AUX FANS turn on at start-up

The aux. fans on my 91 560sel turns on as soon as i start the car and stay on for about say 15 minutes before turning off.

Is this normal or is there something wrong that i need to check out. It does not matter whether I start with the engine cold or hot, or whether the AC is on or off...

would appreciate your help...

[Benzmac]

There is a small blue switch on the front of the engine that MAY have failed...Have it checked.

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Benzmac:
Donnie Drummonds
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

BENZMAC:
I am looking at the aux fan schematic, and are you referring to S32 ( refrigerant High pressure switch) as the small blue switch?

The diagram says that this switch is closed
20 BAR/ Open 15 BAR. What does this mean, how can i test if this switch is bad?

Do you know exactly where this switch is located at?

thx
emmy mendoza
91 560SEL

[MB STARS]

The auxiliary fan switch is blue color with two wires (B10/8)part number 008-542-45-17

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MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner

now i got it...it is described as a Coolant Temp Sensor located at the front top of engine.

Any tips on how to test this sensor?
The spec says that it is 130 degrees, and it looks like a temp sensitive resistor.

What about removing this sensor? Is there anything that I should know and be careful about?

thx again.

[stevebfl]

The first thing I would do is identify whether you are running on the low speed fan or the hi-speed fan. I would identify the two relays and pull them one at a time to see which one is working. the k10 is the low speed and the k9 is high speed.

The low speed fan is controlled matter of factly by the S32 high pressure switch. The idea being when the refrigerant pressure gets too high the fan comes on.

The high pressure fan relay is controlled by the A/C pushbutton control unit under some program. Look for the ground signal at pin 5 term 85 on the K9 relay. If this is grounded both switches S25/5 and B11/7 need to be evaluated as the pushbutton controller is telling it to come on for some reason. These relays should be the standard silver 5 pin relay. There should be a number of them in the box. Try different ones to evaluate the possibility of a stuck relay. Check the number on the side of the relay if its the same then it will be interchangable.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

stevebfl:
first of all pls clarify S25/5 and B11/7.
In my diagram, it shows that S25/5 is applicable to MY1986 and MY 1987 only. The component location photos seems to indicate that this S25/5 is in the same location as B10/8. Also, I cannot locate B11/7.

This is what I did with engine basically cold:
1. I isolated K9 as the relay that turns on the aux fans. This is relay code C in the fuse/relay box.
2. Pin 85 of k9 is grounded with the ignition on.
3. Pin 85 of k9 is still grounded with B10/8 wires disconnected and ignition on.
4. B10/8 ( color blue installed on top of t/stat) tested with high resistance.
5. K9 is the only black relay, so i could not test symptom with any other relay.
K9 contacts pin 30 and pin 87 tested NO (Normally Open)

would appreciate further assistance...

[stevebfl]

If I have time I will go back to the shop and look at the diagram again, all my comments now will have to be from memory.

The S25 switch in the diagram is a two-fold switch in 86-87 after 88 I interpreted the diagram to infer that only the 115deg side of the switch was used.

The B11 is a variable resistor type sensor and keeps the control unit apraised of the engines temp all the time.

I forget the program but the controller starts altering the A/C cycle rate at around 105deg. At 115 deg the A/C is shut-off and the high speed fan comes on. I think both sensors take part.

Since you have a ground to pin 85 relay K9 you need to find out why the controller is asking for the fan. Unfortunately disconnecting sensors to the controller could initiate a fail safe type "limp home" strategy. If the controller sees an open circuit it likely will realize it to be wrong and it will substitute. In the case of something as critical as engine temp, it may just default to the fan on position.

Locating temp sensors on these motors is tricky as there are a number of them and their functions often change by the year model. The best way to test them would be from their input position to the pushbutton controller, it also allows checking the installed circuit. The problem on temp sensors is often the connetion itself. If you disconnect and check with an ohm meter the test may temporarily fix the problem leaving you no where.
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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 04-08-2000).]

stevebfl:
just to followup if you have had the chance to verify the S25/5 and B11/7 for a 915 560sel.

My diagram shows that S25/5 is for 86 and 87 models only, and B11/7 does not show on my schematic nor on the component location page.

One temp switch for aux fans referred to in the ACC function page is referred to as a switch in the receiver dehydrator. But again, it does not show in the schematic diagram at all.

Could it be an intermittent grounded connector in the ACC push-button control unit?

am quite puzzled and hope you could help...

thx again

[stevebfl]

I took another look at the diagram and
the S25/5 IS only used on the early cars (86-87).

The later cars have the sensor B11 which gives actually temp reading to the control unit so that it can use programing for all the various functions it controls: high speed Aux fan and A/C clutch on siganal.

The fact that you have found the 85 terminal of the K9 relay to be grounded is key here. I would test the temp circuit at the pushbutton controller, but then again I have the testor. It probably would be easier under the hood. The B11/7 sensor is right by the engine lift bracket by the oil dipstick. If you have problems with this let me know. I am looking in the printed ETM and being that you have the last year of that chassis it is possible I'm missing a supplement. I have numerous other ways of viewing this if this isn't the way it is.

The problem could be the pushbutton controller but I would definitely check its imputs first.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

steve thx;

is it possible that you are referring to B10/8? B11/7 does not exist in my diagram. This sensor is blue and it is located on top of the t/stat by the lisft bracket. Together with B10/8 is B13, the temp gauge sensor.

I have tested the fuse box term 85 grounded with the wires from B10/8 disconnected. Is there any other test from undere the hood before I open up my center console?

[stevebfl]

Another diagram I have shows the switch as B10/8, strange that the diagrams are the same other wise and the theory is the same.

You do not want to test this with the sensor disconnected as I suspect the controller recognises this as a fault and turns the fan on as a default (by grounding the relay). I do not know the resistance of the sensor but if its like most engine coolant sensors the resistance will be around 300ohms at around 80deg C.

The inside pushbutton controller does have self diagnostics. The codes can be gotten without special tools. You will need one of the following tools to do this; a logic probe, an LED, or a DSO (scope). If you can handle and have one more more of these items I can tell you how to do the code retrieval.

BTW the code manual for that model refers to code 12 and 13 as being short and open circuits in the B11/8 temp sensor. Strange!

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician



[This message has been edited by stevebfl (edited 04-11-2000).]

Steve;
seems like testing the resistance of B10/8 is the most practical for me at this point,
and replacing it if needed.

thx

i just want to give an update on this issue.
I tested the resistance of the sensor, and found to be erratic. it can go from 5 ohms to infinity without changes in temp. I then replaced the sensor and solved the problem finally. No more aux fan coming on during start up, and temp stays between 80 to 95.