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peter dowdle 13-09-18 09:17 PM

411 Heater Controls
Hi All , I am after some advice on how my heater works on a 411 mk 2.
There are 2 controls on the dash.The demist defrost knob and the heat knob. Clearly the heat knob controls the hot water tap on the side of the heater box. does it do anything other than that?
The demist defrost knob in the demist position clearly sends air to the front screen but my question is what is meant to happen to the air in the defrost position ?
The reason i am inquiring is I get very little hot air in the cabin as I am driving along the road and get no heat out of the fresh air vents.
Strangely when the car is at idle in the driveway and the 2 speed fan is working I get lots of hot air in the cabin at the screen and the under dash trap door, but also through fresh air vents. The car becomes almost too hot which suggests I dont have a water flow problem.
My guess is that at speed the cold air is mixing with the hot air and lowering the overall temperature, which suggests a flap in the system isnt doing its job.
Should I at any time be getting hot air through the fresh air vents?

Any thoughts or suggestions much appreciated.

PEU186F 15-09-18 09:29 PM

There was an excellent article a little while ago in the BOC Journal which described the rather complicated (clever) workings of a 408(?) heater. Do you have access to back issues of the same?

The 411/12 heater is, I understand, from the viewpoint of a 410 owner, basically similar, with some clever fresh air variants, but trying to explain its workings and the clever variants feels rather too much after a glass or two, so let me know the answer to the above and I'll have some idea where to start from.


peter dowdle 16-09-18 06:04 AM

Rodger thank you for that. We are off on a 5 day Bristol Rally starting tomorrow morning . Brother Geoff is a member of the UK club so should have the article.
A bit of a look at the heater today made me realize how complex the system is, It would appear that there are at least 3 flaps inside the box one of which is activated by the heat knob which also turns on the hot water tap.
As you said the 411 has the flow through fresh air vents as well. I may yet have to pull it out and apart to work it out,which i suspect cont be an easy job.

PEU186F 17-09-18 01:41 PM

You will find the article I mentioned in the Bristol Owners Club Bulletin Number 142, dated Autumn 2011 and the good news is that it actually describes a 411 heater and ventilation system, not an earlier model which is what my memory told me.

I found it extremely useful when I had the 410s heater out to replace seals etc. The heater box itself is apparently identical.

Two points

I found that there was a very simple gravity flap on the 410s heater which opens a direct path into the heater box from the air intake in front of the windscreen when the pressure in the former overcame the pressure supplied by the fan. So at speed the air doesn't all have to pass through the fan. I don't see any reference to this in the article, perhaps the 411 was different. I also discovered that the foam on this flap had long since perished causing it to close at intervals with a surprisingly loud, rather alarming and previously untraceable clang.

I see I marked up the copy that found its way into my workshop manual, at the bottom of page 27 to read "Slacken screw E the point F and retighten screw E"

Trust you have a good tour


PEU186F 23-09-18 03:28 PM


Am having difficulty responding to your private message with a private message, quote

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It would certainly be easier to see how everything interacts with the heater out, but you will need to dive under the dash to disconnect the demister ducts. I highly recommend taking the relevant seat AND its runners out first which will save a lot of pain, not that it will be a doddle then. And then some of the ducts may turn out to be perished and so it goes on

2BA spanners are essential, an adjustable will drive you nuts!

I seem to recall that the fan mountings are no more than substantial sleeved grommets so these are probably worth investigating further before paying high prices

My regards


peter dowdle 24-09-18 09:30 AM

411 heater
Thanks again. The rubber mounts are actually a block of rubber with a threaded stud attached on either side like a very small engine mount.

PEU186F 26-09-18 10:19 AM

These are called bobbins and an Internet search for " bobbin vibration mounts " (to differentiate them from the sewing machine variety) will bring up inumerable suppliers

Kevin H 30-09-18 10:27 AM

Did you resolve your heater issues?

There are a couple of photos in this thread that I took many years ago of the heater linkages in situ. It appears that the heater control cable only acts on the hot water valve.

I have the heater box out of my (your old) 411 S5, having since fitted air conditioning. I can't find it at the moment, but I know I have it somewhere. Let me know if you still have questions re the heater and I will endeavour to find it.


peter dowdle 30-09-18 09:02 PM

411 heater
Thanks Kevin

Im now on top of what to do with my heater, The article in the UK Club magazine that Roger pointed me to is excellent. I have established that my controls are set correctly and that i have a good flow of hot water yet when the car is out on the road very little warmth comes through.While the car is stationary hot air pours into the cabin even through the fresh air vents, Clearly the seals on the flaps inside the heater box have collapsed causing the cold air to mix with the hot air. This week I plan to pull the heater box out and recondition it.

Regards Peter Dowdle.

NB When i get it apart and cant put it back together I might come calling,

peter dowdle 09-08-19 09:27 AM

I can finally report that I have plenty of hot air finding its way into the cabin of the 411. I followed the instructions as set out in the excellent article mentioned earlier. The heater box was removed , pulled apart ,cleaned and painted, the heater core reconditioned and all seals replaced .After much twisting and discomfort it was refitted to the car.

To my horror still no hot air. I eventually discovered that a flap which is activated when the hot water tap turns on allows air from the fan to flow through the core. This flap was closing rather than opening blocking the air flow to the internals of the heater box. A simple adjustment and all works well.

The first control knob on the dash controls the temperature in the cabin. It has three functions as it is rotated it opens a flap which allows cold air to flow into the heater box. At the same time it opens a second flap allowing air to flow through to the cabin above the the passengers feet. Its third function is to turn the hot water tap on to allow the core to heat up.

The second knob marked DEMIST and DEFROST if turned half way allows cold air to flow to the windscreen without impacting on hot air to the cabin. If fully rotated it cuts off the cabin air and directs hot air to the windscreen. All very simple Ho Hum.

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