'81 380SL lack of heat
 

Hello everyone. I just purchased an '81 380SL last week and I'm glad to say that just about everything is working flawlessly. Aside from a stuck rear pin on the hard top, hoping some curious screwdriver prodding will find the release hook and fix that, the heat doesn't seem to be working. The a/c works and I've read that the monovalve seems to be the culprit for most heat/cooling related issues when the rest of the system seems to be working. Only problem is, I don't know if I have a monovalve. I know it is supposed to be on the firewall but the only thing I can find is a large box with vacuum lines going into it and I'm wondering if that's the dreaded servo unit. It has the auto climate control, which I thought was the monovalve setup, but read another post here contradicting that. Any ideas? Is there a temp fix to get heat if it is the servo unit? Its not extremely cold around here (50-60s during the day) but my girlfriend is a bit of a wimp when it comes to cold so won't ride in it with me till the heat is fixed. :P

Your car doesn't have a monvalve. It has the older-style climate servo. If you have no heat in any position including DEF, I would guess that the water valve has failed or corroded shut. There's no easy fix.

For five bucks, George Murphy at Performance Analysis will sell you an excellent diagnostic manual.