w124 Duo valves - heating.

[Frank O' Phile]

I've been trying to resolve the temperamental heating in my 300E.

It doesn't always blow cold. Hot is no problem but when switching the dials to cold, warm is the best i can get. Sometimes it will stay on hot.

I've had the duo valves apart and clean up all the parts (twice) and initially it went cold but after switching to hot and back again (after waiting the necessary time) warm air was the result again.

I've measured the volts across the terminals and get 13.9 when set to hot and 13.1 for cold. The resistance across both coils is 14.9 ohms (each).

At the moment I now have the drivers side cool, but the passenger side hot\warm.

I would consider replacing the duo valve but would like some other options before parting with the cash.

However there is a 300SEL in the local scrappy, any ideas where the duo is on that i.e. it's not by the master cylinder.

Ta.

 

[Dieselman]

The temperatiure selector dials could be faulty. Twiddle them like mad and give the potentiometers a spray with contact cleaner.

 

[Frank O' Phile]

The temperatiure selector dials could be faulty. Twiddle them like mad and give the potentiometers a spray with contact cleaner.

Thanks DM, had the unit apart and measured good variable voltages across both sides 0 - 5v.

Still looking for some clues, may have to go with the new duo valve.

 

[SL300-24]

Have you checked the temperature sensor is sucking in air from inside the car. Do the tissue test........Put a small (10p size) piece of tissue over the small grill near your interior light with the heater on. If it sucks it up and holds it,then it should be okay and your problem is elsewhere. If it does not hold the tissue up and it drops onto the floor, then there is a pump behind the passenger side glove box that is connected to a plastic hose that is attached to the grill. The hose may have become detached, or the electric pump may be faulty. The interior temperature is controlled from this sensor, if the system cannot read the interior temperature, then you will get similar problems to the ones that you are experiencing.

 

[ms500]

Have you checked the temperature sensor is sucking in air from inside the car. Do the tissue test........Put a small (10p size) piece of tissue over the small grill near your interior light with the heater on. If it sucks it up and holds it,then it should be okay and your problem is elsewhere. If it does not hold the tissue up and it drops onto the floor, then there is a pump behind the glove box that is connected to a plastic hose that is attached to the grill. The hose may have become detached, or the electric pump may be faulty. The interior temperature is controlled from this sensor, if the system cannot read the interior temperature, then you will get similar problems to the ones that you are experiencing.

I have a similar problem with mine, so I might try this.

The right and left vents constantly blow hot air out even when both dials are set to cold, but the center vents are fine. I too suspect the Duo-valves, which I think are about £130 from MB.

 

[Dieselman]

Just stuff 12v across each valve. If it closes the valve then the problem is elsewhere, if it doesn't then it's the valve.

Frank. I didn't really take in what you had written earlier, it appears you have duff valves as with 12v across them they still allow hot water through.
Are you measuring correctly.? Not saying you aren't.

 

[Frank O' Phile]

Have you checked the temperature sensor is sucking in air from inside the car. Do the tissue test........Put a small (10p size) piece of tissue over the small grill near your interior light with the heater on. If it sucks it up and holds it,then it should be okay and your problem is elsewhere. If it does not hold the tissue up and it drops onto the floor, then there is a pump behind the passenger side glove box that is connected to a plastic hose that is attached to the grill. The hose may have become detached, or the electric pump may be faulty. The interior temperature is controlled from this sensor, if the system cannot read the interior temperature, then you will get similar problems to the ones that you are experiencing.

Been in there and done that. It isnt the most powerful suction, but the motor is working and no leaks in the pipe.

Then, typically replaced everything and noticed I hadnt replaced the ducting to the passenger vent. Bummocks, everything out again.

 

[Frank O' Phile]

Just stuff 12v across each valve. If it closes the valve then the problem is elsewhere, if it doesn't then it's the valve.

Frank. I didn't really take in what you had written earlier, it appears you have duff valves as with 12v across them they still allow hot water through.
Are you measuring correctly.? Not saying you aren't.

When poking around yesterday, I had the unit apart and could watch the solenoids working when the ignition was switched on. (that would \ should stop the flow of hot water) Switch off, and the solenoids retracted. Having reassembled everything, went for a drive and both sides were blowing cold. But, as previously posted, altering the dials, and then switching back to cold left me with warm\hot air from the vents.

It raining outside so happy to discuss any theories.

 

[Dieselman]

They work on 12v being shorted to Gnd on the return side to operate. They are pulsed as opposed to proportional, so It's 12v or nothing.

You could rig up a meter or test lamp semi-permentantly to see what is happening.

 

[Frank O' Phile]

They work on 12v being shorted to Gnd on the return side to operate. They are pulsed as opposed to proportional, so It's 12v or nothing.

You could rig up a meter or test lamp semi-permentantly to see what is happening.

Thanks DM, I think I'll go with the 12v straight at them, and see if i can force them closed.

Will update if\when I find anything

 

[Frank O' Phile]

Just to update this problem, it seems to have been cured.

Following Dm's advice, I put 12v direct to me coils and couldn't hear any clicking.

I, once again dismantled the duo valve unit and found that the valves weren't seated properly. (that explains the flow of water) and again put 12v across each coild while removed. Both actuated correctly.

I carefully reassembled the Duo valves, using a different procedure to ensure the valves were in the right position, and connected my circuit tester (looks like a screwdriver with a bulb in the handle, great tool) across the passenger side coil.

I started the car and let it run. switching the knob from hot to cold turned the lamp off and on, so power was getting through ok. When it was blowing cold (lamp on) I moved the switch to very hot (not the hottest setting) and eventually the lamp was pulsing.

Repeated this for the drivers side and same results.

So now I can get cold and hot and cold again. I know it takes a while for in-between temperatures to kick in and cant be sure that they are correct yet, but for our impending trip to the foreign, it can blow cold and thats good enough for me.:bannana::bannana::bannana:

Thanks for the avice and tips from those that replied, I think I'm an expert on Duo Valves now, so if anyone needs any help, I be happy to lend my experience.

Next, the air con.......