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  #1  
Old 07-01-2005, 06:16 AM
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Question regarding aux fan

I'm currently in the process of repairing my wrecked 300SD. I purchased a new auxillary fan but the one I got from the dealer is larger than the one on the car. While I can get mounting brackets for the new, larger one, the body shop guys don't know if I should use it as it may draw more amps from the car than the old, smaller one that came with the car. So my question is, can I put the larger one on the car without worrying about excessive battery drainage or should I just stick to the old, small fan? Thanks!

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  #2  
Old 07-01-2005, 06:30 AM
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Check the amp draw on each.... either written on it or by measuring under a load...the size may not make any difference... other things can effect amp draw...
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  #3  
Old 07-01-2005, 06:35 AM
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Thanks for the quick reply, but what do you mean by checking the am draw by "measuring under a load?" How do I subject a fan in a box with load to measure the amp draw?
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  #4  
Old 07-01-2005, 10:23 AM
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There is some way with a volt/ohm meter or amp meter to hook the two fans up to 12 volts and see what they draw. Even same size fan blades with different ' bites' would use different amounts... also the way the actual motor is constructed effects that... I am just saying you can't tell until you get more info ... you might get lucky and the larger one be the same load as the old one.... one ball bearing hysteris compared to old fashioned brushes and oilite bushings...
Load just means ' running with the fan blades attached ' in this case....
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  #5  
Old 07-01-2005, 11:35 AM
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It would be of interest to see the current draw from the new fan.

I'm just guessing, but I would estimate that the fan will draw something around 10 amps when operating. You probably can find an inexpensive DC ammeter that can measure this.

Set the fan up so that it can't fall over and connect one terminal to ground. Connect the other terminal to the ammeter. Then connect the ammeter to a 12V supply. The fan will run and you can read the ammeter.

If it pegs above 10A, then you might have some additional questions regarding current draw. If it gets up there in amperage, you will have fuse and wiring issues to contend with.

I can't recall offhand. What's the stock fuse for that circuit? Probably 16A?
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  #6  
Old 07-01-2005, 11:44 AM
LarryBible
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As Brian says measure the current draw and also find what the current draw was for the original fan. If you find it to be less than the value of the fuse don't assume that all is well. You will have to know what other accessories are fed by the same fuse and add up the draw.

If it turns out that the new fan draws no more than about 10% more current than the original there should be enough margin in the circuit to tolerate it. If not you will need to use the original circuit to energize a relay and use the contacts of the relay and a fuse to feed the new fan.

Good luck,
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  #7  
Old 07-01-2005, 01:47 PM
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Thanks all for the tips. I've been assured that the bigger fan uses the same amount of power as the old, smaller one. In fact, all '85 300SDs have the larger fan, and all '85 and up MBs have that same larger fan. So all is good and I'll use the larger fan. Furthermore, MB doesn't even make/sell the smaller ones anymore.
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  #8  
Old 07-02-2005, 08:27 PM
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Quote:
Originally Posted by braverichard
Thanks all for the tips. I've been assured that the bigger fan uses the same amount of power as the old, smaller one. In fact, all '85 300SDs have the larger fan, and all '85 and up MBs have that same larger fan. So all is good and I'll use the larger fan. Furthermore, MB doesn't even make/sell the smaller ones anymore.

I have the big fan on my SD and it works fine.

Jim
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  #9  
Old 09-01-2005, 07:19 PM
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I put the big fan on mine and it works well. In fact, I just love the sound it makes at full speed as I idle my car in the driveway for just a few seconds after a long highway trip. From about 10 feet away, it almost drowns out the diesel clattering. LOL
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  #10  
Old 09-01-2005, 07:39 PM
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Quote:
Originally Posted by braverichard
I put the big fan on mine and it works well. In fact, I just love the sound it makes at full speed as I idle my car in the driveway for just a few seconds after a long highway trip. From about 10 feet away, it almost drowns out the diesel clattering. LOL
Don, when you say "big fan" versus "small fan", can you provide the diameters for reference?

I probably have the "small fan" on the '84 but wish to confirm it.
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  #11  
Old 09-01-2005, 08:25 PM
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Brian,

Unless the fan on your '84 has been replaced, it is probably the small one. My '84 300SD with the new large fan is currently not here with me so I can't measure the fan, but the diameter of the small one currently on my Blue '84 300SD which is here with me now is about 11.5 inches.
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  #12  
Old 09-02-2005, 04:14 PM
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Brian, why not compare with the fan in the SDL. That's about as big a fan as you can fit.

Sixto
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  #13  
Old 09-02-2005, 05:05 PM
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Quote:
Originally Posted by sixto
Brian, why not compare with the fan in the SDL. That's about as big a fan as you can fit.

Sixto
95 S420
87 300SDL
Yes, Sixto, I should simply do that. I was just curious about how much larger the later fan was. The SD doesn't normally have cooling issues so I'll probably leave it alone until the fan gives up the ghost.
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  #14  
Old 09-02-2005, 10:48 PM
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Any chance you have the wiring from an old aux fan laying around? I still just need the wiring harness for the fan.
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  #15  
Old 09-02-2005, 11:50 PM
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Quote:
Originally Posted by JimmyL
Any chance you have the wiring from an old aux fan laying around? I still just need the wiring harness for the fan.
Fortunately, these two have not given me any trouble yet, so, I haven't relegated any of them to the spare parts bin.

Think you could use a cracked 617 head..........got one of those for 'ya.

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