the parallel resistor of the mod is still in the circuit [ bridging the sensor connector wires/circuit with a larger R value than the computer uses for trip]
Remove the mod , then unplug the connector from the ETC sensor , and see if fan then comes on.
If it does , you have a bad mod resistor/and/or connection...
When you short out the sensor , you are simply putting zero resistance in parallel with the mod resistor and in a parallel R circuit, the RT value is always less than the smallest resistor..
so this shorting will bring on fan as the circuit has now gone from high R to low[actually none] . This triggers the fan..
Do a resistance/continuity test on the mod Resistor...

JimF · #5 05-24-2003, 02:04 AM

he said that the engine temp is 100+ with and without the mod resistor, and it still doesn't come on. It can't be the mod resistor.

He's asking what module (forget the temp sensor) that 'reads' the resistance and then engerizes the fans???

Arthur Dalton · #6 05-24-2003, 07:06 AM

I read 100+ to indicate it is breaking 100, but the temp sensor
cut-in on that car is for 107c..

So, the question is , how high are you over 100?

If the engine coolant is not reaching 107, you do not know if the module
is being triggered at correct spec..

JimF · #7 05-24-2003, 11:59 AM

What module makes the 'decision' to turn the fans on???

So let me answer the question for my car. That may trigger an answer for his car.

The Climate Control Module, N22 does that. It's the main push-button panel located on the dash. The B10/8 sensor feeds this module and when the resistance gets to about 315 ohms, the signal is given to turn the fans on.

So what's the equivalent for his car???? Same module in the dash???

Corey, it could be that the wire harness is broken at or near the end of the plug that mates with the CT sensor. That would explain you problem nicely since an open circuit appears as high resistance. If you see that it's 'brittle', it's very likely to be open.

Could also be a bad sensor: measure the resistance of it when the car is warm. Remove the plug and measure with an ohmeter: sb about 320 ohms or so at about 95C. If way off, then something is wrong with the sensor.

Arthur Dalton · #8 05-24-2003, 02:51 PM

I doubt a sensor problem, as he has just replaced it ..he was thinking the same ..
The 300e has to get to 105-107 to trip N22, so until you know if that trip point is reached, true diagnosis can't be completed..
If a trip is found at spec without the mod harness, the next test is with the harness mod plugged back in. If the fan still has to go to spec [105-107] than the mod [ be it open, bad connection, R value, etc] is at fault . [ as if it was not there.. i.e.-open circuit]

Verify aux fan operation w/o mod .
If no change with mod, Check mod circuit..

pesuazo · #9 05-24-2003, 03:54 PM

Our cars (1986 300E) do not have a "module".
The sensor drops its resistance as temperature rises. Once the resistance is "low enough" the coil of relay K9 is grounded (via the sensor), this "pulls" the contactor and closes the circuit, which sends 12 volts to the auxiliary fan via fuse D (18 Amps).
The fact that grounding the sensor connector makes the fan work tells me that the relay K9 is good. This relay would be the "module" that makes the 'decision' of turning the fan on.

"My new theory is that whatever component measures the resistance value must be faulty."

Unfortunately, that would be the sensor.

Remember, the sensor that reads the engine temperature for your instrument cluster (the last one on the head) is not the same that drops its resistance with temperature for the auxiliary fan (the middle one on the head?). (Although some models have the 4 pin type, which includes both)

I would follow the troubleshooting indicated by Arthur Dalton, and JimF.
It appears one or more of the wires is so brittle and corroded the circuit "thinks" (high resistance) is open unless grounded directly.

Arthur Dalton · #10 05-24-2003, 04:36 PM

The sensor drops its resistance as temperature rises. Once the resistance is "low enough" the coil of relay K9 is grounded (via the sensor), this "pulls" the contactor and closes the circuit, which sends 12 volts to the auxiliary fan via fuse D (18 Amps).
The fact that grounding the sensor connector makes the fan work tells me that the relay K9 is good. This relay would be the "module" that makes the 'decision' of turning the fan on.

"My new theory is that whatever component measures the resistance value must be faulty."

Unfortunately, that would be the sensor.<

The ECT on the early models that had the 105/115 On/Off
switch worked that way, but the units with a Thermistor [negative/temp ] do not complete the relay coil to ground .. They feed the ACC -N22 Control and that trips ground to the coil side [primary] of the relay via a transistor in the ACC control at the proper ohms .
So, the N22 is both the reader of the thermistor and the grounding switch for the coil of the relay...
Some S class cars have different resistor trip points [ usually V8s]
than do some of the smaller 124 cars. [ lower temp cut -in]
What the Mod does is parallel up the ECT to fool the N22 reader into thinking it is at trip time , as it only sees resistance [ mod actually puts the ECT out of spec.] , allowing a lower temp/resistance option...

csnow · #11 05-27-2003, 10:03 AM

Wow.
So much input. This is great, thanks.

Let's see if I have this right:
Coolant sensor drops resistance at 105 degrees, which kicks (relay?) N22, which in turn kicks relay K9 to turn on the fan.

If I am understanding this right, one possibility is that the circuit has some 'static' resistance, such as a corroded wire, terminal. or somesuch. If true, the resistance may remain too great even when the sensor drops to exerting whatever ohms at 110 degrees, which is the highest temp I tried.

Jumping the sensor, (theoretically Zero resistance) starts the fan (just one fan in my case), but perhaps this is not really Zero resistance, but a small enough R value to trip N22.

Unless the harness has conductivity problems, it would seem that I need to replace Relay N22. Have I proven K9 is working, or can it partially fail?

Anyways, I will take actual resistance measurements and publish my results.
Thanks for your help.

sjsfiji · #12 05-24-2004, 07:47 PM

"Coolant sensor drops resistance at 105 degrees, which kicks (relay?) N22, which in turn kicks relay K9 to turn on the fan."

csnow,

I think your car is hooked up like mine. Here is how mine works (though I'm having problems with it)

There are 2 circuits: 2 sensors, 2 relays, 1 resistor, 2 mods of operation... (at least i think it's how it works via the schematic).

When the refrig. pressure switch located at the rec/drier get's to some press., it activates the fan in LOW speed via pre-resistor relay (relay B in fuse box) throught the RESISTOR.

When the Engine temp sensor (3 pole/2 connectors) located next to the cam cover @ left side gets to a certain temp (105 deg C i think), it activates the fan in HIGH speed via the aux fan relay (relay C in fuse box).

Hope this helps... if not, check CD1, Electrical Diagram, page 55.

late,