'85 300SD - No Heat from Vents - PeachParts Mercedes-Benz Forum

treiberg · #1 12-10-2009, 09:38 AM

I just replaced the thermostat - this fixed my coolant low temp problem, and now the car runs around 80-85C, just where it should.

I also replaced the mono valve and now have wonderful heat - but it only seems to work when I have the defrost button selected. If I use the EC mode, I get very cold air coming through all the vents. The manual said heat should be coming from the side vents, and it looks like the center vents should be nearly closed in heat position.

I speculate a bad climate control unit, or maybe some venting actuator problems. Any ideas?

latitude500 · #2 12-10-2009, 05:13 PM

I’ve been told that it could be the climate control unit but most likely it’s the vacuum pieces under the dash that have gone bad. They have a plastic diaphragm inside them that go bad over time. Make sure your climate control unit is good, then look for the diaphragm devices to see if they are still working.

Graplr · #3 12-10-2009, 05:22 PM

I've had the same problem on my SD since I've owned it.

I plan on replacing all the pods when I change out the dash for one with no cracks sitting in my basement...whenever I get around to it.

I have always assumed that was my problem.

For the past few winters I depress the defrost button and one of the arrow buttons (usually the one right next to the defrost) and I get heat out all my vents. If I depress any one of the arrow buttons by themselves I only get cold air.

toomany MBZ · #4 12-10-2009, 05:32 PM

I don't think the 126 has that foam hose, the sending unit is above the middle rearview mirror.
The CCU itself may have one or more cold solders.
Defrost defaults to full heat, often this will be the last item to fail.

SD Blue · #5 12-10-2009, 08:24 PM

If there is no redirection of air, I would suspect the pushbutton controller. Internally, the circuit boards are arranged in an "H" pattern and develop cracks where they join together. Try a search for "resoldering" with photos.

If there is some redirection, but not necessarily the correct vent, then the likely culprit is one or more of the vacuum pods. Fortunately, on the W126 pulling of the dash is not required and most are fairly easy to access (The exception being the center pod, used for air conditioning from the center vents.)

treiberg · #6 12-11-2009, 12:19 AM

There is redirection of air from the defrost to the dash vents when I go from defrost to economy mode, but the temperature of the air goes to ice cold after a few seconds (I feel just a second or two of heat).

SD Blue · #7 12-11-2009, 11:34 AM

From my schematic, it could be either the pushbutton controller or the temperature control electronic unit. In defrost mode, the ground side of the mono heater valve is wide open, giving full heat. In EC, Norm, T/V, the monovalve ground is controlled by the temperature controller (it is routed through switches,etc. in the pushbutton controller). Have you done any resoldering in the pushbutton controller?

Probably the surest way to check is to place a voltmeter (dc) on the ground side (brn/wht) of the monovalve and see what the changes are when switching from Def to E/C. In Def, you should see supply voltage (12vdc) and in E/C, you should see a variation, after a slight delay, when moving the temperature dial.

(Note: This is what I gather from the schematic and experience in electronics, so don't expect the results to be set in stone.)

Graplr · #8 12-11-2009, 11:50 AM

Quote:

Originally Posted by SD Blue

From my schematic, it could be either the pushbutton controller or the temperature control electronic unit. In defrost mode, the ground side of the mono heater valve is wide open, giving full heat. In EC, Norm, T/V, the monovalve ground is controlled by the temperature controller (it is routed through switches,etc. in the pushbutton controller). Have you done any resoldering in the pushbutton controller?

I know your answering for the OP, but since we have very similar issues I'll answer for his benefit.
-I did on my original unit. Fried it and bought a used one. Granted I bought the car in the spring and fried the CCU then so I don't know definitively if the issue existed with the other CCU. The reason I attempted to solder it was because I was getting heat out of the defrost always, especially under hard acceleration. I eventually found the issue to be a bad aux water pump. Unplugging that solved that issue. But I was quick to try soldering as everyone on here said it was easy and had many state it fixed their issue. It didn't work out so great for me. If I had to do it all over again I would just buy a quality rebuilt CCU. I still may if replacing the pods doesn't help.

Quote:

Originally Posted by SD Blue

Probably the surest way to check is to place a voltmeter (dc) on the ground side (brn/wht) of the monovalve and see what the changes are when switching from Def to E/C. In Def, you should see supply voltage (12vdc) and in E/C, you should see a variation, after a slight delay, when moving the temperature dial.

I have my monovalve unplugged so it should be full heat no matter what the CCU is telling the monovalve what to do. I still get no heat out of anything except defrost or defrost derpessed along with another button simulatneously. I did check the voltage at the monovalve (which was replaced BTW) and it went up and down with the dial. I don't remember what buttons I had depressed but I noticed a change in the voltage (12V or so when the dial was as cold as it would go and close to 0V when full heat on the dial was selected).

This is why I suspect it is a vacuum/air flow issue. I know I checked the vacuum at the kickpanel on the passenger side and they all held air, but this was a long time ago. I should revisit them but I'll just inspect them individually when I have the dash out.

Graplr · #9 12-11-2009, 11:51 AM

Quote:

Originally Posted by treiberg

There is redirection of air from the defrost to the dash vents when I go from defrost to economy mode, but the temperature of the air goes to ice cold after a few seconds (I feel just a second or two of heat).

Exact same things happens on my 84SD.

treiberg · #10 12-11-2009, 02:08 PM

Thanks for all the input. I will do some more testing and let you know what I find.

Graplr · #11 12-11-2009, 02:10 PM

Quote:

Originally Posted by treiberg

Thanks for all the input. I will do some more testing and let you know what I find.

Please do. I've heard a few others state the same problem as us, but none with a resolution.

We'll see if the dash pods help (when I get around to it...most likely not for awhile.

).

micalk · #12 12-12-2009, 01:21 AM

If the air is not redirecting, it could be anything from bad ACC to switchover valves to vacuum pods. You can test the pods and the switchover valves if you pull the ACC - the valves sit right behind it. You shouldn't have to remove the dash to get to the pods themselves, though that may make it easier. But you may have to remove the center console (not sure - I'm lucky enough to have a working system - I only had to replace a single pod for the fresh air flap to bring it up to full operation). One leaky pod could affect others that need the same vacuum to operate so you'll need your mity-vac and vacuum diagrams to isolate it. Electrically speaking, your flaps are switching, so at least some of them are fine.

When you hit the defrost button on the ACC, you open the circuit for the monovalve. This works. The monovalve allows hot water to flow. When you hit the Economy button, the monovalve circuit is routed through the ACC to the electronic temperature control unit. The TCU takes inputs from the interior temp sensor and the heat exchanger temp sensor to determine if the monovalve should be cycled. The cold engine lockout supplies ground for all the relays in the ACC to operate (I think - at least some of them - that damn schematic is so bad in the manual I can barely make some of it out!). But it appears to be bypassed when the defrost button is selected. So, your defrost works and the the monovalve allows flow when there's no ground applied. So that's our starting point.

If you have fans when the Economy button is pressed, your cold lockout is probably good. But let's check anyway, basically, because it's simple and it rules it out as a suspect. This is a temperature controlled switch on the engine block next to the EGR with a single wire going to it. With the engine cold, pull the wire and check continuity between the tip of the switch and ground. It should be open. With the engine barely warm (~100 deg F), it should be closed. I expect you'll find that this switch is operational. Alternatively, you can ground the wire and see if that changes any of your symptoms. Inspect the wire going to it and make sure it is viable at least up to where it enters the wiring harness with the glow plug wires (IIRC).

I'm assuming now that the cold lockout switch is good and you still have your problem. When the ACC Economy button is pushed, you get no heat, which means that the monovalve is energized and no water is flowing past the valve. Since it is getting a ground that means the circuit through the ACC is good (it would be highly unlikely for the ACC path to short to ground), so I'd guess the only way for the monovalve to be energized is either for the electronic temperature control unit to have a short in the transistor that controls the monovalve, or the sensors that feed it are sending the wrong signals and forcing the transistor to conduct. My first inclination would be to test both the heat exchanger temp sensor and the interior temp sensor (83-603 in the manual - you can use a standard digital volt-ohmmeter; just plug into the controller plug using the meter at the pins.) If those both test good, then I'd try replacing the temp control unit. You might try testing the resistance between pin 8 and pin one of the controller to see if there's a short or not. (edit - test for short with power off! You may damage your ohmmeter if power is on.)

good luck!