This Mercedes world is wacky man!

Just got this 240d. It needed new belts. Got it done, but what a headache...ac unit is now gone too. Next problem is the defrost.

Front defrost. The fan seems to come on and turn off of its own volition. My logic tells me that if I turn the defrost knob, the fan should turn on. It's got three speeds and off. However, I have found that if I fiddle with the ac (COLD) knob, which is lower on the dash, the fan will sometimes come on, but not always. Seems the "colder" I set the cold knob the more likely the fan is to come on. Does this sound like the defrost switch is punky, or am I missing some combination setting intention that my buddies from Schwabenland designed in? I have felt hot air come out of the vents, so I don't think the fan is working just on cool.

Rear defrost: This has worked like twice. I haven't found any button or knob on the dash that explicitly controls this. It makes sense that my suspect defrost knob also controls the rear defrost and that when I turn on the front, the rear automatically comes on as well. The fact that it has defrosted tells me that the filaments are probably just fine.

That $115 cd with all the info keeps looking better and better. I've never felt so blind working on cars before.

Another goofy thing is the starter. The guy I bought the car from claimed to have just changed the starter and glow plugs--both seem to work fine. The catch is that the starter grinds if I don't give the glow plugs enough time to heat. On cold starts I have to hold out the start plunger for like 25 seconds before I engage or else the starter just makes a sound that would turn any cog's guts. Once the engine is warm everything is a lot easier. Is this a dud starter or was the system actually designed to be heat sensitive--turning when the glow plugs are hot and complaining when they are too cool? My glow plug bulb is out so there is some guesswork as to when the glow plugs are hot.

Does this sound familiar to anyone?

This does sound familiar!

The front defrost is a manual control and the control should be in moved all the way to the left to vent air to the dashboard.

The AC knob is a vaccum control and if you have low vaccum or leaking lines you're not going to get much movement out of the "system". Turning the AC knob all the way counterclockwise will put the system in fresh air (non AC mode) and air from the outside will flow through the vents. Now when the AC knob is turned clockwise from its resting position it will engage the AC compressor and pneumatically actuate a flap in the dash the reroutes the air through the ac system core thereby generating some cool air.

For heating make sure the AC is all the way counterclockwise and slide the heater levers to the far left and right and that should slowly build up heat. Remember to close the center vent by moving the black lever all the way to the right to force the fan air through the system and out the side vents, if you don't the cold air will just rush out the center vents which initially confused me as I thought heat should have been coming out of all vents but that's just how the system is!

Now with that being said I would check the vaccum controls in the dash because it seems like the vent controls are stuck or stuck inbetween hence no air moving through or just very weak becuase the air is exiting through all vents simultaneously. THis also might be a relay issue....

As for your rear defroster there is a knob right next to the ashtray, pull that and the heater is on pushed in it's off. Does your car have that?

There is always a certain amount of guesswork when starting my car in the mornings. If it's cold it may be 25 seconds or more even though my GP indicator does work. Even then I still require a lot of cranking. at 65F it still requires 30 seconds or so of GP and then 10-12 revs of the cranking with the idle adjust at maximum. In 90-100F weather it only takes 15 seconds with 5-8revs.

Hope that helps a little

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

The front heat blower/defrost knob is actually more than 3 speeds..... 3rd speed uses variable resistor to gradually increase speed. Try 2nd speed and it should be more consistent. Otherwise remove the switch and clean it with brass brush and electric parts cleaner. The rear defrost knob has white center dot that lights up. AC should be completely shut off.

Never in my life heard of starter motor sensitive to glow plugs, probably coincidence but take it easy with quick turn of the key and hope it doesnt grind.

Are you sure you're reading the salt-shaker glow plug indicator properly? They hardly ever burn out. 30 second glow is typical and the same knob controls idle and engine shut down, if memory serves me correctly. Hell I wouldnt be embarassed to head to the dealership and ask advice for using the proper start-up technique unique to the old world diesel 115.

You've got alot of guts getting into a primitive diesel this time of year. Summertime cold starts are easy, Wintertime is ancient ritual that can take 15 minutes before the car is drivable. Fuel additives, 1/2 gallon of kerosene in the tank can help.

If equipped with AC, these cars have TWO blowers - Heat/defrost and AC - working off the same fan switch. The AC blower is in the center housing below the radio, while the heat/defrost blower is mounted against the upper firewall.
There's a vacuum-operated change-over switch mounted above the gas pedal, which selects either blower, depending on the position of the AC knob. With the AC knob turned completely off, the heat/defrost blower should operate. If a blower runs only with the AC knob full on, it's probably the AC blower.
Hope this helps instead of complicates.

Happy Motoring, Mark

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DrDKW

Thanks for all the responses!

Rear Defrost: Indeed there is a knob down next to the ashtray. It lights up when I pull it out, a buzzing noise follows down by my left knee, and then the light goes out when I let the knob slide back to middle position. Rear window still doesn't defrost. I assume that buzzing in some relay that is faulty. Change that first and then if the window still doesn't defrost it's probably either a bad wire or a break in the filaments.

Front Defrost: With the ac knob completely off and the defrost knob at second position, the fan comes on like five seconds after I start the car. It doesn't blow hot air, even when the engine is at operating temp. I've got the left horizontal sliders all the way left and the right sliders all the way right, but air just never heats up. In an American vehicle that would make me think the heater core is plugged. However, I do feel hot air coming out down by my feet. Is that just ambient heat from the engine, or is that the only outlet for hot air?

Starter: I completely believe you that there shouldn't be a connection between warm glow plugs and the starter's cooperation to engage, but after three or four cold starts, I am convinced my starter is hexed to only engage when the glow plugs are warm. In this car the glow plugs are not activated by the key switch, rather there is an ignition plunger to the left of the key switch past the cold start mixture enrichener knob. The plunger has to be out half way for the vehicle to operate(with the key turned on). When you pull the plunger out to 3/4 way, the glow plugs start warming. The glow plug lamp should come on and go off when the plugs are warm, but my lamp is totally dead (brake light indicator comes on, but no glow plugs). So I completely have to guess at when the plugs are warm. If I count to 15 and pull the plunger, the starter motor spins but doesn't engage the flywheel (or whatever it should engage on the crank). If I sing the Battle Hymn of the Republic all the way through with chorus at the top of my lungs to awaken the starting gods(30 seconds) the starter is very likely to engage the flywheel. Once the engine is warm I only have to hold out the plunger 10-15 seconds. I imagine this would all be simplified if I just got that glow plug lamp fixed. I leave the house at 5:30 every morning and my neighbors are probably getting tired of my singing!