1984 MERCEDES BENZ 300SD NO HEAT ON BOTH SIDES

  • Tiny
  • seweavers
  • 1984 Mercedes Benz 300sd

Heater problem
1984 Mercedes Benz 300sd 135000 miles

Some mornings I have heat on the passenger side and not on the drivers side. Other days its the opposite. This car does not have climate control for either side. Air blows on both sides but one is cool and the other hot. I had everything apart and cannot find any doors that are not operating properly. The puzzling thing is why does have hot on one side and not the other and the next day it will be reversed.

Tuesday, October 19th, 2010 AT 12:21 PM

15 Answers

  • Tiny
  • KHLow2008
  • Expert
  • 41,976 posts

Hi seweavers,

Thank you for the donation.

The most likely cause of the problem would be vacuum leaks or insufficient vacuum.

Air mix doors are operated by vacuums and if there is insufficient vacuum, either due to leaks or kinked lines, the vacuum is only able to operate a single component and whichever side happens to operate first would be the one to have the desired temperature.

Check the vacuum lines and reservoir for leakages or kinked lines.

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Thursday, October 21st, 2010 AT 10:55 AM
  • Tiny
  • seweavers
  • Member

New vacuum pump and new lines. I have removed the ducts to observe what is actuating. When the doors are actuated they open simultaneously.

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Thursday, October 21st, 2010 AT 12:30 PM
  • Tiny
  • KHLow2008
  • Expert
  • 41,976 posts

When both doors are opened simultaneously, does the problem of heat and cool air still occur?

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Thursday, October 21st, 2010 AT 12:34 PM
  • Tiny
  • seweavers
  • Member

Yes. When both doors are open, one side is hot the other is the same as the outside temperature. I'm refering to the doors that allow the air to flow to the vents below the dash. Is there a door somewhere behind the heater core? Some days the heat will be on one side and the next it may be on the other.

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Friday, October 22nd, 2010 AT 12:01 PM
  • Tiny
  • KHLow2008
  • Expert
  • 41,976 posts

I could not find anything in our database on the heater air flow paths that can help and will try to get some of my fellow technicians to assist on this.

Hope they can come up with something.

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Saturday, October 23rd, 2010 AT 8:42 AM
  • Tiny
  • seweavers
  • Member

That would be greatly appreciated. It will be getting cold here soon and my morning drives into work will be a bit uncomfortable. It's not bad now but I know it will get worse. Something else to note, I never get hot air to the back vent.

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Saturday, October 23rd, 2010 AT 2:46 PM
  • Tiny
  • Dr Loot
  • Expert
  • 2,332 posts

I only have one question for now, do you have a 300SLD 6 cylinder diesel or a 300SD 5 cylinder diesel?

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Saturday, October 23rd, 2010 AT 11:52 PM
  • Tiny
  • Dr Loot
  • Expert
  • 2,332 posts

There is a heater control valve mounted next to the battery ti is electrical, make sure the hoses are hot on both sides, on the left side of the engine there are additional hoses coming out of the heater core make sure they are hot also and there's an additional hoses right behind me engine make sure all hoses are hot to determine if you have circulation through the heater core.

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Sunday, October 24th, 2010 AT 12:11 AM
  • Tiny
  • seweavers
  • Member

I have the 5 cyl diesel. Checked the hoses, all hot. Added antifreeze last night due to colder weather and this morning I had no heat. Possibly clogged heater core? I will check all of the hoses again tonight. It seems that there are two heater control valves, the one next to the battery and one below it. I don't think the one below has any wires going to it, just hoses. How can I check the valve with the wires running to it?

What directs heat to the vent to the back seat?

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Friday, October 29th, 2010 AT 9:50 AM
  • Tiny
  • Dr Loot
  • Expert
  • 2,332 posts

If I am correct, on the right front fender down by the flame there is a circulation Pump, it looks like this


http://www.2carpros.com/forum/automotive_pictures/307270_BMW_079_1.jpg



if that pump is not circulating when you turn the heater on you will have no heat. also there is a heater control the valve that looks like this next to the battery


http://www.2carpros.com/forum/automotive_pictures/307270_BMW_080_1.jpg


that valve is easy to check, just make sure the hoes going in is hot and the one coming out is hot, if the heater core is plugged the hoes on the left side of the engine coming out of the firewall will be cold. report back with your findings.

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Friday, October 29th, 2010 AT 10:42 AM

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