No heat

[Vintage]

Hello everyone.

My "new" (to me) '79 450SL was a bit chilly on my drive to work this morning. We had a warm weekend in SC and I actually used the AC yesterday for a bit, mostly to check it out. All vacuum switching works well, the compressor runs, etc. But no heat. Blower works fine but I get cool air.

I am thinking monovalve, as the heater lines to the unit are hot and the outbound/return lines to the core do not get hot. Rotating the temp dial does not seem to make anything click, but there is another (potential?) issue here. My temp gauge does not work. Moves maybe a quarter inch at startup but never registers the true temp. I was planning to ohm out the sensor (once I figure out what it should read) and replace it if bad. Is it possible the climate control system needs to see a baseline temp before it will open the valve?

Just a thought. Anyone?

[Walrus]

Originally Posted by Vintage

Hello everyone.

My "new" (to me) '79 450SL was a bit chilly on my drive to work this morning. We had a warm weekend in SC and I actually used the AC yesterday for a bit, mostly to check it out. All vacuum switching works well, the compressor runs, etc. But no heat. Blower works fine but I get cool air.

I am thinking monovalve, as the heater lines to the unit are hot and the outbound/return lines to the core do not get hot. Rotating the temp dial does not seem to make anything click, but there is another (potential?) issue here. My temp gauge does not work. Moves maybe a quarter inch at startup but never registers the true temp. I was planning to ohm out the sensor (once I figure out what it should read) and replace it if bad. Is it possible the climate control system needs to see a baseline temp before it will open the valve?

Just a thought. Anyone?

No monovalve... Much worse. You have the dreaded Climate Control Servo, AC II system. Search here and at benzworld 107 forum for lots of information and get ready for an odyssey... The servo performs multiple functions ; It switches the blower, it regulates the hot water, and it directs vacuum to actuators and/or electric switches... Prone to failure and pricey to replace. Electronic substitutes are available, Unwired Tools, for example, but also spendy...

Good Luck!

[meltedpanda]

perhaps you have a bad tstat? stuck open, Id check that , if not ...

yea , the potentially dreaded servo. If your lucky one of your vacuum lines is off . Check carefully the servo for signs of leakage or corosion. there is a way to bypass for heat , but it will not be directed by flaps

[Mike1953]

Take it to MB mechanic for new Amplifier
MB mechanic says it’s not the Amplifier, it’s the Servo

I pull the Servo out and return it (strangely I recovered all of my money on that)
Determine auxiliary pump is frozen and plumb hoses so it is out of the picture.

Buy Unwired Tools Controller and it arrives just as promised...
Install it perfectly; your right it is not that hard...
But still no heater...
Everything works perfectly with the Air conditioner; fan speeds up to cool car down; defrost works, Bi-Level works; Auto Hi/lo works, etc.; side vents work, center vents work, you can freeze meat with this air conditioner.

Start car set dial to 85F push defrost; AC switch on off
Still no heater; in fact it is cold air.

Check thumb wheel on Environment Control unit in the dashboard according to your instructions; 1200 OHM differential; checks out OK

I take new Unwired Tools valve out of the picture and flush heater core to determine if there is a blockage; no blockage.
Run a flush on the engine block inlet and outlet; no blockage.

No matter what I do I have the exact same thing; full on cold air regardless of the temperature; regardless of the AC on/off switch and regardless of the tumbwheel postion and never, never any heat or breath of warmth coming out of the dahboard.

I run car and check Unwired Tools heater valve. It opens and closes as promised.

I am no longer convinced I have a controller problem, but then again I don’t really know…

I need some coaching on exactly how to start checking vacuum hoses, but all dashboard vents seem to work OK.
Is there a flapper valve behind the dash that exposes the heater core to the blower that refuses to open when the car is running?
Is there an auxiliary heater fan rely that is bad?

Can anybody coach me how to trouble shoot this from here?

[meltedpanda]

are you getting flow to the heater core ?. Sounds like you are not . IIRC the auxillary pump circuit is the way fluid gets to heater core

[Mike1953]

As stated check flow through heater core.
Contacted UnwiredTools for help. You klnowwhat the fix is?
Disconnect vacuum from UnwiredTools valve (this puts the valve in open position to allow hot water into the heater core).
Drive the car for 10 minutes to bleed the air out. Viola! Car now has heat and cool.
nvironment Control System workls perfectly now.
The blog intel is right... UnwiredTools is the fix.

[meltedpanda]

ahhhh - the ol burp...shoulda thought of that , always had to do that on my W126 good job!