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MG MGB Technical - Heat and Air control

Neither my heat or air control knobs will turn properly on my 1975 BGT. How does the air control mechanism come off and is the connection at the heater box accessible?
R E Merrall

Having just finished a mechanical rebuild of a 72 GT I can tell you that heater controls are not a lot of fun !
Get a Moss parts book - the diagrams will show you what the control mech looks like .
Access is limited but can be achieved with much swearing ,however the heater box may not be working and be jammed
This means removal.
If ,like mine, it is stuck -more cursing is needed
Take your time and walk away and then go back to it
If I can do it at 73 years old it can be done .
Mind you the engine and gearbox were out at the time !
Andy
bxnsphjii

Thanks Andy you,ve made my day I'm 71 so should be a whole lot guicker!

Rod
R E Merrall

The heater valve control knob is easy to sort. Either the cable is rusted in it's outer, the knob itself is stuck or the valve on the RH engine block is seized. All are easy to strip, free and rebuild.
The air distribution box usually fails simply on efficiency terms, because the direction flap seals have fallen off, it seldom seizes up completely unless the matrix has been leaking badly. You can get at the RH flap pivot through a grommet in the footwell, give it a good squirt!!! Again check the cable and knob are free to move, otherwise the heater unit has to come out.
Allan Reeling

You can get at the back of the heater knobs by removing the glove box - I'm assuming the knobs are up on the dash, not down on the console. They are rotary devices that wind the cables in and out and may need lubricating.

The heat knob controls the valve on the side of the engine so it is easy to see if it is operating properly by watching the valve under the bonnet while someone turns the knob.

The air distribution knob controls a flap inside the heater. In the central position (Interior) the flap should point vertically down allowing warm air to come into the footwells. In the OFF and Screen positions it moves either forwards or backwards to either cut off the air flow, or direct it up to the screen.

To see the flap and the other end of the cable, you need to lie on your back on the floor on the (UK) driver's side with your legs up the seat and your head pushing against the pedals. With a lamp look into the air outlet in the footwell and you should see the flap and the cable clamp. Working on it is not at all easy. Totally removing the heater is even harder. Ghastly design, and not very effective even when it works properly.
Mike Howlett

I've had the heater control mechanisms off the dash on both my 73 and 75 (V8) and didn't need to remove the glovebox.

Paul Hunt

Taken the air control mechanism off and it appears free to move. So either the cable or flap are seized. Can the flap end of the cable be reached to detach it? Haven't looked in the outlet yet; not at all sure I can twist myself enough to do it!
R E Merrall

It's a real fiddle and usually attached to the distribution box before the heater is fitted. It can be done. Probably better to leave the inner screwed on to the flap control and slacken the outer, that way you can ascertain whether the cable is seized.
Allan Reeling

Thanks Allan. Where is the access?
R E Merrall

Drivers footwell, there's a grommet above and to the left of the air outlet.
Allan Reeling

Took the grommet off but the screw that was revealed seems to be the attachment of tom's knob rather than the cable connection. Any ideas, I don't relish stripping out the heater.
R E Merrall

It's through the footwell outlets, not the grommet, and the screen vent outlets behind the console, see here on a 75 http://www.mgb-stuff.org.uk/heatunit.htm#4
Paul Hunt

You've got me guessing now, relying on a distant memory is not good!! You're right. Access is actually through the air outlet itself, as I said a fiddle!!!
Allan Reeling

I'll have another go but even if I can reach it getting the inner back through the trunnion must be a nightmare!
R E Merrall

" getting the inner back through the trunnion must be a nightmare".
"Trunnion"??
Leave the inner clamped and release the outer.
But try spraying the hinges through the heater outlets first.
Allan Reeling

Fixed heater control but unsure which is on or off. When the cable is pulled and the tab on the control unit on the engine block is down, ie towards the front of the engine, is hot water on or off?
R E Merrall

Rod, if the tab is down the heater water supply is off. See attached picture, where the heater valve is turned on.

Andy

Andy Robinson

Andy

That's what I feared. It means the control knob on the dash is in the wrong position; it shows hot when the heat is actually off!

Rod
R E Merrall

Are you sure Rod? I'm a bit confused by Andy's photo and comment. Having just looked at mine, the control cable is extended when the control is in the HOT position. Twisting it back to OFF pulls the cable back in. So in Andy's photo the heater valve is in the HOT position. I can't see how it could be back to front.
Mike Howlett

As Mike says. Look at the pin in the end of the assembly. When that is out the seal is off its seat i.e. the valve is open.

There were at least eight different types across the years and markets. Originally you had to turn the heat control clockwise to open the valve, which is counter-intuitive when compared to most other radiator valves and taps, whereas the direction control was turned anti-clockwise - 'wonderfully illogical' as someone described it. It comes about because both controls operate the same, but at the other end the valve has to be pushed to open but the flap pulled. On my 73 the heat control is stamped 3/366 and the direction control is 3/347, whereas that on my 75 V8 which operates the more logical way the heat is 3/533 but the direction is the same at 3/347. These controls are like a mirror image of each other so one pushes and the other pulls when they are turned in the same direction.

Pictures at http://www.mgb-stuff.org.uk/heatercontrols.htm#4
Paul Hunt

I didn't know that the control rotation was altered Paul. Mine being a 1969 car the heat control is turned clockwise to go from OFF to HOT. I don't mind that, but I can never remember which way to turn the direction control when I'm driving at night. I have fitted a courtesy light above the mirror in my GT and I generally have to flick it on briefly to see which direction I need. As you say "wonderfully illogical".
Mike Howlett

I have the heat on full and I attach a picture of the control knob. Doesn't look right.

R E Merrall

Rod,
Are you sure the geared quadrants haven't been swapped at some time, and are orientated correctly in the dash. Also the knob and shaft are hexagonal but the retaining pin denotes the position.
Allan Reeling

Don't know but a good thought. I'll have a look.
R E Merrall

Don't know but a good thought. I'll have a look.
R E Merrall

"I can never remember which way to turn the direction control"

With one of each I get even more confused.

R.E. certainly has a mis-match somewhere. The knobs only fit on the shaft in one position as it is keyed with the release peg, and the controls should only fit in the dash in one position is there are pips on the control and holes in the dash as well as a flat on the threaded shaft.

As a 75 I'm assuming it's a rubber bumper, is that correct? That's when the control and the knob changed, but you have a chrome-bumper knob i.e. clockwise for heat. It's not possible to tell from the location holes for your direction control as they are at 12 o'clock and 6 on both chrome and rubber bumper. Some cars at a cross-over point did get new bits if they had run out of old, or maybe even old bits if they had run out of new. You shouldn't get a new control and an old knob or vice-versa, but who knows? The two styles are here - http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/knobs1.jpg - chrome on the left and ruber on the right.

I don't think we have established yet which way you have to turn the knob to open the valve, but attached are my chrome bumper control at the top, rubber bumper in the middle, and chrome knob on rubber control at the bottom. You can see that the chrome control locating pips are at 2 o'clock and 8, but the rubber at 11 and 5. Also the chrome knob securing button is at 5 o'clock, whereas the rubber is at 9. The chrome knob on the rubber control when in the fully hot position looks almost exactly the same as yours, but it all depends on whether your knob is fully clockwise or anti-clockwise.

When you say 'heat full on' do you mean the heater is blowing hot? And it blows cold (given time to dissipate the heat in the matrix) when fully turned the other way? Or that you simply turned the control to what you think should be hot?

Paul Hunt

Managed to get the heater control working tolerably well. The photo shows the control with the heater fully on and the knob on the heater valve out. To turn the heater on the control needs to be turned anti-clockwise. The pins on the mechanism are at 11 and 5. Any obs?


Rod

R E Merrall

Do you guys actually get heat from your cars??? I get "warmth" at best. Everything works. I did make the effort to see if the heater valve in the engine block is operational as well as the dash board controls.. Yes it does defrost the windscreen, but not too generous with keeping us warm.

cheers

Gary
79 MGB
gary hansen

Rod, I have had a similar problem. You can get some adjustment on the heater temperature control knob position by adjusting the relative positions of the inner and outer cable at the rotary control on the dashboard. Also I was able to adjust the position of the outer bezel to get it in a better position.

Here's a picture of my heat control on nearly full heat. My control knobs are original - hence the faded lettering.
Unfortunately replacements for my year (June 76) are as rare as hens teeth!

Andy



Andy Robinson

Rod - as I mentioned before you have chrome bumper knobs. You've not confirmed that it is a rubber bumper, but having to turn it anti-clockwise to get heat does confirm that it does have a rubber bumper control. You need to fit a rubber bumper heat knob - at least - to get the correct indications.

As for heat, there are at least ten aspects of the system that can impact the efficiency of the heater, but someone with experience of a lot of MGBs has said that some just seem to work better than others, and even dismantling has not shown any reason why. I've measured the output of mine at 60C/140F when the ambient was about 10C/54F, and it burns our feet. Having said that it is never going to be as good as a modern car, don't forget the MGB doesn't have through-flow (by design!) ventilation.
Paul Hunt

Paul, yes I have a rubber bumper car. Thanks for all your help. Surprised Guy needs heat in Florida!
R E Merrall

Now that we live in Florida, the heat issue is rather moot. Having lived in the Adirondack Mt. region of New York, it gets well below -30C. Did not use the MGB in winter as the car could not produce enough heat to thaw me out.

Cheers

Gary
79MGB
gary hansen

A clean heater matrix and properly functioning heater valve go a long way in improving a marginal heating system. For winter driving, a hardtop makes all the difference in the world. RAY
rjm RAY

Paul's comment "don't forget the MGB doesn't have through-flow (by design!) ventilation." is the main problem with my GT. The heater can produce a reasonable warmth, but because the cabin is effectively a sealed box you don't get a decent air flow through the heater, and the rear of the car is VERY prone to steaming up. You can always crack open a window and put up with the extra noise and cold draught, but that does little for the rear window. The rear quarter windows might have more effect, but they are a pain to use and mine won't stay open once the car is moving.

My other old car is an Elan coupe and it has extractors on the roof pillars, and they make quite a difference. Has anyone ever tried to fit such things to a BGT?
Mike Howlett

The HRW and cracking a front quarter-light in cold wet weather does keep my GT clear, especially since I boosted the HRW performance significantly with a relay. The V8 always powered it off the green circuit, along with the twin cooling fans and all the other fused ignition stuff, so the voltage on that circuit can get pulled down quite a bit 40 years on.

Whilst 4-cylinder cars did have a dedicated relay in 1976, and an ignition relay from 1977 on that was supposed to do the same job, because of problems with the relay sticking on the HRW and a lot of other ignition powered stuff was moved back to the green circuit fuse in 1978. These cars, and those prior to 76, would benefit from a relay.
Paul Hunt

Paul I have a 1975 rubber bumper bgt. How is a relay fitted?

Rod
R E Merrall

A timer relay is even better!!
Allan Reeling

Rod - have a look at http://www.mgb-stuff.org.uk/electricstext1.htm#hrwrelay

A timer relay won't clear the screen any better than a simple relay. Like many so-called driver aids I'd rather decide for myself when something should be turned on and off.
Paul Hunt

Switch and forget Paul, just in case I FORGET while carried away by the rapture of a burbling V8!!!!!!!
Allan Reeling

I agree Allan, it is far too easy to forget to switch the screen off, and with the warning light right down on the tunnel, I just don't notice it. I didn't fit a timer, but wish I had!
Mike Howlett

I get fed up having to repeatedly turn the ZS one on again, wondering why the rear window hasn't cleared yet. By the same token I notice the V8 screen has cleared so turn it off. But then I was taught to cancel my indicators after having made a turn, and not rely on the self-cancelling feature that so many do these days. Other than a few groat's worth of fuel having it on a bit longer than perhaps needed there is no harm.
Paul Hunt

Paul,
But you have a intermittent wipe function. I, on the other hand, decide when the screen needs a flick wipe! You're only a Luddite when it suites your argumentative nature! Sounds like your ZS has a problem. My timer has a 15 minute duration .
Allan Reeling

"But you have a intermittent wipe function."

If you are referring to the ZS it's crap. Single-speed which is rarely the right rate for the precipitation. I've seriously considered putting a Smartscreen on ZS as on the two MGBs - that is a brilliant piece of kit. Similarly the poxy horn buttons which need a finger-tip to operate and move around as you turn the wheel, and the dial illumination that changes the display from dark characters on a light background to light characters on a dark background which means in twilight they are illegible.

If you are referring to the MGBs one can hardly be a conditional Luddite, I would have thought. Recently you said you disagree with everything I say - and you call me argumentative :o)
Paul Hunt

I never said any such thing!! I can document several examples when we have been singing from the same hymn sheet! It's just that the difference between the automation of a rear screen timer and a Smartscreen could be barely measured by a thin feeler gauge!
Allan Reeling

This thread was discussed between 30/10/2015 and 16/11/2015

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