Need help with a 75 240D heater. I have not worked on one of these before, so I am not sure even how the controls work. The fan in this car only comes on when I rotate the AC ****. The fan does not have high speed which I assume is just a blower resistor. Shouldnt the fan come on with just the defrost ****? There is one other **** below, I guessed it was the dash dimmer. I get no heat at all. Heater valves work Ok and Heater cores are getting flow. Anyone have a wiring diagram& vacuum diagramfor the heater system on this car. The one book i do have shows 300 heater systems but leaves this one out. Thanks in advancefor any help

 

is this a 115 chassis or a 123?

can you post a pic of the controls?
[align=left] [/align]

 

The car is a 115 chassis. The top **** works the fan, but only when I turn the AC ****

 

Im looking through my Owners Manual for my 1975 300D. This was a shared manual for the 240D and 300D.
First to describe the levers.

1. top left with blue- this controls air to the windshield. move left =on
2. top right with blue- this controls air to the floor. move right = on
3.bottom left with red - this controls heat to the driver side. move left for more heat
4. bottom right with red - this controls heat to the passenger side. move right for more heat
5 center **** blower speed.

There are letters DEF and below the letters a picture of the maximum heat/windshield setting.
1 left: 2 left: 3 left: 4 right

Above this setup are vents and a lever that moves left/right. these provide unheated outside air.

I am driving my 1975 300D to work for the first time today. Its about 50F and raining here. I noticed as well that the heat was...well there wasnt much heat. Also my windshield did not get completly clear driving all the way through Atlanta, 26 miles I-75 @65mph one way. Yes the blower was blowing semi warm. I felt deja-vu just like my old 1964 Mercury Comet back in high school.
Anyway I will post as I work on getting some heat.
My temp guage never got up to 175. I wonder if the thermostat might be an issue. I checked the coolant and it is about an inch below the top- fine for a non recovery system. Certainly its not overheating and that is good. Perhaps the big problem is not now but in the summer keeping these cars cool enough. I will have to get a towel and start wiping windows...ahh the good old days.

 

I truly hate to tell you this, but it seems you may have a nearly unsermountable problem, at least according to Kent Bergsma of mercedessource.com ... In his book, "Mercedes Diesel Buyer's Survival Guide," he says, "My best picks are the 1974 to 1976 240D with 4 speed manual transimission and the 1975 50 1976 300D with the 5 cylinder non-turbo engine. .. My big pet peeve on these models ( all of them ) is the heater blower motor. There is an old joke floating around that 115 assembly line production started with the heater motor assembly. The worker's hung it on the assembly line as the first part to go up and then built the remainder of the car around that single unit. If you ever have the opportunity to replace on of these heater motors you will understand the job. Labor can exceed 20 hours and if you have it done at a shop it will most likely cost in excess of $1000. "

 

I'm still getting to know my car and I have found I have to turn the AC **** on in order for the defrost/heat fan to come on as well.

I had no heat this morning. I think my thermostat must be stuck open.

 

Thanks for the info. I have an update on my car. I did hear the one about the assembly line.I also found out some info about testing the blowers. there are 2 blowers, one for AC, one for heat. There is a change-over switch on the left
(drivers) side of the heater casing operated by the vacuum
change over link. It is above the gas pedal, it switches between ac/heater blower. you can watch it work. I jumped the heater blower and comfirmed it was dead.