vettebk

The weather just turned cold here and my S600 started to blow heat this week. A couple of times this week, it was blowing really cold A/C; so cold I had to turn the climate control to off. No heat at all from any of the vents.

I checked all of the settings and it looked just like always. I did notice the rear climate control system was on and usually I don't run it at all. Jumped into the back and turned the system off using the the rear controls - Auto, Man, Off - I selected off. Hopped back up front and heat is blowing just fine. No codes.

What's up with that?

auburn2

The only thing I can think of is it had not heated up yet. What was your engine temperature? If you turned up the temperature on or both sides of the front that side should have started blowing warm air once you car warmed up.

DaleB

The possibilities are almost endless. Water flow for the heaters in these things can be complicated. Having dissected the old changeover valve (AKA AC water valve, heater valve, etc.) I can see several failure modes that could occur. This is the one that is located just forward of the firewall, on the driver side (assuming you have a right-hand drive car like you'd find in any civilized country). The valve consists of a small DC electric motor with a worm gear that drives a large spur gear, which is on a shaft with a small pinion gear that's inside the water flow. That drives a rack up or down to open or close the valve. The AAC control unit tells the front SAM to open the valve if it thinks heat is needed AND once the engine is warmed up. The actual control of the valve is done by the front SAM, which is why you won't find that in the AAC actuations in DAS.

So how does the SAM know the position of the valve? Well, really, it doesn't. There is no feedback, there are no limit sensors. The best you can do is to drive the motor on one direction until it stalls (easy to detect by sensing the drive current), and assume that's the fully open or closed position. Problem is, if there's any debris or crud built up on that rack and pinion assembly, it will seem like the motor is stalled when it's not actually fully open or closed. That's what the problem was with my old one, I believe. The motor was OK, but there was a chunk of crud wedged in the teeth of the rack. I would guess it was part of the plastic hose nipple on the valve assembly, which snapped off and literally crumbled as I removed the old valve from the hoses. Old crappy plastic, and apparently a poor choice for being in the flow of hot engine coolant. Anyway, I can see where it would be possible for this valve to be either completely stuck, or have only partial travel in either direction or even both directions. That could completely screw up water flow to the heater. It could also be intermittent, especially if water has seeped into the motor side of the assembly and corroded parts of the motor.

Here's where it gets fun. Suspecting my valve was bad, I ordered a new aftermarket valve and put it in. Not a terrible job, took me maybe an hour or less. Afterward I and STILL had cold air. Some poking around with DAS manual actuations proves that with the valve OPEN, I get cold air. With the valve CLOSED, I get nice toasty warm air. The manufacturer apparently connected the motor backwards. I got a refund on the part (thanks, Ebay) and am installing a new genuine Mercedes part at double the cost.

Then there's the "duo valve", which is located in one of the wheel well areas and apparently controls water flow to the rear heater. I haven't fully explored how that part of the system works, but the duo valve is substantially more expensive than the AC water valve.

BOTUS

if you have rear AC controls, there are extra bits my understanding was they have "2 Duo valves". Which I thought were the expensive $400 dollar things. But both front and rear parts doc show they share a change over valve component at $50 each - then some totally different bit for the back that does sit by the lower door hinge inside the inner wing and that's silly money ($400). This bit has a pump for further circulation of residual heat when the key has been turned off.... and two solenoids that do each side of the car in the rear by moving strange pintels that wear both itself and the seat of its conical face. These pintels are listed with a repair kit for the DUO valve and the part normally fails HOT


front 170 and 150 at the rear are "change over valves"
https://nemigaparts.com/cat_spares/e...71/63v/83/181/

rear 350 "timing valve" and this is big money
https://nemigaparts.com/cat_spares/e...71/63v/83/183/


part 350 does have bits similar to these inside
https://www.ebay.co.uk/itm/Heater-Co....c100008.m2219


.

vettebk

Thanks, Dale. That was my thought. I suspect my water valve is original. It happened again today. Fine in the morning. Went out again in the afternoon, no heat. Turned climate control off. Drove for ten minutes and turned climate control back on. Auto. Heat.

I just got a Thanksgiving $25 service coupon from the Mercedes dealer (LoL). And I still have another year on my extended warranty.

vettebk

Car was at normal op temp. Something is definitely not right. Suspecting that water valve near the firewall as Dale described.

It is a shame because she was running really good.

DaleB

I was fairly shocked at how brittle the valve assembly was when I removed it. I'd say it's a time limited part that probably ought to be replaced every 10 years or so anyway. The replacement isn't too expensive; I just ordered one from a M-B dealer for $49 and change. My son had to replace the same part in his '08 E550, which did fix most of his HVAC problems. In the summer his AC was blowing hot air part of the time, with inexplicable periods when it seemed to work OK, then went back to blowing hot air. I will admit to having used a phrase not entirely complimentary to our German engineering friends more than once over the past few weeks. There are simple and reliable ways to do things, then there's the Mercedes-Benz way... "Let's complicate it until it breaks, then add a few parts..."