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Old 06-27-2002, 12:06 PM
Registered User
Join Date: May 2000
Location: Georgetown, MA
Posts: 84
Auxiliary fan operation

In my 86 300E, the auxiliary fan does not come on when the temperature exceeds 115c. The fan works with direct power from the battery. There is power on the 30 terminal from fuse 7 and from fuse D on the K9 and K10 relay. Apparently the relay is not moving from the 87A to the 87 position because when I manually jump the 30 and the 87A the fan works. What am I missing? Are these relays prone to failure? The fan also works when I jump the high pressure switch. Any help would be appreciated.
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Old 06-27-2002, 01:44 PM
Posts: n/a
Coolant temp is not what causes the aux fan to come on. High side A/C pressure causes the fan to come on to bring down excessive high side pressure to prevent A/C system damage. There is a high pressure switch, probably mounted on the filter drier.

If the A/C charge is a little low, the fan may not come on at all.

Good luck,
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Old 06-27-2002, 02:18 PM
Senior Member
Join Date: Apr 1999
Location: Walnut,CA,USA
Posts: 554
The auxiliary has two operating modes
Low speed activated by pressure switch at a\c drier pressure over 20bar with relay K10 and resistor.
High speed activated by temperature switch S25-5 at 105c coolant temperature with relay K9 without anyresistor.
MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner
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Old 06-27-2002, 03:19 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,768
You have high fan when jumping 30-87 and this is the load side of the K-9 relay, so the coil side of the relay is not pulling in or you have a bad relay.

You know you have power as the load fuse feeds both high/low speed fan. So the test is to ground br/gn wire at pin #2 at temp sensor for 105 cut-in. This completes coil side circuit for relay latch.
If high speed now comes on, change ETC sensor.

If it does not , you have a bad relay/open wire.....

I can send you the schematic , if needed
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Old 06-27-2002, 04:23 PM
Registered User
Join Date: May 2000
Location: Georgetown, MA
Posts: 84
Arthur, Thank you for the offer of schematics. I do have the MB Electrical Troubleshooting Manual. I agree with your diagnosis of a bad relay or an open wire. I forgot to mention that I jumped the temperature switch and the fan did not come on. Since both the K9 and K10 relays have the same part number, what role does the R15 auxiliary fan preresistor play? I do not remember seeing one when I traced and tested the wiring. Brutally hot and humid out and I do not think I will get back to troubleshooting this afternoon. I live across the border in MA. You know what I mean. Thanks for all the responses.
John O
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Old 06-27-2002, 04:29 PM
Posts: n/a
Not too sure on your car - but normally the fan will kick in low speed when one sensor activates ( hence the resistor) and kicks in at high speed when the other one activates - hope that helps.
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Old 06-27-2002, 05:14 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,768
The resistor simply allows K-10 to put the resistor in series with the fan circuit for the LOW speed [ a/c high pressure circuit]
This R is behind the drivers headlamp.
You can exchange the relay for the test of the High speed [ K-9] circuit.
You have to have a relay/ relay coil circuit fault because you stated the fan ran when jumpered 30/87.
And you know you have relay coil feed for both K-10 and K-9,
cuz they are the same fuse [7]. So the completion of the circuit is a switched ground [ This is what the ETC does].
My guess is open relay coil or coil pulling contacts in, but load contacts are burnt to point of non conduction.....
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