Revision [405]
This is an old revision of W140BlowerMotor made by WebMaster on 2006-05-12 13:56:56.
Diagnosing the Blower Motor and Cabin Filter
on the 1998 S500 (W140)
by Greg Baxter
4/14/06
This is a how-to guide to fix the a/c blower motor on the Mercedes 140, AKA "S" Class 1993-1999. This article is of my own making, and from my own research. If you use any of the material within, then like me, you're on you're own. You can't hold me or anyone else responsible for whatever, so if you don't feel comfortable doing this sort of mechanical work, then take your car to an authorized mechanic.
Air conditioning and heating can be problematic in the S class of Mercedes production autos. Dozens of sensors, pressure, temp, evaporators that require the entire interior to be removed, condensors, pumps, compressors, driers, fans, valves, on and on. Even the fuel on a 140 is cooled through the a/c before it goes to the injectors.
Some of my earlier problems with a/c were electrical in nature. Like the fan motor. It's quite common for the motor to only put out say, 25% power at the highest control setting. On a hot day, this can be a major issue. So after doing my share of reading on the forum, I came up with some superior advice from the usual plethora of talent that hang thereabouts. The results of my reading are displayed herein, implemented by me.
25% power barely moves any air through the plenum. What was causing this, a short? A bad motor? Bad brushes? Was the controller itself sending bad "bias" voltage to the blower motor controller (a TTL transistor to amplify voltage). Should I just replace the entire fan unit, motor controllers and all? I came close.
The problem ended up being a bad TTL blower regulator. You can find them from Phil (mercedesshop.com), or if you're lucky you may get one that works at a junkyard. But let's dig through the diagnostics I went through to isolate the problem - we'll take apart the firewall-forward air handlers to expose the motor and electronics. It's really not a big thing - especially to those that do them all the time. But for me it was fun, and even some redemption that I only spent $200 for the solution - when the shop had quoted $1,280.00 for labor and the entire assembly.
You'll need a voltmeter. And REMEMBER - there is power to the motor even when the switch is OFF. The blower motor on a 140 is controlled by the NEGATIVE pole. I understand that a lot of cars use this same technique. So the closer the negative on the motor is to a short circuit (0 volts), the faster the motor runs. Keep that in mind.
And finally, since the cabin/charcoal filter sits right on top of the blower assembly, I thought a simple relabel of the title would suffice. The other two filters are another story, one under the passenger side dash - easy to get to. The "pollen filter" is buried behind the evaporator, and is a significant job IMHO. We're going to do the big filter, the charcoal cabin filter.
Ok - here we go
First off - you're supposed to disconnect the battery, which I didn't do because I wanted to do my diags on the car (not on the bench). I do not recommend this. Remember - the positive to the motor is a hot wire even with the key off. If you ground the motor, it'll at best run at full capacity, and at worst arc, burn or melt your jumper. So that being said, if you don't feel comfortable working this way then DON'T. Take it to the dealer.
Here's the wiper cover manifold before it comes off.
This cover has to come off - bunch of screws. Those screws that go down the middle are fairly long, like 3" or so.
----> Note: if you're just replacing the filters, then if you're careful you don't really need to pull the plenum. Just be careful not to jerk out any connections like the vacuum hoses etc, and slide in your new filter, button it back up and you're done.
Notice that little plug? You DON'T have to undo the drain hoses for this piece to come off. Just give it a short CW twist and it pops right out of the manifold.
Here's the bottom, where the plug fits.
Just pop this little vacuum line off the manifold.
Unplug the "Smog" sensor and it's off - just set it aside for now
Notice my new shiny air filter. This is one of the charcoal filters, but NOT the real expensive one that sits in front of the evaporator - that one [not shown] is called the "pollen filter" and goes for around $380.00 cheapest I can find it. I've replaced the pollen filter once, and don't plan on doing it again anytime soon. Not only is it a pain becaused the wiper assembly has to be removed to get to it, I simply don't use it that much. In order to use the pollen filter, you've got to press the little button on the console, and then it times out after 15 mins. I think the filter is rated for 24 hours continuous use, but will literally trap particles down to 1 micron (that's small).
Anyway, I replace the main cabin charcoal filter often - maybe once/twice a year - Phil has them for about the best price around. Like $100.00
Bottom of the charcoal filter. Again this one is new. I can say firsthand that this filter works. I have yet to replace one that hasn't been absolutely filthy after just a few months. They will also clog if neglected. The symptoms are similar to low fan rpm, just restricted airflow.
This fan is the original in my 98 S500. Notice the corrosion here and there - and I removed it to clean it up about a year ago. The motor bolts and the aluminum case itself seem to be subject to corrosion.
The Blower Regulator can be seen at 3 o'clock. Try doing the obvious diags first - like the little controller that sits on top of the fan. Look for loose wiring, corroded connections to the motor, anything like that. You can actually replace the brushes on these motors if you're up to it, but the comments I've heard are it's not worth it. A new motor is like $150, and of course it has new windings, new armature, new brushes, connectors, bearings etc. If you've got to pull the blower assembly, just pull the screws all the way around the fan case and it will lift out.
Here I'm testing the GROUND wire that connects to the motor (my red lead, the car's blue lead) against the frame ground (black). See results below. Remember - these motors are controlled by varying the ground potential voltage - NOT the positive voltage. The positive voltage to the motor is always a direct circuit to the battery (via a relay/fuse I believe). It's also hot all the time and never switched.
With my fan control inside the car set to "full", this is all the voltage I got to the motor on this car. So, 12V on the ground wire means the motor is shut off. 0V on the ground wire means it's running wide open. Actually, I think it's more like 1V. So with 6V control signal (bias) , I was supposed to be getting bias * 2, or 12V to the motor. This is why it was only turning about 1/3 speed. For the more technically minded: power-voltage = ( bias voltage ) * 2.
Another shot of how I attached the scope leads to test full potential between the two poles on the motor. It showed 12V, so I assumed at this point the motor was beginning to look good.
Through the motor. I'm not sure how a different value would be diagnosed, especially if there were diodes on the brushes. So maybe some of the experts reading this could comment.
The culprit, the "Fan Resistor", aka Blower Regulator. When putting it back together, I used a generous amount of silicon heat-sink paste on the bottom of the new controller. You can get a little tube of it at Radio Shack, it's used for computers to heat sink CPU chips. Did I tell you this thing gets HOT? That motor pulls an awful lot of current.
Anyway - if you were here just to replace the filters then place your new one on top of the motor housing, charcoal down, and replace the cover in reverse order. Don't forget to do the wires and the drain plug, and the long screws.
This is the second one I've done. the first one I just purchased the entire assembly from Potomac Mercedes (junkyard). I got lucky and found one that came off a 99 S500 that had been totaled. He sold me the whole assembly for $240.00, so now I've got a spare motor and fan case. The second time I just purchased a new blower regulator (140-821-83-51) from Phil. I hear this is a common part - perhaps I'll call Autozone one day for pricing, but I doubt I'd find it any cheaper.
Fan now runs 100% - what a difference! , and responds to inputs from the control panel in the car.
I wish you success!
-Greg Baxter
CategoryDiy
