124 - bad smell when heat on

[AlexTheSeal]

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.