Did the simple spin test on the dual aux elec. cooling fans on my 93-300E 3.2 and they both spun freely.

If I am not mistaken, this has been said before to be a sign that the brushes are shot and the fans either will die soon or are likely not to start up when needed.

It's getting into the time of year that when I am sitting in stop/go NYC traffic my eyes are glued to the Temp gauge....

Yes - No - Opinions - Brickbats?

Just don't want to have a toasted 3.2 M104 as a door stop!

Find the plug for your fans,put 12 volts to it momentarily ...do fans spin?

There is a plug in a clip (mounted on the frame rail,below and to the right
of the Reciever/Dryer(site glass and electrical sensors [and AC hose connections])
In this picture ,I'm leaning over the driver's side fender looking down.
The plug is just above center frame of the picture.
This is on a 300D ,but yours is similar.
Separate the two halves of the plug(after removing from the clip)
and apply 12 volts to the Pins on the male part of the plug(the wires of
which lead in front of the Radiator/Condenser and out of sight.)

All the usual disclaimers apply...If you don't know what you're doing,do
not attempt without direct ASE supervision.

Attached Thumbnails

 

__________________
'84 300SD sold
124.128

The question was about the condition of the BRUSHES not if the thing works. Believe me I KNOW where the fan power input is...the question is about the CONDITION of the fan motor.

Previously somone posted an item stating something like:" If you spin the electric fan and it spins freely, this indicates the fan motor brushes are not making complete contact and fan motor is not operating at full output and fan speed will be lower than spec" also stated was the statement that the short brush condition can lead to the fan working either intermittentkly or not at all but not a consistent pattern.
I am asking if this statement is CORRECT, an answer that addresses this statement will help me determine if they need replacing ($350) or have some life left. I don't want to be stuck in traffic and not have them work consistently when needed.

AND here I was logging back on to ask you if you were referring to the Viscous
Fan Clutch on the Mechanically driven Fan(Between the Radiator and the Engine).
You can sorta test the Viscous Clutch by spinning it by hand...BUT you have to
do it with a warm engine (80 degrees centigrade or higher)...that, you have just Shut Off. (And I mean within 5 sec.)


Pick any electric fan you can find...If you (or anyone else on Planet Earth)can
determine the Wear Length [percentage of the original brush length remaining]
by spinning the fan blades,and sensing by some as yet invented process,the
friction co-efficient of the remaining brushes as compared to the friction
co-efficient of new brushes...You have created a new Mechanical cottage
Industry and will retire a rich person.

The rest of us fumbling DIYers have to remove the brushes and eyeball them
with a measuring device to determine usable life left in them.(Heck,it's so
much hassle,ONCE they are out you just replace them as a matter of course.)

I am not even sure the difference between worn and new brushes could be
measured electrically [Ohm meter or a DC amperage meter] (with the motor still intact)

But you've got me thinking about that possibility...NAAH,the resistance for
new and worn brushes should be the same...Unless worn brushes draw more
current than new ones?

member TOMJ posted this excellent aux fan bearing R+R (its for a single fan
but would apply to the dual aux fans also).If you get this far the brushes
would be a piece of cake.

Bosch Aux Fan - New Bearing/Rebuilt - 123 Body

Jim Forgione has The Best web page for cooling system "Fixes",I would
especially recommend #s 17,18 and 20 on his page for you!

http://pages.prodigy.net/jforgione/MB_S500.html

I've relayed your inquiry to stevebfl (an expert),who will hopefully reply
in the A.M.

It is my personal belief that man was allowed to create automobiles,primarily
as a transport device for "Angel's Breath". (Air Conditioning)

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'84 300SD sold
124.128

The quantitaive answer in the evaluation of a working fan, would use the technique called current ramping.

Current ramping is the plotting of the instantaneous current over time with an oscilliscope. It would be done mostly with an inductive amp probe. Using such a technique current rises as the brush enters the commutator post. It builds till it leaves the post. This happens quickly but is easily caught on a scope. This technique is usually used on fuel pump motors and a quick google of "current ramping" shows this example: http://www.wrenchead.ca/HTML%20Presentation%20folder/sld111.htm

The amount of current, the uniformity, and individual sector issues can be addressed.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

New Lesson

Thank You,Steve!

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'84 300SD sold
124.128

Steve,

Are you connected to a turkish engineer named Hussein by telepathy?
He said the EXACT same thing and showed me a method using an DM O Scope to do it.

But the problem is I don;t have time to rip it all apart and do it.

A simple test using same method as answer #1 (which I had intended to do anyway) came up with Left Fan DFM (dead as an Efing Mackerel) and Right Fan at maybe 30% of max power and also kinda intermittent. This is with 12V direct from the battery using 12 ga test leads. I figure that the feed from the Aux Fan Relay was not eniough amps to kick on the Right fan and the Left is doing nada. When I applied power it took a few seconds until the Right fan came on, the Left fan will not come on even with a little help (starter spin in correct direction of rotation).

I have a supplier who is shipping a good tested used assembly (whole shebang of fans, shroud, and all fasteners) on Monday. the price is 25% of new (he's an old College bud).

When I get the time I will tear into the dead fans and do the brushes and see what is what..

Thanks for the help and the education!