Lost heat in 300d
 

Hi all,

Cruising down the highway two nights ago, suddenly the heat stopped working in my 300d. It just started blowing cold air. Interestingly, before it stopped, when the heat did work, it only blew out of one of the air vents. When it stopped and only blew cold air, it blew it hard out of all the air vents.

I've been reading here about the monovalve. Where is it and is that the problem? Any other ideas are greatly appreciated. It's getting cold.

Many thanks,
BS

On the firewall right hand side near the battery. Try unplugging it.

Thanks, Kerry. I'll give it a try.

Guess I'm just not seeing it. Is it under the battery, perhaps?

What year 300D is it?

From what you describe occurring with the air vents, it sounds more like you have a problem with the acutuators for the vent flappers (ie, vacuum problem), and/or a problem with the CCU itself.

A problem with the CCU itself would explain both symptoms - if the CCU went funky and decided to shift from heat to vent mode, that would explain the change in air flow from the vents, and commanding the monovalve shut would explain the sudden lack of heat.

The monovalve by itself going south wouldn't explain the change in air flow out of the vents.

A loss of vacuum to one or more of the vent flap actuators, depending on which ones, could explain both symptoms as well, but after experience with my two 87 300D's, I'm sort of doubtful of that.

I'd first do a vacuum test on the vent flap actuators, to eliminate that possibility. If they check out, check the monovalve and aux water pump to ensure they work and eliminate that possibility. If they're OK as well, I'd say you're left with a flaky CCU (for whatever reason) to blame.

Thanks, Mitchell. I'll dig in deeper and look at the CCU itself.

On a working version of one of these CCU's, how do you activate vent mode, anyway? I never could figure out how to simply get fresh air flowing through the system. Very confusing.

Also, where exactly is this monovalve? Kerry said on the firewall, right hand side near the battery, but all I see are hoses.

Thanks,
BS

Monovalve pic
 

Try this link. Very good reading
whunter has provided pics of the monovalve in post #20

http://www.peachparts.com/shopforum/diesel-discussion/246273-damaged-mono-valve-pictures-please.html

Thanks for the pics. But I still can't find the monovalve housing on my car!

Need year and model.

Sorry - it's a 1977 300d non-turbo. At least that's what the VIN # and the manuals indicate; the license plate people say it's a 78. Thanks, Kerry. I can't find the monovalve anywhere, but I think it's the CCU.

77 explains it. You don't have one. You have the evil servo. Search on evil servo or servo for lots of info.

If you need immediate heat without spending a lot of money for a new servo, search on servo bypass.

It's far less likely to be the CCU with your system and far more likely to be the servo. The servo is far more dependent on vacuum and has a central vacuum control switch, which , if it fails, won't let the system operate. If you search on my user name and 'servo manual' I posted a link to anonline manual fo rthe servo system.

Answer
 

This may help you.


Evil Servo service manual
Evil Servo, service manual - PeachParts Mercedes ShopForum

Evil Servo bypass
http://www.peachparts.com/shopforum/diesel-discussion/286770-evil-servo-bypass.html#post2567364

Maybe pictures will help. Trying to get my heater working. Where to start?
http://www.peachparts.com/shopforum/diesel-discussion/238588-maybe-pictures-will-help-trying-get-my-heater-working-where-start.html#post2029228


ACC servo unit = Heater Control Servo



.

Thanks so much, folks. Seems a complex and thorny subject.

I'm gonna bypass. Does anyone have pictures of a completed bypass?