1972 280SE 4.5 no heat - PeachParts Mercedes-Benz Forum

Scotto52 · #1 12-06-2020, 10:07 AM

So as I’m crossing projects off my list, one of them is no heat.

I’ve looked at the two cables going to the valves behind the dash, they both move, but are very ugly looking. When the car is running, both heater hoses on the fire wall are hot. There is literally no warm air blowing from the vents. I feel some warm air when you put your hand behind the glove box area, just seems like it’s radiating from the heater core. When I replaced the heater hoses I blew water through the hoses in both directions. The blower motor does work on all speeds.

I’m guessing I have a blend door that’s stuck. Can anyone steer me in the right direction? Or maybe have some tips and pointers?

Thanks again everyone, stay safe!

rumb · #2 12-06-2020, 02:10 PM

Those cables can break. The heater valves get stuck and need cleaning. With the fan on high you should feel air coming out somewhere. Can you feel air flowing somewhere but not hot?

I would get a good view of the valves under the glove box area and see if they are actually functioning.

Tomguy · #3 12-09-2020, 04:20 PM

I don't think the 108/109 has a blend door (the AC is a completely separate system that uses only the 2 front center vents). It's been a while and I don't have one anymore, but I'm pretty sure you have fan speed, floor or cowl (and far left/far right vent) distribution, and 2 valves, 1 for passenger's side heat, and 1 for driver's side heat.

When you put the "Defrost" setting on it's full heat straight at the windshield and the top 2 vents. Can you feel any air coming out of them? Note that nothing will come out of the 2 center AC vents when using heat.

801mbz · #4 12-10-2020, 03:29 PM

check the red levers they have two cables each one for the valve and one for a flap that lets air go past heater core

Fitts07 · #5 12-11-2020, 02:14 AM

I have a W110, we have very similar HVAC systems.

Most likely your heater valves are not opening. Possibly the cables are broken or are slipping on the valves. Have you verified the valves actually move? They do have a small bypass built into them that allows a small amount of water through the heater core at all times. Have they ever been serviced? Its possible to reinstall the heater valves out of "phase" the valve may still move but the holes dont line up for water to flow. Be careful flushing heater cores with a hose. City water pressure can be very high, locally i have over 80psi. The heater core never sees that and can blow the joints on it.

There is a great write up on the Ponton website about heater valve servicing. Its kinda funny that you can take a spark plug and it will screw into the valve to pull it out.

Mercedes-Benz Ponton Heater Core Valve Repair © www.mbzponton.org

Scotto52 · #6 12-11-2020, 05:50 PM

Ok, so I was in the garage for a bit today. It seems the only airflow is through the center vents. Regardless which way I put the levers. As stated before, all the cables work and the valves turn. The blower works at all speeds. When I stuck my head up under the passenger side dash, I can see the mode doors move with the cables. I can actually touch and see the core with a mirror. There’s just no air flow from there. It looks like the duct work comes right from the blower motor to the center vents. I’m guessing there has to be something that stuck on the vents to be open all the time. I’m not sure. I was trying to find an exploded diagram of the system. Still looking though.

Scotto52 · #7 12-12-2020, 05:36 AM

Is there a separate blower motor for the heat, and one for the A/C?

Scotto52 · #8 12-12-2020, 01:45 PM

Ahh, there is! Operator error on my part. I guess the correct statement now would be “heater blower motor isn’t working”. So I took the cowl off and looked down in there. The heater blower motor can spin by hand. I also put my test light on each connector while clamped to the (-) batter post. And now that I think of it, I’m not sure when the blower runs KOEO or KOER. I didn’t have engine running and I had no voltage present. We’ll get there, all in due time.

801mbz · #9 12-14-2020, 05:44 PM

yes ac and heater blower are different they built the car around the heater motor

Tomguy · #10 12-18-2020, 02:36 PM

The top left lever is the fan speed. All the way to the left is high / full blast. If the fan can turn by hand but the switch does nothing I hope for your sake it's the switch or a fuse!

p.smitty · #11 12-29-2020, 12:25 PM

Fitts is probably on the right track. When I had the dash out on my 4.5, I had the heater core "recored" cleaned and put new o-rings in the heater valves that allow water to cycle through the heater core. And yes, you have to be careful to put the valves back in correctly to allow water flow when selected. The valves can get very "gummed up" over time and is the main cause for the heater controls to be troublesome. The heater core can also get somewhat clogged as well, restricting water flow.

It's a PITA to rebuild this system and the worst part is the heating blower fan. I already had the dash removed for refinishing and found a new NOS MBZ blower motor $$$ and replaced that "while I was there".

Unless this is a really nice car, you have time and patience, you're a contortionist, and you're planning on keeping it. I would almost just consider driving it on pleasant days.