W211 Heating Failure - I'm Puzzled; Advice Needed

E55BOF

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My '03 S211 is not blowing hot air, only cold. To check it wasn't one of the cabin temp sensors, I put the iCarsoft MB II on it; the overhead sensor read 11.5 degrees, the dash sensor minus 99 degrees. Bingo! (The dash sensor was loose on top of the lower panel, and the only thing I've found online states it can be replaced in its correct location without further dismantling. Maybe by Inspector Gadget it could, but not with my big mitts it can't; I couldn't get closer than best part of a foot near it).

Replaced the sensor, left it on top of the lower panel, with a foam collar to stop it rattling around too much. and now both sensors read within a degree or two of each other. :banana: then ?

Er, no; it still only blows cold...

There's no difference between front and rear; both only blow cold.

No fault codes now, but as far as I can find, the Carsoft doesn't provide OBD for either the duovalve or the aircon/heater control panel.

All four heater demand temperatures were set at 30 degrees throughout, except where indicated otherwise. The aircon was set to off at all times. I still think the duovalve is the most likely culprit, but some of the heater flap positions seem odd, depending on whether the manual air distribution is set to screen, chest, footwell, or mixed footwell/screen. I'm assuming that 0 means a flap is closed.

The charcoal filter and recirculation flaps function normally, though the recirc warning light was flickering on and off irregularly without being touched.

In case it helps, I've attached a chart of the various readings.

I'm assuming that the system is bright enough to discount the implausible -99 temperature reading from the u/s dash temperature sensor and use the upper sensor's reading, so something else has gone wrong. I still suspect the duovalve, but the system has done this two or three times before, and fiddling with the controls brought it back to blowing warm again.

Is there any easy way to check the duovalve, and do the flap settings all look OK?
 

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bolidemichael

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I hope that perhaps you're over thinking it.

Twice I have experience cold air blowing irrespective of the temperature. One was en route to dropping someone off at Milan Malpenza airport on a rather cold January afternoon!

On both occasions, it has rectified itself without having threaten to persist.

If it is the duovalve (which I found pushed high temperatures to the rear), I found that it could unsieze by cycling to either extreme of the temperature range.
 

Jobsworth

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One or two...
Duovalves are common for trouble. It’s possible to put a bar on it and give it a tap while the fault is occuring. A tap will often set it working again, then you have your smoking gun. Only other possibilty that springs to mind is the outside air temp sensor. You can see the reading in the middle of your dash cluster if you select the correct menu. I’ve seen bad ones adversely affect the climate control before
 
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E55BOF

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I've tried cycling the temperature Low to High and back several times; no change. One thing I've never understood, though, is how a failed duovalve could result in a high temperature in the back and normal temperature in the front; surely that should be a flap issue?

The OAT sensor reads correctly. If I have to do the dismantling to get at the duovalve, I won't bother trying to unseize it; it will only seize again, so while I'm in there I'll replace it anyway - they're not expensive.

It looks as though I'll need to wear a coat in the car until I get round to doing it; good thing we're not in the depths of winter yet.
 

Jobsworth

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One or two...
Ah, you’ve moved the goalposts now. In your original post you state both front and rear only blow cold...
 

bolidemichael

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Do you have heated seats, at least?

As a word of caution, the duo valve replacement involves the removal and replacement of the servo.
 

Jobsworth

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One or two...
Do you have heated seats, at least?

As a word of caution, the duo valve replacement involves the removal and replacement of the servo.
Well you say that... Cole always used to unbolt under the dash and move the whole MC unit up and out the way with the brake lines still attached. I am blessed with the wrists of a ballerina, and can get my hands in there without. Still fiddly though ;)
 

bolidemichael

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Well you say that... Cole always used to unbolt under the dash and move the whole MC unit up and out the way with the brake lines still attached. I am blessed with the wrists of a ballerina, and can get my hands in there without. Still fiddly though ;)

The indy that changed mine wasn't so dexterous... it was a £400-£500 job!
 
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E55BOF

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Ah, you’ve moved the goalposts now. In your original post you state both front and rear only blow cold...
No, front and rear do blow only cold. What I'm getting at is that I've seen a couple of comments that blowing only hot from the rear while the front works as normal is a sign of a failed duovalve, but I can't see how that could be correct.
 
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E55BOF

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The indy that changed mine wasn't so dexterous... it was a £400-£500 job!
Cor blimey, you've 'ad your leg lifted, guv'nor, aint'cha? Mine's going in to Terry Gates on Friday - about £150 + vat all-in. That'll do me nicely!
 
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andy27168

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Hi,
Probably £400 - £500 just reflected the official MB book time, so no rip off intended, and as implied by the contents of this thread it is known for a being time consuming especially if the known short cuts cant be employed because of other factors.
I am dreading the duovalve failing on my CLS I have already on several occasions had to cycle the the temp through the range when I have noticed inconsistencies in the temp control in the car.
 

MikeInWimbledon

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Before jumping into large scale faffing about, what's going on with the external temperature sensor? Is that giving a normal reading?

It could be that 21st century car design thing: it's not the car that's faulty, it's a stupid £5 sensor giving a false warning.

.
 

MJJ

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Have you checked the coolant level? Very low coolant level typically means that the pipework and in-dash heater matrix are starved of hot coolant to provide cabin heat.

Martin.
 
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E55BOF

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Coolant level is fine. Duovalves are a known failure item, and on a car that is seventeen years old almost to the day, and has done 130K miles, if the duovalve is original, and I suspect it is, it's overdue for failure.
 

pmcgsmurf

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You can change these without taking the master cylinder out, very fiddly but can be done.

It is probably quicker though to unbolt it and move it to the side. ;)
 
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E55BOF

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You show me the invoice first!

The duovalve it was; the car is warm again.

Terry Gates came up trumps, as usual; 1.6 hours labour (professional; it would take me at least twice as long...) and the part, £149.80 + vat. There's not actually much dismantling involved to get at it, but it's extremely fiddly - with big hands, between 'nightmare' and 'f'ing impossible' on the difficulty scale - actually getting it out and replacing it, and at that price I'm quite content to avoid the cursing and swearing, and the scrapes and cuts on my hands...

I suspect your indie just didn't want the job, really.
 
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Smart320

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+1 for the Gloat 🛠
 

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