85 300D Heating - PeachParts Mercedes-Benz Forum

tobybul · #1 11-04-2006, 01:09 AM

Noticed today that the right or passenger side vents do not consistently spit out heat when called for. My son drives this car and has not mentioned this to me until I drove it today and noticed it. We went to attend an event 30 minutes away and noticed only the left side including the floor vents had heat. The push button controls was set at the ventilation mode (2nd button from the right) although it responded the same way no matter which button is pressed. Parked the car for 2 hours while attending event. On the way back, all vents were producing heat. I'm in Michigan and its kinda cold here now. It was 30F outside air when we were driving. The a/c temp controller was set at 26C.

Have not gotten into what controls the heating vents yet. Would appreciate any information...

The other thing I noticed with this car is it takes awhile b4 the air coming out of the vents heat up even when the engine is already at operating temp. Plus the air is luke warm - not hot like it should be. I wonder if something is worn out or maybe needs cleaning. And I don't think my heater core is dirty. Thanks...

biopete · #2 11-04-2006, 02:32 AM

I believe that the side vents are always open unlike the center and floor vents. So maybe you had something blocking the right passage. If you take out the glove box (easy to do) you can see the duct for that vent and make sure air is going through it and trace it back to the air supply. The main air supply is there too on the right side. There is a flap with vacuum pod that controls it. You can watch it move when you turn on or off the cc system. Not sure about why your air is not warming up except maybe you are not getting much water to your heater core. It could be a clogged monovalve or something.

good luck and post back.

tobybul · #3 11-04-2006, 08:34 PM

Quote:

Originally Posted by biopete

I believe that the side vents are always open unlike the center and floor vents. So maybe you had something blocking the right passage. If you take out the glove box (easy to do) you can see the duct for that vent and make sure air is going through it and trace it back to the air supply. The main air supply is there too on the right side. There is a flap with vacuum pod that controls it. You can watch it move when you turn on or off the cc system. Not sure about why your air is not warming up except maybe you are not getting much water to your heater core. It could be a clogged monovalve or something.

good luck and post back.

Thanks... been quite busy today atending my son's water polo state tournament so haven't had time to do anything. Just to clarify, I was getting air thru the vents but its just that the right side vents were cold air instead of hot. But I noticed today that it worked fine. It was last night that it did not. But still not getting hot enough. I will chk the mono valve.

I didnt quite understand what you meant about air locked hose or heater core. Could this happen even if I had not messed with the system? I saw your response to my post in the other thread and will read it again to understand the procedure. Not quite sure what this tool is that you are recommending.

Jeremy5848 · #4 11-05-2006, 01:32 AM

If you have not drained the cooling system, it's probably not an air lock. You could have a partially clogged heater core (not good, must take the whole dash apart or so I have heard), a partially clogged or otherwise unhappy monovalve (easy to fix, much info posted here, do a search for 'monovalve') or a failed or failing auxiliary water pump.

This aux pump was added by Mercedes (so goes the legend) because at low engine speeds, the cooling water circuit doesn't move enough water through the heater core -- the aux pump helps it along. If this pump fails or clogs, you will get less or no hot water to the heater core. This pump is attached to the base of the right side inner fender. It's fairly easy to get at. You can unplug its electrical cord and wire it directly to 12 Volts for a test. You can hear the pump running if the motor is OK.

By the way, if that pump siezes, it will draw too much current and potentially fry the ACC. It should have been fused from the factory. I added a fuse of my own, just in case.

tobybul · #5 11-05-2006, 08:39 AM

Quote:

Originally Posted by Jeremy5848

..........It should have been fused from the factory. I added a fuse of my own, just in case.

Thanks for this info..... with regards to fusing this pump, how and where did you mount the fuse and what size did you use??

Jeremy5848 · #6 11-05-2006, 11:12 AM

Quote:

Originally Posted by tobybul

Thanks for this info..... with regards to fusing this pump, how and where did you mount the fuse and what size did you use??

The power leads from the pump run into a bunch of wires on the right inner fender. I located the hot lead and spliced a fuse holder into the lead. The fuse is 1 or 2 Amperes. Let me go look...[looking]...it's a 2-Amp fuse. Somewhere in this forum I have posted pictures....[searching]...look here:

http://www.peachparts.com/shopforum/showthread.php?t=157679&highlight=aux+pump

I need to experiment with a 1-Amp fuse, 2 Amperes maybe too big. My pump drew 0.9 Ampere when I tested it.

The kind of fuse doesn't matter, just remember to carry spares for that kind and size!

Jeremy

dieseldiehard · #7 11-05-2006, 12:08 PM

Quote:

Originally Posted by Jeremy5848

The power leads from the pump run into a bunch of wires on the right inner fender. I located the hot lead and spliced a fuse holder into the lead. The fuse is 1 or 2 Amperes. Let me go look...[looking]...it's a 2-Amp fuse. Somewhere in this forum I have posted pictures....[searching]...look here:

http://www.peachparts.com/shopforum/showthread.php?t=157679&highlight=aux+pump

I need to experiment with a 1-Amp fuse, 2 Amperes maybe too big. My pump drew 0.9 Ampere when I tested it.

The kind of fuse doesn't matter, just remember to carry spares for that kind and size!

Jeremy

Use a fuse that is greater than the normal current to allow for surges, a 2 A fuse is OK, even a 5A would protect it better than nothing.
I agree that MB blew it on this one, they should have added a fuse locally. The 108, 114 and 115 and so on used small fuse holders in various places around the engine compartments, why didn't they do that in the 123 we'll never know. I used a 5A ATC fuse in a small rubber inline holder.

carnut · #8 11-05-2006, 01:20 PM

It is probably the aux water pump but I would also check for a torn rubber diaphram inside the MONO valve. If torn, the heater will be somewhat warm at idle then go cold or will work in max heat then go cold when the temp wheel is moved from max heat.