I have a 1985 Mercedes 300cd 123. I am having trouble with my heat ac fan staying on. When I put my climate control on defrost and fan high and turn my ignition on with out starting the car I hear a clicking noise near the glove box. The fan comes on sometimes and sometimes it does not. The times when I turn my ignition on without starting the car and the fan comes on it works good in all the vent settings. When I go to start the car the fan shuts right off.

There is a blower fan controller located directly behind the glove box. It is black in color and has a multi-wire plug in. It is located to the right of the HVAC controller. You might want to remove the glove box liner and listen or touch the controller to see if this is where the sound is coming from. The black controller is easy to replace (one screw). If this is the problem I would recommend a used one from a wrecked car as new controllers are $300+.

Thanks for quick reply. Ok I will check that out. Is there a way to bypass the controller that is located by the glove box temporary to see in fact that it is bad.

Is there a name for the black controller or part number. Thanks

The last 4 part numbers are 0303 and it is called a blower control unit. Made by Bosch. I do not know if there is a way to by-pass the unit. The other possibility is that the brushes in your blower motor are almost worn out. If your car has more than 150,000 miles that is a good possibility and will produce the same symptoms. This is a do it yourself job. There is a vendor on ebay that sells the brushes. You will need a soldering gun. The new brushes will need to be shaped to fit but this is easy using the included Emory board. They are under $25.00. You should be able to buy a used blower control unit or less than $20 at a pick a part.

If its the same controller as in the 1st gen w126, then it takes variable voltages to control fan speed. You can put 12 volts to pin 12 on the controller to see of the fan runs. The fact that it runs at all tells us as the fan is at least semi functional and that the controller is able to send it high voltage at least( judging from the fact that it works with the key in run position.)

Once the car is started, try backing off the key just a touch to see if your switch is worn. Just turn the key counter clockwise without turning the car off to see if it makes contact.

The age of the brushes and fan motor will delay it starting on low speed and then not start at all on low speeds. Then it will not start unless you tap it while on defrost mode.

It sounds like a contact issue to me from the fact that it shuts off directly after startup of the engine.

Another issue that you could be going through is low voltage across the system. You may want to simply check the voltage at the battery or cigar lighter while the engine is running to rule out the charging system as a culprit

You also have a Klima relay which most likely could use a wire brush at the contacts. That relay is located in the engine compartment near the fuse box on most mercs.

The walk through of how to troubleshoot the fan motor is posted by Brian Carlton. If you search "Brian Carlton that's what we are here for" you'll get his excellent walk through.


Thayer

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

I just tested one off the suggestions Thayer made. I put the ignition in the on position and played with the key a little and the fan came on. I then started the car and the fan turned off. While I had the car running I then backed off on the key counter clock wise just a bite and the fan came back on. Not sure but i think the ignition could be the problem. Is there another way to double check and make sure it is the ignition switch.

My ignition switch had to be changed due to similar issues. The electrical contacts degraded and built up resistance that lead to electrical gremlins. The tumblers were OK.

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1989 300E

2005 Acura TL

2011 Hyundai Elantra Limited

Swing hard! Take chances!

I removed the glove box and the lower kick panel on the passenger side to check for the blower control unit but could not find it. I looked behind the glove box and all around but could not see it. I tried to listen for the clicking noise I here when moving the ignition switch still no luck. the clicking noise seems to be coming from the engine. I removed the blower motor and tested it and it worked just fine. I wanted to see if the brushes were good and eliminate the blower as the problem. I went ahead and installed a new ignition switch (not an easy job)and still the climate control does not work. I could not find the blower control unit behind the glove box? Thank you for all the help so far. I want to get the heat working its starting to get cold in NJ. Tomorrow there saying it will be 20 degrees. I would like to make this fix so if there are more suggestion please let me know.

The blower control unit is mounted to a metal bracket directly under the windshield behind the glove box . It is fairly close to the right side. It is a little tough to see as there are wire looms in the same area. You should see a silver box control unit for the HVAC system and directly to the right is the black control unit for the blower. I have both a 1982 CD and a 1985 300D and they are both the same.

Thanks for the quick reply. I located the blower control unit. Its right were you said. The numbers on mine are 000 822-11-03 does that sound right? I recently purchased my 300cd and I'm trying to get the heat working so I can drive it trough the winter. It has a couple other items that it needs, but will get to them later. Thanks always for the help.

Now that I know were the blower control unit is I turned the ignition switch on while i was touching the blower control unit and there is no clicking noise coming from the blower control unit. Does that make a difference. Should I go ahead and change the unit?

The number is correct. I do not know to to determine if the blower control unit is faulty other than changing it. I would recommend if you are able to get one from a wrecked car such as Pick- a -Part. Were you able to check the condition of your blower motor brushes?

I pulled the blower and the brushes looked good. I bench tested the blower and it works fine with 12v to it. If there is a voltage drop to the blower that I dont know. I will try changing the blower control unit for know, if you think of anything else give me a shout. Im going to take her out for a ride even thought she has no heat. 25 degrees here. Talk to you soon.

I replaced the blower control unit (located behind the glove box) and still the blower does not come on. Still no heat. Could the climate control unit be the problem? Happy Thanksgiving to everyone.