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  #1  
Old 05-12-2007, 10:41 AM
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88 300E Relay Box Question

I am trying to get my aux fan to come on. Opened the relay/fuse box and all of the relays are the same except for position E which has a two-fuse top on it. They are all the silver ice cube relays. Is this right? I thought that the fan relays in positions B and C were supposed to be different. My car was built in 9/87 and chassis #A592186.
The fan works with 12 volts from the battery. The blue 2-pin temp switch cuts off the AC compressor when jumped. The strip fuse has been checked and cleaned. Am I in the right direction here? Any help much appreciated.
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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out
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  #2  
Old 05-12-2007, 10:52 AM
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You need to identify the K9 and K10 relays, low and high speeds. The low speed relay powers the fan through a resistor which I think is close to the drier on your model. Looks like a ballast resistor for an old ignition set-up.

Be sure power gets to it and through it.

K10 should be position "B" and K9 should be "C" K10 is low speed which is the one that comes on for AC pressures high.
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Continental Imports
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  #3  
Old 05-12-2007, 10:58 AM
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Should the relays in B and C be different than the 20A/30A ice cubes? I have seen relays online that are specific to the high speed circuit.
__________________
1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out
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  #4  
Old 05-12-2007, 11:09 AM
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The relays you refer to are for newer models I think after 93. The "B" and "C" position relays should be the standard silver 5 pin relays.
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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  #5  
Old 05-12-2007, 11:14 AM
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Very helpful, thank you. Now I have switched the relays in B and C as suggested in another thread and still no fan. I also read that the fan circuit went through the CCU. Is this true of my year and model?
__________________
1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out
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  #6  
Old 05-12-2007, 11:31 AM
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The low speed relay is turned on based on a ground signal from the AC high pressure switch. It is a simple off/on circuit with no electronics.

To verify the fan portion, pull the "B" relay and verify power to the "30" socket position (the positions are noted on the relay match the pin to the socket). If you have power then locate the 87 socket the same way and use a fused jumper wire to bridge 30 and 87 and the fan should work. If it doesn't verify that the new circuit does indeed have power still and then LOOK AT the resistor!!! The power will be on one side. If there is indeed power on both sides then you either have a bad ground to the motor, a bad circuit from the resistor to the motor, or a bad motor!

Continuously monitor what you think is power because as soon as there is a load on the circuit that unloaded voltage may go away. If it does walk the testing right back to the source and find the break. The test points would be the motor input, resistor output, resistor input, pin 87 of "B' relay, pin 30 of "B" relay, and fuse "D".
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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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  #7  
Old 05-12-2007, 11:54 AM
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Thanks Steve, that should keep me busy for a while!
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1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out
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