W115 heater fan - PeachParts Mercedes-Benz Forum

AlexCole · #1 10-14-2008, 02:48 PM

Believe me when I tell you I have read as much as is available on this subject, just want to run some ideas by yall.
My blower/fan does not work properly.
When the fan speed selector (round switch between dash grill vents that says "defrost" and has 3 positions) is turned on with the heat on, no fan/no air at ANY of the speeds.
When the fan speed selector is turned on with the A/C turned on, no fan/no air on the first 2 speeds, but appropriate amount of air when turned to speed 3.
I suspect that it is a switch problem because there are 2 separate blower motors involved (one for A/C and one for heat). If it were a fuse problem, I would expect that one or the other would be out (whichever had the blown fuse), same for the motor (whichever had the non-functioning motor) and even the grounds.
So I think it is the switch on the dash that I referred to above. What do yall think? Anyone ever done troubleshooting on this switch, or does it just have to be changed? Seemed expensive ($100 +) when I looked it up. Alex

Mike D · #2 10-14-2008, 04:19 PM

Man, oh man, you are opening a can of worms.

First things first. The fan speed switch works both motors. The motors are selected by the change-over switch located on the center console just above the fuel pedal. The change-over switch is operated by a vacuum diaphragm which is controlled by the A/C knob. The default setting is for heat.

Check to make sure the change-over switch is indeed switching. You can manually depress it for testing.

The wires for the heat/defrost fan run in front of the firewall to a 4 pin connector going through the firewall just about directly above the brake master cylinder. If you have power there, then it's defrost motor time. I'm thinking it's the lower left pin, looking from the front of the car, which is power. The different speeds are controlled by varying the GROUND connections. The speeds are selected through the use of resistor coils on the back side of the firewall plug.

250 Coupe · #3 10-14-2008, 09:04 PM

Mike D's comments reminded me that on my blower resistor plug, there was a "spare" wire taped to the bundle that turned out to be a non-changeover valve source of power. IIRC, my blower didn't always work but switching to that wire made it work.

Somewhere way back in the archives, I mention the computer fan conversion I did on my very rusty '76 280C. I went in from the top side and fiberglassed in a really big computer fan. Worked great, had a ton of air flow and was quiet.

Michael

AlexCole · #4 10-15-2008, 08:47 AM

Quote:

Originally Posted by Mike D

Man, oh man, you are opening a can of worms.

First things first. The fan speed switch works both motors. The motors are selected by the change-over switch located on the center console just above the fuel pedal. The change-over switch is operated by a vacuum diaphragm which is controlled by the A/C knob. The default setting is for heat.

Check to make sure the change-over switch is indeed switching. You can manually depress it for testing.

The wires for the heat/defrost fan run in front of the firewall to a 4 pin connector going through the firewall just about directly above the brake master cylinder. If you have power there, then it's defrost motor time. I'm thinking it's the lower left pin, looking from the front of the car, which is power. The different speeds are controlled by varying the GROUND connections. The speeds are selected through the use of resistor coils on the back side of the firewall plug.

Okay Mike, here's what I know (it ain't much):

The change over switch works fine, when the A/C knob is turned on (this is the only time that the fan will blow, and only with the knob in position 3).

Imagine the four pin connector as such: pins connected to the following: red wire, green wire, green/black/yellow wire and green/black wire. I assume the red wire (lower left, like you said) is positive and the other green combinations are the grounds you mention above. When connected to a voltmeter, I get a power response across the rd to gn, rd to gn/bk/yw and rd to gn/bk when the defroster knob is in the 1 and 2 position, but when I put it in the 3 position, I only get a power response on the voltmeter at the rd to gn/bk/yw, the other 2 give no response.

Don't know if this helps. Any more suggestions/homework for me?

Also, how do I get the grill off the dashboard (the one that has the levers and the defrost knob associated with it)? I am real hesitant to just start prying it off, since I am sure that it will break if I do. I'd like to take it off so that I can get to the back of the defrost knob, where I envision there is a switch, which may be malfunctioning that can be tested. Is this correct?

Thanks, Alex

Mike D · #5 10-15-2008, 09:32 AM

The defrost/heater fan is worn out/ frozen. Some people have had success with reviving the motor by removing the air inlet cowling (the chrome grill in front of the windshield), spraying the backside of the motor with a lube and gently spinning the fan blades using a bent coat hanger/ wire. It's worth a try.

There are Phillips head screws in the corner of the air deflector (the grill in the dashboard). Pull the speaker grill, the speaker and the air flow rubber boot for ease of access. You'll need to disconnect some of the air flow control cables.

The entire deflector will slide out with some judicious tugging, tilting and swearing. The fan switch is mounted in the body of the deflector.

I told you it was a can of worms.

I'd try to free up the motor first. You need to pull the wiper arms and there are a series of rivets along the front bent edge of the fresh air cowling. They are plastic "pop-in" rivets from the factory. They may either be simple push in style which can be gently pried out or have a center pin which needs to be pushed through.

This would be a good time to renew the weather stripping, paint the center grill mesh, and clean out all the crap which has accumulated in the space. Use a garden hose on low pressure to flush the gunk out through the front drains.
I'd give a good spraying of Febreeze, Simple Green or some citrus based cleaner to remove the collective grime and diesel odor which is there.

Enjoy!

AlexCole · #6 10-15-2008, 09:47 AM

I'd give a good spraying of Febreeze, Simple Green or some citrus based cleaner to remove the collective grime and diesel odor which is there.

Enjoy![/quote]

Thanks, I'll give it a try. Does that mean that the switch behind the grill is not an option for troubleshooting?

And you mean to tell me that you don't like the diesel odor??? I thought that was part of the charm..............Alex

Mike D · #7 10-15-2008, 10:20 AM

The fact you are getting power to the different leads to the heat/defrost motor tells me the switch is working.

The "charm" of diesel soot wore off for me many, many years ago. My profession is fleet maintenance for a fleet of delivery trucks, all diesel!

250 Coupe · #8 10-15-2008, 12:37 PM

The wiring diagram is a 716Kb PDF. Want a copy?

Michael

AlexCole · #9 10-15-2008, 12:53 PM

[quote=Mike D;1993580]The fact you are getting power to the different leads to the heat/defrost motor tells me the switch is working.

Okay, maybe I'm just not understanding something (and I never claimed intelligence), but..............

If the switch serves both the defrost/heater fan AND the A/C fan, wouldn't it make sense that the problem is probably within the switch itself since there are problems with BOTH the A/C fan(only blows at position 3) and the defrost/heater(doesn't blow at all), which started at the same time, rather than problems with both of the fan motors themselves?

And the unit on the firewall that is described above (four pins/wires) is a good foot or more away from the knob on the dashboard grill panel, so shouldn't there be some kind of switch/wiring behind that knob that may be faulty?

Is the power running from the battery to the firewall unit (4 plugs/wires) and then to the switch on the dash then to the fan motor, OR does it go from the battery to the switch on the dash to the firewall to the motor? The latter doesn't make sense to be, but again I never claimed intelligence..............

250Coupe--I appreciate that, but I have the shop manual on CD and have downloaded and printed the wiring diagram and will admit that I hate them mostly cos I can't follow them, but thanks. And just for fun, here is that link: heater fan replacement 1970 280SE W111.026 this sounds like it might be the way to go if nothing else works out, it's tough to be a purist when it is 9 degrees and snowing. Alex

EDIT: Okay, I reread the earlier post and do see that there is a switch in the dash, I'll pull it and have a look, need to grease the cables anyway they are a little stiff, and do the coathanger thing with the motor. thanks.

250 Coupe · #10 10-15-2008, 04:47 PM

Alex, since you have the diagram, we can give you points to measure and what to expect when measuring them. That way, we will all be on the same page which will make it a lot easier.

I'll take a look later and get back to you. I've been up since 5PM. Yesterday.

A quick thought is that the heater fan burned up and took out the two lower speed resistors ( the bit screwed into the firewall), that would explain the heater blower not working at all and the AC blower only working on high.

Michael

NewToW115 · #11 10-15-2008, 11:31 PM

Hey i could really use that wiring diagram. I recently aquired a free 1976 300d in decent shape
Unfortuately, it looks like the old owner decided to fix the blower fan, then lost the center part of the dash, and then moved to switzerland.
I got the whole dash/fan controls this morning from a wrecker, and i have searched with no luck for the diagram.

250 Coupe · #12 10-16-2008, 07:57 AM

The diagram is on the Mercedes CD that comes from mbusa.com. It's $20 and covers all the W114 and W115 cars.

PM with your email address and I'll forward a copy of the diagram.

Michael

AlexCole · #13 10-16-2008, 08:25 AM

I got the center dash grill and vents off last night (at my son's soccer practice, the rest of the soccer parent's are starting to wonder about me since I pull up and unload the toolboxes and start disassembling my car at every soccer practice and game).
I noticed that when I shut the defrost fan off and on it "sparks". By sparks I mean that it was rainy and getting dark, and I had all the vents and grill pulled so that I could see the switch, and when I turned it on and off, it would click and produce a blue light, which I could only see as a reflection coming off the metal housing from around the switch, but it was blue and pretty bright. Had it been high noon and bright/sunny, I don't think I would have noticed it. Is this normal or a great clue as to what is going on? That was the only evenful thing that happened since practice was called early and I didn't get a chance to pull it any further, cables looked too complicated to do properly at practice. Alex

250 Coupe · #14 10-16-2008, 09:51 AM

Sounds like the heater blower is frozen.

Pull the grill at the base of the windshield and see if you can get your fingers in there to spin the fan. Don't try it with the blower on as you can't get your fingers out quickly if it starts to spin.

Michael

250 Coupe · #15 10-16-2008, 03:06 PM

Alex, I've had a chance to look over the wiring diagram.

The AC fan speed is controlled by the same resistors that the heater blower uses. It is possible for the AC fan to only run on high if both of the resistors are open, the heater blower would also only run on high

The blower speed switch has two separate sets of contacts. It's hard to make out the numbers but it looks like #5 and #6 control when 12 volts is applied to the AC blower. When the switch is off, no power to AC blower, at any of the 3 speeds there will be 12 volts at the blower. This assumes the started relay is de-energized and that the change over switch is in the AC position.

The second set of contacts control the ground part to the blowers.

Speed one grounds the blowers through what might be a 25 Ohm resistor and speed two grounds it through what might be a 7.5 Ohm resistor. It's really hard to read the fine print.

Speed three directly grounds the blowers.

The heater blower always has 12 volts on one side when the ignition is on and is just waiting for the speed switch to supply the correct ground.

The fact that you see a spark when switching speeds shows that the winding in the motor is probably good and that there is a complete path for current flow from the key to the speed switch. What is odd is that you only have one speed on AC and still see sparks at the switch when AC is off. Unless you are seeing the sparks only on the transition from high speed to a lower speed. That would lead back to open resistors.

As usual, I hope this helps more than confuses,

Michael