R129 Permanently blowing hot air

teabag

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I've just aquired a 1995 SL280 without air con and the heater vents on both sides are constantly emmitting hot air regardless of the settings on the 2 rotary controllers.
I have replaced the 3 port "Duo-Valve" (A000 830 65 84) but the problem still persists. after reading various threads on the same subject, I think it may be the interior temperature sensor in the roof lining at fault but I am unsure as to how to access this to check/replace it, please can anyone provide some advice.

Also the engine coolant temperature seems to be very low at around 65 degrees according to the gauge, is this correct.

Thanks in advance
 

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Something odd here, the only way hot water can get to the car is via the duo valve. I would check the voltage on the 3 pin plug, center pin is ground and the 2 outer ones the live from the control unit in the car.. There should be between 0v and 12v depending on the setting in the car, on the coldest setting the hoses leading to the heater box should be cold.

The heater box is all vacuum controlled and maybe an element not working.

The engine should be at 85c so the thermostat has failed
 
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Something odd here, the only way hot water can get to the car is via the duo valve. I would check the voltage on the 3 pin plug, center pin is ground and the 2 outer ones the live from the control unit in the car.. There should be between 0v and 12v depending on the setting in the car, on the coldest setting the hoses leading to the heater box should be cold.

The heater box is all vacuum controlled and maybe an element not working.

The engine should be at 85c so the thermostat has failed
Thanks Malcolm,
Both soleniods on the duo valve seem to be working with 15 Ohms resistance at each coil and the valves are moving freely, the one I took out was showing 12.1 Ohms and 14.9 Ohms which is why I changed it.
Am I right in thinking the voltage should be 0v at the hot setting to allow the valves to remain open and 12v to close the valves and shut off the hot water supply?

The thermostat seems to be working as the bottom hose remains cold until the engine warms up but I have a sneaking suspicion the previous owner may have fitted a stat with a lower rated one to try and combat the heater fault, so I need to check it.
 

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These yes they stay open till the 12 volt closes the duo valves.

What does happen to these valves is the rubbers can swell up making the closing impossible, they are in two parts, the little valves in the ends of the solenoid that never give any trouble and they just fall under their own weight where the water flow lifts them to allow the water to flow.

The problem lies in the solenoid part where if the rubbers have swollen, then the valve cannot close. There are extensive threads on Benzworld on rebuilding them.

One test that I do is to apply 12 volt from the battery directly to the valve and if good it makes a healthy clonk, when poor just a dull thud, this could wake up a sticking solenoid and worth a try.

One can also feel the hoses leaving the valve that go to the heater box, when the valve is working well you can feel the pulses as the valve opens and close about 3 pulses per second on medium setting, plus if these pipes leaving the valve that go to the heater box are warm, then you know that the valve is not closing.

It is easy to change the engine thermostat and I have that info if you need it
 
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These yes they stay open till the 12 volt closes the duo valves.

What does happen to these valves is the rubbers can swell up making the closing impossible, they are in two parts, the little valves in the ends of the solenoid that never give any trouble and they just fall under their own weight where the water flow lifts them to allow the water to flow.

The problem lies in the solenoid part where if the rubbers have swollen, then the valve cannot close. There are extensive threads on Benzworld on rebuilding them.

One test that I do is to apply 12 volt from the battery directly to the valve and if good it makes a healthy clonk, when poor just a dull thud, this could wake up a sticking solenoid and worth a try.

One can also feel the hoses leaving the valve that go to the heater box, when the valve is working well you can feel the pulses as the valve opens and close about 3 pulses per second on medium setting, plus if these pipes leaving the valve that go to the heater box are warm, then you know that the valve is not closing.

It is easy to change the engine thermostat and I have that info if you need it

The valves seemed to moving freely when I took the assembly to bits before fitting it and the solenoid rods slid smoothly through the rubber gasket when extended against the springs. the old one seemed ok as well but I'll check them all by applying 12v as you suggest.

I'll check the water pulsing now but my mutimeter wont be able to detect voltage with less than 300ms impulses so I'll rig up a test lamp to check this later today.

If I am not getting any voltage at the solenoids, does this suggest the internal temperature sensors are at fault and if so how do I test them?

I'll pick up a new thermostat today and change it as a matter of course.

Thanks
 

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The control comes from the control panel to the duo valve, so the voltage comes directly from this, the OCP controls what the control panel is set for
 

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Try the ac or dc setting to get a reading
 
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Try the ac or dc setting to get a reading
Mine is just a cheap multimeter, it takes almost a second to register any change so it is unlikely to detect 3 pulses a second, a test lamp should show me if there are any voltage pulses.
 

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I should point out that the hot water flow is such that the inlet supply comes from the single pipe from the nearside cylinder head and divides internally into the 2 sides of the heater of the passenger cabin. The duo valves act on the 2 separate outlets [ exhausts if you like] This doesn't negate anything that's been said about testing the valves but should be born in mind when assessing the temperature of the single inlet and two outlet pipes from the heater assembly. If the duo valves are OK then its most likely that one or more of the pneumatic actuators has failed/developed a leak. IRRC if you look underneath the dashboard you can see a vacuum manifold at the side of the heater unit supplied by a vacuum line from the the inlet manifold
 
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I should point out that the hot water flow is such that the inlet supply comes from the single pipe from the nearside cylinder head and divides internally into the 2 sides of the heater of the passenger cabin. The duo valves act on the 2 separate outlets [ exhausts if you like] This doesn't negate anything that's been said about testing the valves but should be born in mind when assessing the temperature of the single inlet and two outlet pipes from the heater assembly. If the duo valves are OK then its most likely that one or more of the pneumatic actuators has failed/developed a leak. IRRC if you look underneath the dashboard you can see a vacuum manifold at the side of the heater unit supplied by a vacuum line from the the inlet manifold

Yes they are reversed flow through the heater matrix and out to the duo valve.

The duo valve is the only thing that shuts off the water supply from flowing through the heater matrix, the vacuum actuators are used to direct the flow of air through the chosen path.

The center vent on a 129 is fresh air only I think.

Actually to test all this if the heater is set to cold and the air con turned off,then the footwell temp should be the same as the side outlets, if the footwell is cold and the side vents warm then is the air being pulled from the wrong place, ie as you say a actuator not working.
 
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Yes they are reversed flow through the heater matrix and out to the duo valve.

The duo valve is the only thing that shuts off the water supply from flowing through the heater matrix, the vacuum actuators are used to direct the flow of air through the chosen path.

The center vent on a 129 is fresh air only I think.

Actually to test all this if the heater is set to cold and the air con turned off,then the footwell temp should be the same as the side outlets, if the footwell is cold and the side vents warm then is the air being pulled from the wrong place, ie as you say a actuator not working.
The footwell vents and the side vents are all constantly hot but the footwells can be shut off by using the rotary knob which controls the airflow either upwards or downwards, so it looks as if the vacuum actuators are ok. This tells me the duo valve solenoids are not being activated.
 

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Then that does confirm it. I cam measure the voltage on a very cheap meter, it may show 10 volts in place, of the 12v pulsed.

A test lamp should indicate what is going on but it will add possible more load than the control box can supply, I am not sure on that. But with the plug out it would work
 
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Thanks Graeme,
I've checked out all of the vacuum actuators and they are working fine, so this definately points to the water circulation.
 
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Hello Malcolm,
I've checked the operation of the vacuum actuators and they are all ok, see my reply to "grober" above.

I've managed to get a voltage reading to both solenoids, they are 10.8 volts when the rotary selectors are set to cold and fall to 0.4 volts when set to hot, a test lamp from the plug shows they are pulsing at about 300 milliseconds so every thing seems to be OK here.

I supplied 12 volts to each of the solenoids and heard a resounding click which suggests they are also fine.

I was scratching my head at this time so decided to resort to old fashioned methods and dug out my hammer from the garage, with the selectors both set to cold and the engine running with the heater fan at full blast I gave each solenoid a few light taps and waited a few minutes, the passenger side started to pump out cool air (as cool as it could be with an ambient temperature of 26 degrees) but no change to the drivers side.
A few more taps with the hammer and still no change, I moved the control wheels to hot and tried a few more taps in this position then back to cold and even more taps but still no joy on the drivers side.

I have now given up for the night but at least I can move the wheels to cold and then tap the rear solenoid to get cool air to the passenger side vents and shut of everything on the drivers side to make the inside cabin temperature a little more bearable.

It looks as though ebay has struck again with a valve assembly which tests ok electronically but fails in service due to a distorted gasket as you suggested earlier.
What makes this even stranger is the valve I removed yesterday was marked with blue paint as if it had came from a breaker also.
At least I now have a spare valve to "play with" so I can attempt to repair it and get it moving freely.

Thank you for all of your help
 

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I have tried taking them all the way down but you need special washers. what you do not want is water getting inside the actual solenoids

The rebuild if you fancy trying it is in the 140 section on benzworld

At least you are getting somewhere, even if you have got cold feet for one side ;)

This was my point on the DIY here for the duo valve, it does nothing at all to deal with the problem and it should be taken down
 
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Just to recap on this problem.
I decided to bite the bullet and have just fitted a new duo valve and thermostat obtained from MB at a total cost of £214, the engine is now running at the correct temperature of around 85c but the heater is still blasting hot air from the drivers side with the control wheels both set to cold.

I have 12 volts at each solenoid with the control wheels set to cold and both solenoids have 14.5 ohms resistance.

The internal cabin temperature sensor has a supply of 12 volts and a good ground but both outputs to the controller are only showing a nominal 0.04 volts regardless of the temperature control wheel settings, is this correct as I would have thought a control voltage would be apparent at one of them depending on the settings from the control wheels and the ambient temperature inside the car.

All vacuum operated divertor flaps can be heard to be working and seem to operate correctly as all areas can be isolated.

To simplify the problem: hot water is entering the heater matrix regardless of what the controller is demanding, so what else can be wrong.
 

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I cannot see anywhere where water can get in if you have 12volt on the plug

Is it possible just for testing that the 2 pipes can be changed over to test to see if the passenger side gets hot.
 
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They are a pig to get at so it may be easier to swap the pins on the plug so drivers control will operate the passenger side and vice-versa if this would have the same effect
 

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They are a pig to get at so it may be easier to swap the pins on the plug so drivers control will operate the passenger side and vice-versa if this would have the same effect

I thought of that but 12 volt is 12 volt,, can you clamp the pipe to see
 


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