Author Topic: Defroster/blower motor  (Read 7217 times)


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Defroster/blower motor
« on: November 10, 2015, 22:25:03 »
Hi, on my 71 280se I noticed this morning when I turned on the defroster the speed of the
defroster/blower motor was fluctuating with the rpm of the engine.
As I went faster, the defroster motor would speed up. As I slowed down as when
coming to a stop, the defroster motor would slow down. It's a variable speed switch
but regardless of where I had it set, the symptoms where the same. I hope it's not the
switch. I recently saw it online for over $700 bucks. At least it was new.
Any ideas what's going on here.


  • Guest
Re: Defroster/blower motor
« Reply #1 on: November 14, 2015, 19:58:40 »
Hi Jeff,

The fan motor appear to be really sensitive to the voltage supplied to them.

I have just had my fan refurbished and on a low battery charge it is incredibly noisy. Once the battery is at normal charge it is fine.

I'd suspect that as your engine speed increases the voltage reaching the fan increases.

Regards, Ian


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Re: Defroster/blower motor
« Reply #2 on: November 16, 2015, 20:36:04 »
That does seem to be what's happening. But how does the voltage regulator figure into all of this?


Benz Dr.

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Re: Defroster/blower motor
« Reply #3 on: November 17, 2015, 00:02:13 »
Your voltage regulator may be cutting out at a voltage that's not high enough to maintain at least 12 volts. Most are set around 13.8 volts. Check yours with a volt meter. If your idle speed is too low you may also find that your alternator won't be spinning fast enough to keep up with current draw and you may have to increase it a bit.
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  • Guest
Re: Defroster/blower motor
« Reply #4 on: November 17, 2015, 22:43:37 »
I had the voltage regulator checked today, tested at the battery with a digital multimeter. It checked out fine. More or less right in the 13.8 range.
I saw a little more fluctuation first thing this morning when testing with an old analog multimeter. Sometimes at night when
I turn the headlights on, they will dim momentarily before coming back to normal. It was the same with the analog meter.
Momentary drop, then back to 13- 14 range, more or less. Kind of hard to read accurately.  Is it possible it's fine at times
but not at others?
Any other ideas?


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Re: Defroster/blower motor
« Reply #5 on: November 26, 2015, 15:19:48 »
Hello Jeff,

Try putting a good charge on your battery. A fully charged battery should reduce voltage fluctuations. RPMs of DC motors varies directly with voltage supplied.  Even your electric fuel pump will change pressure and rpms depending on the DC voltage in the system.
Joe Alexander
Blacklick, Ohio
1969 Dark Olive 280SL
2002 ML55 AMG (tow vehicle)
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1963 404 Mercedes Unimog (Swedish Army)
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Re: Defroster/blower motor
« Reply #6 on: November 26, 2015, 22:35:55 »
I have a Mercedes Benz brand battery and I've been trying to find out how old it is. I posted just that question in
a different section of this forum and after more than 100 views, I haven't received a single reply.
Maybe it's time for a new battery?
When I had the voltage regulator checked they also checked the battery. While not in like new condition
he said it was OK. Thanks for your suggestion.