MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Heater Box Repair Inquiry

I'm in the process of replacing the heater core on my 72B. While I'm at it, I'm now having to deal with other complications I ignored in the past.

Like, when I turned the floor/defrost knob on dash it didn't change air flow at all.

I see (through the right side floor heat vent)that the rusted cable at the air flow flap doesn't move when the knob is turned. So of course the air flow flap doesn't move with the knob either.

I also see that when moving the flap from above by hand that there appears to be a small metal tab that perhaps should be attached to the flap, thus enabling cable movement to be transferred to flap movement.

Question 1: Is the metal tab at the box end of cable suppose to be welded to the flap that controls air flow?

I imagine some of this will be clear to me once I get the heater box out, but first I have to get the cable off. Which isn't going easy I can't get the control value out either.

Any ideas on how to remove a stuck control knob from dash, so that I can remove control valve.

Once I get the rusted nut holding the cable to the heater box off, I can then wrestle with getting the control valve out so I can replace what is obviously a rusted in place cable. I foresee lots of things breaking in the process.

Any ideas? So far I see that pulling engine is easier than pulling heater box out.

Thanks for any assistance.

BobA (Andy)
Stillwater, MN

R.W Anderson

Andy To remove control knob, spray with WD40 behind the knob and also the push button which holds it in place, then wriggle rather than straight pull till it comes free. The cable is easier as it is possible to leave the flap end still fastened to the box, simply disconnect cable from control, and remove heaterbox from car. The cable will bend quite severeley as it comes thro the bulkhead but your going to replace it anyway. This may seem brutal but it works . Jim
jim soutar

It was not a real easy job...but well worth it...The heater/defrost worked so much better after a rebuild...The box and flapper is easy to reburbish/rebuild with sheet metal/etc...and once done...well worth the effort...
Pete

I have my heater box sitting on the bench.

Would any pictures help

Bruce Mills

This might help:

http://www.mgb-stuff.org.uk/wn_heaterframe.htm

The way the cable is attached to the heater box is almost identical to how the other cable is attached to the water valve.
Paul Hunt

If you think pulling the heater box out was fun...wait'll you try
to re-install it!

This is one of those things where I wished someone on the
factory assembly line had a video camera to document how
installing the heater box was done in a quick and timely manner.
D

As well as Paul's site, this worth a look too. http://www.chicagolandmgclub.com/photos/b_heater/
Mike Howlett

APPROVED METHOD OF REFURBISHING MG HEATER

1. Take four glass bottles to your driveway.
2. Hammer the bottles into 1/2" shards of glass.
3. Pour gasoline on spread out glass.
4. Ignite.
5. Walk across your driveway and back.
6. Hire a shop to do your heater.
Dan Robinson

Andy

WS-40 is primarily for Water Displacement and, at best, is only mildly helpful in penetrating rust. Try a good penetrating oil, or PB Blaster, OR Kroil for better results.

HTH

Larry
Lawrence Hallanger

Andy-
One of the tasks that most MGB owners eventually confront is the overhauling of the heater box. The heating system on the MGB may be an antiquated design, but it is adequate if kept in good operating condition. Unfortunately, reconditioning the system is a time-consuming task, so they are almost always in a state of severe neglect. It is not unusual for the heater core to be so loaded with sediment that it barely functions, if at all. Reversing the flow within the closed system by switching the hoses in an attempt to purge the heater core is a very bad idea as it would force the sediment trapped in the heater core into the rest of the system, there to do all kinds of mischief in the coolant passages of the engine, coolant pump, radiator, thermostat, and heater valve. However, you can backflush the heater core with the hose disconnected and hope for the best. Simply pouring a descaler into the system can be risky as it is often the "kiss of death" for an internally corroded old heater core, resulting in leakage into the cockpit. If you want to get its performance up to what its designers intended it to be, you will have to pull the heater box and do a rebuild. While you're going through this ordeal, keep telling yourself that the entire process is just part of the initiation ritual into the ranks of an elite group of MG owners who have "been there, done that". You can consider it to be the MG owner's equivalent of a quaint and colorful puberty rite.

Removal of the heater box is not as difficult as some make it out to be. First, disconnect the battery, then disconnect the wires from the fan at their snap-connecters. Next, remove the three screws that secure the blower motor mounting plate and remove the motor along with its mounting plate. This will make the heater box lighter and thus easier to handle. Now, drain the system and remove the hoses from the heater box. Next, depending on which you have, remove either the radio speaker box or the console and loosen the demister tube retaining clip screws. Remove the screen demister duct tubing and pipe elbows from the lower part of the heater unit, and then remove the pipes onto which they were mounted. They should slide right out.

Next, remove the heater air control from the dashboard and disconnect the control cable from it. Finally, remove the screws that secure the heater box to the bulkhead. Pull the cable out with the box. Make sure that you thread the directional control cable through behind the dash - this has two purposes: firstly, so it won't snag on any wires as you pull the heater out and thus result in a electrical problems and, secondly, since it will come out with the heater, it is easier for the cable to thread through the bodywork without having to "U-turn" itself from where the control knob is located. Be aware that there is a rubber gasket beneath the flange of the heater unit and that it will often be difficult to remove. Normally it is just a matter of wiggling, tilting the assembly forward from the top, and just gradually working it out.

Once you have the heater box out of the car, remove the four cover retaining clips and peer inside. If the air intake is not protected by a screen sandwiched between the heater box and the circular air intake gasket you should not be surprised to find a considerable quantity of leaves and other debris clogging the fins of the heater core, thus preventing adequate air circulation. Take the heater core to a local radiator shop and have it boiled out to remove the usual accumulation of crud from inside of it and have them pressure-test it. If it leaks, a new one is available from Victoria British. To do a proper rebuild on the heater box, you will need the following parts:

Heater Unit Base Gasket (Moss Motors Part# 282-750)
Vent Door Seal (Moss Motors Part# 363-145)
Heater Core Foam Seal (Victoria British Part# 9-9079)
Air Intake Seal (Victoria British Part# 12-5556)
Air Intake Screen (Victoria British Part# 12-085)
Heater Outlet Seal (Moss Motors Part# 363-135)

In addition, you will need thick strips of felt that you should be able to obtain from a piano shop.

When reassembling, pay special attention to the careful sealing of the heater box, so that airflow has to travel through the heater core rather than bypass it.

Now you are ready to reinstall the heater box. Attach the air control cable to the heater box, and then attach the block seal, air inlet seal, and perimeter seal to the heater with rubber contact cement. Pass an 8" length of small metal tube (probably a length of steel fuel line) through the rubber grommet in the firewall and up through the rectangular hole where the heater resides. This tube can act as a cable guide for the control cable so that it won't get kinked as you lower the heater into place. You might consider taping a shop rag over the rear edge of the bonnet aperture so the heater box would not scratch the paint as you lower it into place. Using Wire Pull Lube (either Rotanium or Yellow 77 should work fine), smear it all over the rubber block seal. This material will evaporate over time and, unlike grease, will not attack the rubber. Pass the control cable through the metal tube, then slowly lower the heater box into position. With the wire lube, steady firm pressure is all that it should take to get it into place. If possible, have a friend pull the cable and tube from the inside of the car as you lower the heater into place. Once the unit is in place, just pull the metal tube off the cable. Reinstall the fan with its motor and be careful that you do not accidentally reverse the connections.

Should you still be dissatisfied with the performance of the heater, you might consider installing the uprated heater core available from the MG Owner's Club in the UK and modifying the heater valve in the manner described by Bob Munchenhausen in his website article found at http://members.aol.com/bobmunch/index1.html

If you find evidence that water is seeping into the cockpit from under the dashboard, this means that the drain tube for the air intake plenum is clogged with debris. This can be very bad as the water just sits in there in the plenum and can cause rust. Left uncorrected, water will stand in the air intake plenum and the resultant rust can do serious damage. However, it's not hard to clear out the drain tube.

If the car is on axle stands you could slide right under there and see the drain tube hanging down beside the right front of the transmission. It should be hanging down just below the bottom of the car. On the end of it is a round bulb with a slit-like lip. In MG lore this is called "Tom's knob," supposedly after the engineer that designed it. Supposedly this "knob" was to prevent exhaust gases from working their way up the tube, into the air intake, and from there into the heater box, poisoning the air inside the cockpit. Unfortunately, the bulb can eventually get plugged up with debris, usually a compound of teeny bits of leaves and dust. This part is no longer available, so you are going to have to be careful with it.

Remove the cover and look in the corner. You will find a small hole that connects to the drain tube on the left side of the plenum chamber when viewed from the front of the car. You will want to avoid damaging the paint, so gently push a doubled-over electrical wire into the hole and gently twist it to clear the hole out, then add water and gently push it down through the tube while twisting the wire. Get under the car, squeeze the knob so that the slit opens, and have a partner flush water under pressure directly down the hole in the plenum chamber. This is the easiest approach and sometimes does not work. If it does work, you will get a nasty bunch of crud on yourself.

If that method does not work, there's another that will: The drain nipple for the air intake plenum is located on the right hand side of the transmission tunnel, under the heater and just above the bellhousing. The drain tube is fitted to its outlet tube by a hose clamp and is accessible from the side of the gearbox tunnel down in the right hand footwell area. You might have to remove the center console heater control mounting or try looking behind the hot air outlet in the footwell. It is pretty hard to see and even harder to get your hand thru to refit. Be prepared for grazed knuckles. Remove the 4 screws holding the flap to the tunnel, then you will see the tube that drains the air intake plenum. It has a clip holding the tube to the drain nipple. Undo that and reach under the car to pull out the tube. Now cleaning out the tube itself is easy. This is really the best way of cleaning the tube as sometimes there is a bend in the tube, thus it cannot be cleaned out using a wire.

I went through this joyous ritual with my second wife's car and resolved to never endure it again. To keep debris out of the system, I fabricated a screen using black stainless steel window screen and put it under the chromed intake grill. Nowadays you can buy one from Victoria British (Part# 12-017).
Steve S.

One thing I did that you may want to think about. I replaced the original outer metal cable sheath with one that was just slightly larger. It allows a bit smoother slide. Larger cable attaches to bot the box and the knob. I don't know exactly the size.
Bruce-C

No need to drain the system. Close the heater valve before you disconnect the two hoses from the heater; you'll lose coolant from the heater but not the rest of the system. If you want to run the car without the heater (while you fix it) then just connect the heater valve to the pipe along the top of the valve cover with a spare piece of tubing. You should tape up the hole left by the missing heater to avoid getting too many 'engine fumes' into the cabin though.

N
Neil

Steve S of Virginia, USA has given a great deal of information regarding the heater.
BUT, I have taken out the three screws, and the top bolt under the bonnet. Water and pipes removed. Air control wire removed from the dash. Demist tubes removed. AND NOW? I cannot get the box out. In the Haynes manual and parts book there appears to be another screw shown at the bottom right of the picture, contained within the bracket number 1. It is shown pointing upwards. Is there such a screw? I cannot find one.

If I ever get the box out then I can continue with the advice given. But when, the box comes out. Is it just stuck, as I cannot find anyother clip, screw or anything holding it in place.
Any further advice would be most welcome.
David
D Pughe

PS. I did not mention that my BGT is an early 1967 just incase that makes it different.
David
D Pughe

There is a screw directly beneath the fan motor in addition to those along the front lip. That may be part of the difficulty you are having. It is also possible that the heater box is seriously 'glued' into position with aged sealant around the edges fitting to the car body. About all you can do with that is persevere with prying the box upward/forward to clear the hood lip. Careful not to over-force anything to damage the box or body.
Regards
Roger

Roger T

Roger is right. There is a fourth screw that is a real pain to get at. When I replaced my heater, I left it out - it's not needed. See my picture for the location.

Mike Howlett

That V8 install is looking good Mike
Peter

Thank you Mike for the helpful photo, I did not have one. When I re-installed my heater I used hex headed screws, which made turning that covered one with a spanner a lot easier. From memory I used a 1/2" length in that location, starting it by hand.
Regards
Roger
Roger T

"Gluing" is the big thing, there is a large rubber block between the demister ports on the heater unit and the holes in the firewall, which decomposes into a sticky mass. It's debateable which is worse - getting it out with the old one out or getting it back in with a new one!

Paul Hunt

Peter, that V8 install is finished and has 2000 miles under its belt.


Mike Howlett

GOT IT OUT. Roger you were right. I did have all the screws out. But with brute force and a few odd words, levers etc., it came out. And yes, the thick sticky mass (I expect it was rubber some years ago) seemed to hold it in place. Not any more. I have ordered a new matrix from Abbingdon Spares to replace the leaking one so should be ok for the winter runs.
I am not too pleased to read that getting it back into the car is even worse. Msybe I should have just bought a new sweater and scarf instead?
Anyway, to you all my thanks

David
DAVID PUGHE

David,

Well done. FWIW, regarding re-install.

I gave up on trying to refit my heater with the rubber block attached. My car is a 69 roadster. I did my heater with the engine/gearbox removed so was able to (sort of) stand in the engine bay to do the work. That also gave me a full bird's eye view into the heater box chamber and of the apertures from that into the rear chamber through which the demister tubes pass. What I found was:

1 Heater and rubber block could not physically fit through the mounting space, they could be pressed so far but the combination then locked up,
2 More importantly, test fitting the block alone established it did not, and could not be made to, insulate between the two chambers. There remained three channels of 'space' through which a gale could blow back and forth if it chose.
3 The block did not effectively hold the demist tubes in place against the heater body. What secures the tubes is the 'lip' structures around each tube aperture on the face of the heater body. The openings in the block are of a diameter to fit outside those 'lips', so don't actually touch the tubes - by some margin, and
4 If the block is fitted to the face of the heater body, the heater flap control cable must first be attached to the heater flap section and fed through a cutout in the block structure itself. This guaranteed that when the heater/block combination was forced (as far as I was able to) into the mounting space, in every instance the control cable sheath was severely bent ( near 90 degrees) to the extent that movement of the heater box flap was reduced to about a quarter of its full range at best.

Since re-install the heater and demist functions work perfectly without the block in place.

I know of a number of discussions on the block topic here in the archives. I seem to recall that a consensus developed that the block was probably 'original' fitment only from around 1972. To me that would imply some body modification to enable fitment without damaging the control cable sheath. Whether that is the case or not I don't know.

Whilst your car appears to have had a block it is just possible that that was a PO fitment. I wonder, does the control cable sheath show any signs of a kink/limited flap movement? My car showed no sign of ever having had a block.

I know some here have maintained that they have managed to get the combination installed with all manor of slippery assistors. I have wondered how much flap movement they have managed to maintain as a consequence.

Regards
Roger



Roger T

"getting it back into the car is even worse"

So they say. I've never done it but if my blind pal can do it single-handed without ever having seen one or done one before I'm sure the rest of us can. One could say he 'cheated' by using the original block, but that would be a bit uncharitable.

Apparently later cars had a larger hole for the air flap cable so that it could move into position as the box is inserted without getting kinked in the process. If you test-fit the box without the block you can see how to cut the panel on earlier cars to achieve the same thing.

The cut-away GT at Gaydon doesn't have the block, for what that's worth.

Most oddly the original air-flap cable inners seem to be *stranded*, rather than solid as on the heat control. This could be because a solid inner is more likely to kink on insertion, even though it would be more effective at moving the flap its full range.
Paul Hunt

I cheated and used a home made seal between the heater and firewall. It was made of a compressable foam block. I sealed it with silicone . It stunk for a month but has been perfect for 15 years. RAY
RAY

Happened to visit Gaydon yesterday, this is the cut-away if anyone is interested.

Paul Hunt

This thread was discussed between 17/04/2009 and 17/08/2009

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now