I have a problem with my climate control. The heat works, as in heat comes out of the vents (if I drive fast enough to push air out), but the blower motor will only come in if it is extremely cold out. The only times it has come on have been when it is below 5 degrees F, and then it will only stay on for a little while, presumably until the car warms up inside.

I know this happens in the defrost setting, but I can't be sure about the other settings as it only gets this cold every once in a while, and I didn't test them all.

It worked properly in the fall, but failed some time early in the winter. I just thought it was the blower motor until it started coming on whenever it was really cold out.

Any idea where to start? I couldn't find info on this type of problem in a search. Is it possible to bypass the auto climate control so I can just tell it "ON" and "OFF?"

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Find the little wire/sensor on the thermostat on the engine and then ground the wire, see if that lets the blower come on. Sometimes this sensor fails and prevents the blower from coming on properly. Its intended function is to keep the blower off till the coolant is warmer, but if it goes bad in the open position then it will never allow the blower to power up. Hopefully this helps.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I had the same problem with my 240D. The blower would come on every once in awhile , then less and less. I cleaned the fuse and still no change. It wasn't until I put in a new fuse that it works all the time, every time.

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'83 240D 264k on 100% biodiesel/ 31mpg
83 300D 217K the $800 gem
'82 300D 267K the new WVO project car finished and driving for free
International tractor B275 on B100
Ford 555 backhoe on B100
Sargent 25 ton Rough Terrain crane on B50

Any idea on the location of that part? I have no clue where it would be or what it would look like. Do you mean the same thermostat that regulates coolant to the engine?

Also thanks Biobenz for the suggestion. I actually replaced the fuse a while ago (in the fall) and it worked briefly until it melted the plastic part of the fuse (but not the metal part - learned not to get the plastic fuses!). I actually replaced that one with a fuse was in the spare fuse area in the fuse box and it seemed to work for a while until this problem cropped up. It could be the old fuse in there, though it seems to be very temperature related, which has me thinking it is not the fuse.

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Yes its located on the engine's coolant thermostat assembly. Passenger side front of engine. The connector is a single wire that slips over a pin on the sensor. To ground it, get a piece of wire you can stick into the plug and then to a ground spot on the body of the car or on the engine. (The negative battery terminal works too)

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

If the blower is melting fuses, do you have a good blower motor? It might be drawing too much power.

After it happened I was told by another forum member that the plastic type is no good. The metal actually was not melted in the fuse, just the plastic. I would think that the metal would have burned out if the fan was drawing too much. That's what a fuse is for after all, isn't it?

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Let me get this straight, the thermostat functions both to allow the blower to come on and to aid in the regulation of coolant flow?

So if I ground it and the blower works, does that mean I need a new thermostat? Even if the cooling system seems to work fine in the car?

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Its not the thermostat itself that controls the blower, its a sensor mounted to the thermostat housing. This sensor either provides a ground (when above 34C) or open circuit (below 34C) to the wire which comes from the climate control. If the sensor fails in the open circuit position then the blower will not come on because the climate control never gets the grounded wire signal.

Easiest thing to do is just ground it out somehow, as the sensor costs like $65.

No need to mess with the thermostat at all, unless your car doesn't run at around 80-85C normally....then that might have an issue too....but its unrelated.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Remove glovebox and start engine. When warmed up to the point that the blower should come on then reach in the glovebox with a hammer and tap the blower. Your brushes could be sticking. If blower starts to work then remove it, open it up and check the brushes. On a '79 SD that I worked on I did this and it was just sticky brushes so I lubed them and put it back together. No problems so far, did it last fall (07)

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

Thanks, that clears it up. I will check it out.

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