heat in cabin remedy

[nick350]

Would someone please post the pictures that I just cant find using search of the hose bypass that t's the hot water away from the cabin. I have seen some but they dont have a tap on them and use the forced 90 degree angle to direct the water way from the cabin. Did I imagine that there was version using taps?....I'm dying of heat here!! HA.

thanks a lot . N

[doitwright]

As I work on my car to update the A/C system, the heater bypass is something I plan to do.  The guy at my local radiator shop who also recored my radiator to a 3 row showed me that he did the same bypass in his Jeep and used a valve to open and close the flow to the bypass.  The idea being that you could decide when you wanted to divert the flow away from the heater.  As you probably already know, the bypass also allows coolant to flow from the cylinder head back to the radiator even when the heater valve is not open. This also helps keep down engine temps as they are read on the dash gauge.

From what I have read on these forums, the heater bypass is not as common on the earlier cars.  The later cars have more issues with overheating particularly  if they have A/C.

To do the bypass, it looks to me based on preliminary measurements, that a typical 1/2" copper T fitting available in the plumbing dept at any hardware store would fit the inner diameter of the inlet and outlet hoses at the firewall in the engine compartment.  It seems that you would need to cut each of these hoses to insert the T fitting and then install another hose that bridges across the engine attaching the two T's.  You could install the valve to completely restrict water from entering the heater valve and flow directly to the bypass.

If you decide to do this, the outer diameter of the copper T fitting is what would fit inside the hose and you may find that there is not enough material on the fitting to clamp 3 hoses.  Then I would solder a coupling fitting at each end of the T which means you would also need a small 1/2" copper pipe "nipple" to connect the T to each of the couplings.  This just gives you more insertion of the fitting into the hose and more to clamp onto.

The other thing I would take into consideration is that this is a roundabout way to address your problem with a stuck valve.  The access hole in the firewall is just as non factory as the heater bypass although the heater bypass can always be removed.  The access hole also allows you future access to the valve (they are known to leak) to either lubricate or change the valve seal.  I hope this helps.

Frank Koronkiewicz
Willowbrook, Illinois

1970 280SL Light Ivory

[DavidBrough]

Hi Nick,

Firstly, I have to agree with Frank that if your real problem is a heater that won’t switch off then a by pass hose will not help. A fix on the heater switch is quite easy and can be carried out by removing the large rubber grommet on the bulkhead and there are many posts giving full details on how this should be done.

I had a similar problem but the reason my heater would not switch off was due to a broken operating flap not the switch itself. The passenger side regulating flap has a connecting arm similar to the throttle rods which acts on the on off valve in the heater matrix. As the flap is only plastic the mounting comes under some pressure especially if the heater valve sticks slightly and the flap can easily split and the heater will never turn off. I obtained a new flap from my local MB dealer and fitted it via the fresh air vent in front of the screen. This is easily removed together with the fresh air flap and gives access to both driver and passenger heat/air regulating flaps. It’s quite a while since I did this but I don’t recall having any real problems. My heater now switches off completely.

On the question of the by pass hose fitting, I did this recently as I have fitted A/C and found that it helps to keep the engine cooler. Again, it is a straight forward mod after purchasing the following items

1.   1 meter length of standard MB 18mm heater hose obtainable from main dealer
2.   1 meter of 15.8mm heater hose from Halfords (I’m not sure why they have 15.8mm but they said that was their standard and it fitted well)
3.   1 standard plumbers 15mm on off tap
4.   A short length of 15mm copper tube and two connecting ferrules.
5.   A good selection of hose clips
6.   Two plastic “T” pieces obtained from local motor factors (these will give a loose fit for the 18mm hose and a tight fit for the 15.8mm). I think they were 17mm.
7.   PTFE tape

Fitting is a simple case of breaking into the heater inlet hose on the passenger side with a “T” piece and then running the 15.8mm hose into the on off tap using two short lengths of 15mm copper pipe with the ferrules glued/soldered onto the ends to give the hose some purchase. I also found it necessary to use some PTFE tape to obtain a good seal. You will then have to remove the metal feed pipe on the return side that runs under the manifold and replace it with the 18mm MB hose. You will need to retain the small metal joining piece that actually runs over the exhaust manifold to the water pump for heat protection. The new hose can be run under the inlet manifold and well away from the exhaust. This then joins into the second “T” piece and the heater outlet and by pass hose from over the head by the gearbox filler. Fitting will be different for left hand drive cars as they have the exhaust manifold exiting from the rear and metal fittings may well be required at the back as opposed to the front.

The relative part sizes are quite important and I came across them more by accident than design as I simply purchased what was available. The standard 18mm MB fittings were a loose fit onto the “T” pieces but tightened up well. The 15.8mm hose was a tight fit onto the “T” and a loose fit onto the plumbers tap which was sealed with a ferrule and PTFE tape.

I have just returned from a 1,700 mile trip to Italy and all worked well. In fact, it could be that the on/off tap may not be needed as the heater still worked when needed in Switzerland although it wasn’t that cold and I will have to wait for some freezing UK temperatures to see how it works then.

I enclose some pictures which may help if you can decipher the heater and A/C hoses that is.

David Brough
1969 280Sl Auto with A/C



Download Attachment: Inlet Side.jpg
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Download Attachment: Outlet Side.jpg
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Download Attachment: Top View.jpg
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[nick350]

thank you both, those answers are exactly what I needed. Cheers.