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#1
06-09-2003, 09:13 AM
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Auxiallary Fan question
560sl 1986
Had the R12 topped off at an a/c place that I respect and the tech said that my aux fan wasn't working. He put a jumper to it and it operated OK. It is apparently controlled by an internal switch at the filter/drier. Tech says that most likely the system would have to be open to fix the switch. Does that seem right? Would I be OK just jumping it with a wire in the summer and leaving it off in the winter? Don't really want to change filter/driers and goto the extra expense unless it't going to cause me problems during my 15 minute commute. Thanks. J. Boggs 
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#2
06-09-2003, 09:40 AM
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The switch is almost never the problem.
It is usually the relays or fuses. To verify the whether it is the switch, unplug the two wires going to it and join them. If the fan works either the switch is bad or the A/C has too low of a pressure to activate it (another likely senario). So the next step would be to put a gauge on the high side and verify the pressures. I just looked to see what the pressure was (20bar = 284psi) and looking at the wiring I remembered what the most common problem was: the ballast resistor that makes the low speed. It burns out like a big fuse.
__________________
Steve Brotherton Continental Imports Gainesville FL Bosch Master, ASE Master, L1 33 years MB technician
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#3
06-09-2003, 11:05 AM
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As Steve says, rarely ever the switch, but gages are required to see if the a/c gets to the trip pressure spec of the switch.
If only a couple of ounces low on F-12 charge, this will create that condition. [ cooling, but not enought high side pressure to trip fan] A point to note is that if you are running at low thermal load [ low ambient temp], it is possible for the fan not to be called for . This is something to consider in Northern climates, where one will not see that condition exist in Hot climates [ where high thermal load is constant].. If you jumper the switch and fan works , then all electrical components [ except possible switch] are OK, including the dropping resistor.. With R-12 , [ as compared to 134a],you can judge freon level fairly well by observing the eye-sight at the drier . If it is clear and still no fan, likely the sw, but if foamy/bubbling, likely low on freon and just not getting to sw cut-in pressure spec [most likely probable]..
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#5
06-09-2003, 10:32 PM
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The aux. fan on my 450 slc works when I jump it but even if the engine temp. gets up towards the top of the gauge it still doesn't come on. Is the fam only related to the ac system or will it come on if the eng. temp. is too high?
Where is the relay for this fan? Is it fused and if so is the fuse in the box inside on the pass. side of the car?
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#6
06-10-2003, 09:24 AM
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Ballast resistor?
The way the fan speed is reduced to have a low and high speed in early cars without electronic control is with a Ballast resister. I am not sure where the term comes from. Most have only heard of its similar use with ignition coils. Maybe the term only applies to resistance added to ignition. This resistance is wrong for ignition coils I have seen but standard ballast resistors on MB ignitions have these resistances: 0.4ohm, 0.6ohm, and 0.9ohms. The ballast resistor on most MB fans looks identical to these other ballast resistors but has a resistance of 0.2ohms. I have even seen these resistors catch fire on 126 cars. The resistor is beneath the brake booster in that hole that fills with leaves. Once dried sutiably they can be lit off buy the heat.
__________________
Steve Brotherton Continental Imports Gainesville FL Bosch Master, ASE Master, L1 33 years MB technician
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#7
06-10-2003, 08:49 PM
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Quote:
So if I jumper it, and it DOES NOT turn on, then the possible culprits are fuses/relays and resistor? I just "rebuilt" the resistor, so I am fairly confident that its fine. I just had the AC recharged too, so I don’t think that’s the problem either. Which relay controls the low circuit circuit? (part #, exact location, or even color will help). I know the dark green with built-in 30A fuse controls the high side.
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#8
06-10-2003, 10:52 PM
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ok update, looked at the blue hella relay (position B I believe) the 20A fuse was blown. I didnt have any 20A fuses on hand so I put in a 30 until tomorrow when I can get a 20. I let the car run for about 7-8 min and the fan didnt come on. Not sure if that means anything. What do you think?
BTW 1994 E420
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#9
06-10-2003, 11:29 PM
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Quote:
Slot C is low fan [ pre-resistor a/c fan].. It should be noted that these positions are the oposite on the 123.034 than on the 124.032 cars [ 104 eng]
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