124 - bad smell when heat on - PeachParts Mercedes-Benz Forum

AlexTheSeal · #1 02-23-2007, 02:32 AM

There's an acrid, nauseating chemical smell coming out of the vents of my '87 300D when the heat is on---I've smelled it before, but now it's bad enough to make the car undriveable in cold weather.

It's not the smell of exhaust, oil, fuel, coolant, or brake linings. Could maybe be electrical - it's kind of like the "melting plastic" smell of a short--- although I've noticed no other symptoms of an electrical problem.

I'm inclined to suspect mold, a dead rodent, or some other kind of contamination somewhere along the air-flow routes through the system. However, it's quite clear that the smell only happens when the ACC brain is in a heat-producing mode (not when cooled air or "unaltered" air only is being fed to the vents). I've looked at the diagrams in the CD manual of the climate control system, but it's not really clear to me under what conditions, if any, there are air ducts, passages, etc. involved in heating that would be shut off otherwise. Maybe someone else understands the functioning of the ACC better than I do? If I could figure out how the smell could happen only when heat is being produced, it seems like that would narrow down the possibilities.

I know a leaky heater core is the obvious conclusion, but I've had cars with bad heater cores before (though not Mercedes) and this doesn't smell anything like that did; also I'm not noticing any change in expansion-tank coolant level.

Thanks for any input.

cool · #2 02-23-2007, 11:43 AM

try having your air coooling system cleaned. (if your diagnostic is right and there are no leak from the coolant line. info, if you use the cold side of the aircon, does it smell?

AlexTheSeal · #3 02-23-2007, 12:01 PM

No, no smell at all when the A/C is on (unless it's heating at the same time, like in defrost mode).

brianhi · #4 09-12-2013, 04:53 PM

Hello I am having a similar problem to what you described in this thread but in my W123 300D. I am wondering if you ever came to a conclusion about why your heat smelled bad.

thayer · #5 09-14-2013, 07:01 PM

I have a thought about that. If its been over a year, the decomp of the animal would have passed the stink phase. Mold or fungus is more likely. Especially considering the heater core has been closed off from fresh air all summer. Maybe spray some febreeze down the air intake below the windshield with the heat on? If that doesn't work, start pulling it apart

AlexTheSeal · #6 09-24-2013, 12:49 AM

Quote:

Originally Posted by brianhi

Hello I am having a similar problem to what you described in this thread but in my W123 300D. I am wondering if you ever came to a conclusion about why your heat smelled bad.

Yes I did come to a definitive conclusion, but I don't know if would also apply to a W123.

I've had this problem on three W124s now. Recirculation mode for the climate control is actuated by one of the vacuum pods inside the dashboard pushing a large plastic flap open and closed. This flap is located directly upstream from the heater core. It's padded with a thin foam material, I suppose for sound insulation as it opens and closes automatically. With age, the foam disintegrates and begins to travel the only direction it can--towards the heater core fins, where it sticks, melts, and produces the terrible acrid chemical smell that I described in the first post.

The fix is to disassemble the car until you can reach the front side of the heater core and remove the foam chunks--they come off with a stiff plastic brush, like a toilet brush, and a mild solvent like vinegar, without any damage to the heater core. On the 124, you get to the heater core by removing some trim around the bottom of the windshield, the windshield wiper mechanism, and the heater blower fan. Unfortunately I have no idea what the equivalent job would be on a 123 chassis car.