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MG Midget and Sprite Technical - Curious heater issue

Just going through my car, another annoyance has revealed itself. As I understand, the heater should function as follows:

Heater knob out: airflow off
Heater knob in: airflow on, and blower for airflow at slow speeds

However, my heater does the opposite - the airflow is ON when the heater knob is out, and OFF when the knob is in. This also means that the blower doesn't work properly; while the motor and switch are OK a flap must be closed and so there is no airflow when it is on.

Hopefully this is easy to fix, before I take the heater top off, is there something easy which has been switched round the wrong way?

It sounds as though your cable just needs adjusting.

What year is your car?
Dave O'Neill 2

Sorry Dave - early 1971 car.

Would be good to know how to adjust the cable. I couldn't see when I looked in the bonnet how that could switch the behaviour around! (although I probably am being dozy)

as Dave says, it sounds like your cable or it's relation to fresh air trunking connector/flap valve.

Take a photo of how the trunk connector/valve is and another photo of the inside the connector/valve by take the air intake trunking (usually large black concertina type) off the front tube connector, it will slide off the connector and sit to the side.

Note with the photos whether your heater knob is in or out.

Put the photos up here to confirm what's what.
Nigel Atkins

Handbook would tell you. My '71 car is set up so that the flap valve is closed when the control knob is pushed in. Flap opens as you pull the knob out, and fan is then switched on by turning the knob clockwise. It will only turn when it's in the out position else the fan would be strangulated fighting a closed off air supply!

Above about 25mph the heater works better with the flap open,but the fan not switched on, when it relies on the ram effect from the front of the duct.

here's a photo of my '73. The flap will be fully open in this photo as that's the way it's always set even through winter, fully open, only exceptions would be testing it closes and perhaps if in traffic behind a VW diesel or the like.

Note I've shortened the cable on mine and sleeved it in some rubber hose.

you've turned into me, you've misread the OP and quoted the good book - that should give you a sleepless night or nightmares!

Nigel Atkins

Sorry the angle of the photo isn't great for the purpose.

In the photo as reference, if I closed the valve flap the amount of the cable shown would be less, the connecting arm would rotate anti-clockwise, to the left - as per yellow arrow in photo below.

Nigel Atkins

Nigel, l can see why me referring to the Handbook might give you nightmares, but l didn't quote from it as l don't have it with me. And l don't think l misread the OP either as l only said how MY car is set up.

Don't all the heater switches work that way? i.e. they will only turn to switch on when pulled out? I can see how the flap valve can be connected to reverse the open and close positions relative to the position of the control knob. But altering the position which "allows" the switch to be turned on would be harder to achieve.

My heater control is like Guy's; I pull it out to open flap, and only then can I rotate it clockwise to turn the blower on. Then it will only go back in if it is first turned anti clockwise to turn off the blower.

J Smith

I read the OP differently, I could well be wrong as often happens, I only read it as the flap setting being opposite and the motor switch works.

Switches usually only turn ON when pushed IN (with the flap valve fully open allowing maximum intake air to the motor (impeller)).

Sorry I didn't notice you'd put that yours works the other way round (so I definitely read your post wrongly, sorry).

As I've put before the Driver's Handbook tells (new) owners how to work items like the heater that even many long term Spridget owners may not fully know that's why the book is so useful.

As with the hood folding you've found your own method of working and there's nothing wrong with that and it's fine but for those that don't know or are unsure the instructions in the DH are a good place to start.

You're right "quoted" was the wrong word but I can't think of the correct word, you, like I occasionally do, mentioned the good book (what would be the right word for that?).
Nigel Atkins

that may be the case for you and Guy but it's not how mine is or how it's described in the Driver's Handbook.

Give me a minute or two and I'll quote (from) the Driver's Handbook I have.
Nigel Atkins

"To close the shut-off valve, switch OFF the blower and PULL OUT the knob. The blower motor cannot be switched on again until the shut off valve is returned to the open position."

(note I added the capitals to make it clearer when reading off a post)

My Driver's Handbook is not an original but a later reprint and shown as "Publication Part No. AKD 7596 (6th Edition0 (Includes Supplement for GAN 4 Series car) and above is on page 16 of that.
Nigel Atkins

Ignore my crossing outs and they refer to my car that like some others had a separate switch fitted by a PO because the original switch part of the heater control was broken.

Nigel Atkins

I've only ever seen the heater control work the way Nigel has described it - i.e. the blower will only switch on with the knob pushed in, flap open.
Dave O'Neill 2

Normally the shut-of valve is not in use, so valve open, so the heater knob is pushed in, closed to the dash and out of the way, then only when you use it is it pulled out and obvious to see it is in use (and getting in the way of the washer and taking up precious cabin room) - same as the choke control.

If only more copies of the Driver's Handbook were bought and read and referred to. ;)

Note, Guy and Jim I've already put that if you've found a different way then that's fine.
Nigel Atkins

Thanks Dave, Nigel, Guy, and thanks for the photo Nigel.

A picture painted 1000 words. Nigel was correct in that my heater doesn't work as per the manual, but is "set" in the opposite way, and because of that operation of the blower doesn't do anything because the flap is closed when the switch is pushed in.

I've attached a photo of how my heater cable / flap looks like when the heater knob is pushed IN. As you can see, the flap is closed. When the knob is pulled OUT, the cable goes left as per Nigel's second photo.

After manually moving the flap open with a loosened cable, the blower works well when switched on.

I can see there are two screws on the spindle of the flap which in theory look like they could be adjusted and when loosened, the flap could be moved round to the correct position.

It must have always been set up like this as there is absolutely no sign of disturbance.

Naturally, both screws are seized and the heads are stripped - grrrrrr - does anyone have any idea how to get these out once the intake is removed?? (and any idea of the correct size/type to replace?)


I've often wondered why so many heater switches get broken - now I know!!!
David Smith

I'm not sure if you need to worry about those screws - if you did/do then use a good releasing/penetrating fluid like PlusGas (not WD40) and liberally spray and leave over night.

I *think* this will work, what I'd try first is to completely turn the flap with the connecting arm disconnected and then reconnect the cable in correct position.

Spray the spindle with a good lubricant (or WD40 if you must) and spray the flap rubber seals with a rubber/silicon lubricant so that they force passed the sides of the connector tube easily without damage.

The movement of the connecting arm between open and close isn't much so you should have enough cable, if not just shorten the outer cable cover a bit so you have more inner cable exposed.

This time of year I don't use the blower much not that I use it much anyway, in fact I have to remember to turn it on occasionally to test it and keep it from seizing (not that they do), I usually open a window to clear the windscreen or use a chamois.

When you have the flap open look inside to check you have the wire mesh filter before the heater box as a lot of debris can get up the fresh air intake. If you've not got a wire mesh filter then best to get one to stop the large debris getting inside the heater box.
Nigel Atkins

Thanks Nigel. However, I think I will have to try and get the screws out.

It's difficult to see, but on the RHS of the intake you can just make out a blur - this is a piece of metal part of the intake assembly which prevents the spindle from doing a complete rotation.

Hence, the screws need to come out before the flap can be repositioned.. PlusGas it is, hoping the stripped heads won't be an issue....

Thanks for the link to the mesh. It was missing!

... speaking of which, are the demisters meant to have a reasonable airflow? Mine have virtually none - but I don't know if this is because there is something disconnected within the heater box, or the demisters only work when the heater flaps in the cabin are closed, or indeed they aren't working because of the incorrect set-up!

I was in a rush when I posted last and as I was out I thought of that metal stop and wondered where it was positioned.

I'll go out and have a look at mine later to see what can be done without removing the screws perhaps.

As for screen demisters, they're always open but yes they work better when the footwell flaps are closed - see that page 16 that I posted earlier - as Guy put before, you don't need the blower on below about 25mph.

If you buy, read and refer to a copy of the Driver's Handbook then you will know more about the car than many (most?) other owners even long term ones.

This is the copy you want (unless you can find a s/h copy of the Handbook specific to your year of car) -

(Ref: 0057) -
Nigel Atkins

" Guy put before, you don't need the blower on below about 25mph. "
Wrong way round, Guy said (correctly) don't need the blower on *above* 25mph...
David Smith

Sorry my balls-up, it's as David and Guy have put, you don't need the blower on ABOVE about 25mph (again it's on page 16).

Thanks for pointing that out David (no that was sarcasm, my mind was on the problem and getting outside to look at my car).

I also didn't check and the link to Brooklands Books doesn't work but the MGOC link does.

Having now looked at my heater I certainly forgot how large that metal stop plate is, in my mind it was just a small bump.

Sorry, yes it does look like the screws need to come out. I've no real idea what size or type they but Ashley Hinton, seller of the mesh and much other stuff, might.

I see the flap seemed to be riveted in now but can't imagine how a standard gun would reach.

I wonder if they were soaked in PlusGas two or three times over two or three nights then use some sort of extractor or left hand /reverse drill bit to get them out at least.

Kennedy.NO.1 SCREW EXTRACTOR Order Code: KEN0751010K - £2.74 -

Nigel Atkins

I've not used the Kennedy screw extractor or the left hand drill bits below so have no idea of their quality or performance and only offer them up as an example of what's available.
Nigel Atkins

you probably already know this but it was a surprise to me, you can get a new valve flap 'arm & spindle' complete with rivets (although I still can't see how you'd get them riveted) and even a new flap seal.

See parts 16 and 17 (and 22 for mesh) -
Nigel Atkins

Something else you could also try with the blower motor to see if it could blow more -

. have the flap valve cable disconnected and/or open
. turn the blower on and feel for how much air is being blown into cabin (this is best done by opening one footwell flap and checking air flow there)
then -
. disconnect both battery lead clamps (earth first)
. carefully take the sleeves off the female bullet connectors on the blower motor wires
. carefully work the male bullets out of the female connectors
. make up lead connectors so that you can connect the blower wires directly to the battery
. see if this has improved the airflow out of same footwell flap
if so, then it shows probably crudded or poor electric connections

second test -
. reverse the made up lead connections on the battery (it won't damage the motor)
. see if this has improved the airflow out of same footwell flap
if so then it shows you can reverse the connections and get more air flow, if you get less air flow then good as it means it was wired correctly originally

. clean the male and female bullet connectors before reconnecting them
. as required clean, protect and secure all connections/wires from bullet connectors, switches and spades to fuse connections and fuse.

Nigel Atkins

Thanks again Nigel!

Currently waiting for the PlusGas to arrive so I can get the first stage underway in freeing off the spindle so I can swap the heater flap round...

when yo do the job have lots of time and patience, have short goes spread over time I'd say. Soak in PluGas and leave at least overnight if not two nights before trying to see if you can get those screws out.

Normally with a releasing agent each attempt you'd try slightly tightening first to break the rust/muck/crud seal but if the heads are lost you can't do that.

Don't be tempted to put too much strain on anything, if it doesn't loosen apply PlusGas again and leave again to soak overnight or longer if possible before trying again.

I'd try at least three times before moving on to other methods as it only costs a small amount of PlusGas, some of your time in short periods and perhaps a fair dollop of your patience, so even if it doesn't work this with this you've not lost anything really and the PlusGas and method will bring good savings in the future.

Good luck.
Nigel Atkins

Thanks Nigel.

PlusGas arrived today so I tried getting the screws out. Amazingly - easy peasy!

They moved after about 5 mins, with the screwdriver held in carefully/tightly against the remaining guide on the head.

Can now confirm it's simple to manually move the flap to the correct position with the heater control 'out'.

Next job is to remove the heater intake - one bolt not as easy to reach as the others (grrr) - so I can install the new mesh from Ashley Hinton, and replace the screws on the flap.

See pic attached - this is what I removed. Does anyone know what size they might be?

The screw is about 11mm long, the head is just over 6mm diameter and the thread is about 2mm diameter.

Am asking as they may be an imperial size which I have to order online...


Well done James.

If you can get the awkward screw on the heater intake to come out far enough you could perhaps swivel the tube to the side a bit, or just slide the mesh behind the intake to butt up to the stubborn screw as the mesh is open on its circumference.

But I'd persist with the PlusGas and method to get it fully removed, your patience work last time.

As for the screws I've no idea what thread they might be but if you can't find out or you want to try then as they don't look bad in the photo, you might just cut a slot across the diameter of the heads to use a small slotted screwdriver instead of a cross head screwdriver. Note, I've no idea how study or not those screws might be, whether they fall apart as you cut or install them or later.

Clean off the screw threads and if you can clean/flush-out the threads in the spindle arm. Then if you want, put on the screws before you put them back in again either a very small dab of Copperease to make removal easy next time or a tiny dab of Loctite to firmly hold the screws if you worry they might come loose and go through the fan or matrix - or neither and just screw them back in.
Nigel Atkins

Thanks to Dave, Guy and especially Nigel for the help on this – problem now solved.

I got some screws from the big auction site – machine head M3.5 12mm – which were perfect replacements.

Had some initial head-scratching at first as I still couldn’t make the heater work the correct way. Then I realised that the flap had been installed where one side of the flap is BEHIND the metal stop in the closed position when it actually needs to be IN FRONT of the stop. It must have been this way for many, many years.

Taking the spindle out, reversing the flap and installing together with the new screws and the new mesh filter finally fixed the issue. Heater now works as per the drivers manual, the blower is strong and gives an increased delivery of air even at speed, and the flow to the windscreen vents is now pretty good when the floorwell flaps are closed.


Well done James.

Thanks for reporting back, it's always good to hear progress (or otherwise) and result.

Keep that flap fully open (unless you're choking on fumes of course) even in the coldest of winter as the ram effect and volume of air will get the most out of the heater into the cabin.

With the mesh fitted you'll want to clean it out about once a year as you'll be surprised how much debris can get there, very easy job, an old 1/2" paintbrush is handy.

If your blower motor is like mine it isn't that great to the windscreen but reasonable to the footwell flaps/doors, particularly if only one flap/door is open.

Your heater should be hot even in winter, if it's not we have a method to get it so that's also very beneficial to engine cooling.

Perfect weather to have heater and footwell flaps open now. :)
Nigel Atkins

Thanks again Nigel. It's surprising how effective the heater is. Luckily, mine works very well.

Of course, the tap is off at the moment so the heater system can blow cool air - I don't like frying... :-)

Yes the heaters are usually too hot or warm for the small cabin but its surprising the number of Spridgets with sub-standard heater output.

Tuesday night on the drive back my wife decided the hood could stop down as long as I turned the heater on and even though the footwell flaps weren't open the passenger footwell was too hot to have both feet in there.
Nigel Atkins

something I remembered the other day and have now thought to post - you may already know this, or have yours fitted correctly already, but in case not.

The (heater) fresh air trunking is often left semi-fitted at the front of a Spridget, where it passes through the panel and not fully located to the other panel at the side of the rad shroud (behind the rad grille) instead it's left hanging out of line so the full ram effect of fresh air is lost - see photos below for correct locating.

Nigel Atkins

This thread was discussed between 29/07/2017 and 21/08/2017

MG Midget and Sprite Technical index

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