1972 350SL Heater blows cold air

[Jim Hansz]

I just got my 1972 350 SL off the transporter from sunny California. It is perfect, except for one small thing. The heater blows cold air. This is no small thing here in NW Montana, the thermometer on the porch was at -3 degrees yesterday morning. I have no manuals yet and the control system appears to be somewhat different from my older sedan. Any thoughts on where I should begin looking to uncover the problem?

Thanks,
JCH

[Adamson]

Check the heater valve. It is located in the engine compartment to the rear on the right hand side of the car. There are some rubber heater hoses attached to it.

[ja17]

Hello Jim,
As Adamson suggested check the heater valve. It is vacuume activated and controls coolant flow through the heater core. I believe it is mounted on the firewall right behind and above the air cleaner. Check to make sure the diaphram inside holds vacuume and activates the valve. If the diapram is ruptured you can temporarily wire the valve open until you can replace it.

Joe Alexander
Blacklick,Ohio

[DIAZ-CANEJA]

Jim.

The default position of the heater valve is open (spring return), so if you have no vacuum, the heater should remain open and you should be receiving only hot air.
So your valve must be clogged.
I suggest you remove the valve to see if it's not clogged on purpose to cover a heater core leak (ouch $$$)
Good luck
Jesus
350 SL
230 SL
300 SEL 6.3
220 SE Finback

[SLlover]

Jim, we had the same problem on our 560 Sl. The solution was the mono valve. A easy fix and the price was $20.00. Here are pic from a 300e , should be in the same location. If you have a problem locating a mono valve, pm us.

Real easy take a look. Hope this helps. http://www.zianet.com/SLlover/page62g.html

[Jim Hansz]

Many thanks for the helpful suggestions. I isolated the vacuum supply to the underhood valve and the heat came on when it moved to its default open position. The valve appears to function, but seems to be receiving a faulty signal from the in-dash controls. I now have heat in abundance, but no ability to regulate it via the dash levers. This is not a show-stopper, for now. I intend to dig into the dash/console area to investigate the problem, but will do it when the weather is more agreeable and I have rounded up the requisite manuals and schematics for the in-dash control system.

Thanks again for the helpful guidance.

JCH

[M.F. LASLOW]

I would play with the temp. control wheel on the dash, running it full cold to full hot and maybe turnign the climate control system on and off. Sometimes the temp control just needs alittle jogging ot get it dial in the right temp that u want.

[Jim Hansz]

Thanks, but my '72 does not have automatic climate control. It is the "simpler" manual lever variety that diverts air across the heater core and signals the water valve via my movement of vertically travelling temperature levers, similar to the old horizontal lever system in the earlier cars, I believe. One of the levers will not move through its full travel, as though something is obstructing it. I shall look into it when weather is more agreeable. It may be related to an earlier installation of some burl wood trim pieces that required disassembly of the console control panel. I suspect something has come loose and needs attention. There were originally some lights for the defroster instruction display and the seat belt warning. I believe these were just laid aside inside the console area and now one of them may be fouling up the works.

Thanks again,

JCH

[DIAZ-CANEJA]

Jim.
As Joe sugestted, the mono valve may be more the cause that the internal controls.
Just check you have voltage in both mono valves, they are located on the firewall behind the air filter case.
I suggest you remove them and see if they change their posittion when applying voltage.
The mono valve has three small pipes and it should change the air direction from one to the other when applying voltage

[Jim Hansz]

Again, thanks for the advice, but my car is a '72, and as such, does not use the mono valve that is more common to newer climate control models. Mine uses a simpler vacuum actuated heater valve that is somewhat similar to the old style cars. The 72 model uses a vacuum diaphragm to actuate this valve, open or closed, upon receiving a signal from the dash lever control system. The old cars used a fully mechanical system that had a brass double valve operated directly by cables from the dash levers. This 72 model still uses dash levers, but they send a vacuum signal to the valve diaphragm which then stays open if heat is desired (no vacuum), or closes down (full vacuum) if heat is not required. It appears that the valve responds to vacuum, there is vacuum to the diaphragm almost all the time but it is sometimes off. It seems apparant the vacuum signal is not being properly or consistently modulated by the dash control system. Oh, as an after-thought, I guess the valve on a '72 could be contrued as a "mono" valve becasue it is a single valve rather than a twin sided valve like the old cars. But it is not of the newer climate-control variety.

Thanks again, I genuinely appreciate all this information. I am on a steep learning curve with this car, but fortunately it is confined to one small area.

JCH

[DIAZ-CANEJA]

Jim.
I have a 350 SL and yes it has two electric actuated valves that control the vacuum for the heater valve. I have tha vacuum diagrrams and the mono valves are indicated in all the diagrams.
The heater control lever sends an electrical signal to the valves.
Your turning knob inside only opens or closes the vents in front of the windshield.