R129 Permanently blowing hot air

[teabag]

Thanks, I'll give it a try in the morning

 

[grober]

I may have got this back to front?????? but if the control system is like many other MB units the solenoid valves always get a constant 12v supply---- its only when this voltage is "dropped " across the solenoid that it operates. In other words its operated by being "earthed" this is achieved by controlled time switching in the temperature control unit so points to a possible problem there rather than with the solenoids themselves???

 

[television]

I may have got this back to front?????? but if the control system is like many other MB units the solenoid valves always get a constant 12v supply---- its only when this voltage is "dropped " across the solenoid that it operates. In other words its operated by being "earthed" this is achieved by controlled time switching in the temperature control unit so points to a possible problem there rather than with the solenoids themselves???

The control voltage comes from the control panel giving an interrupted pulse voltage from zero volts up to 12 volts, with zero volts the valves are open allowing the reverse flow of water through the heater matrix. As the voltage is increased from zero the valves start to close reducing the water flow till the full 12 volts is applied and at this point they should be fully closed.

Measuring is done on the 3 pin plug, the center is common ground always, and the pin either side is the voltage in to each valve.

 

[grober]

Sorry I think you have got this back to front As this post illustrates. Its very easy to get the "sense" of what is going on wrong- you will still get "plausible " voltage readings at the duo valve supply pins but may interpret them wrongly if you don't realise what's going on.
QUESTION:-
Why does my w124 e220 estates heater only heat up on one side?

Optional Information:
Year: 1996
Make: Mercedes-Benz
Model: w124 e220te
Engine: 2.2
Already Tried:
changed duo valve and bled system
Hello,
It is possible that the heater control unit is not properly actuating the duo valve. Of course we assume that the duo valve installed is new, or in perfect order (used ones tend to stick and might need cleaning)
So take the plug off the duo valve. Centre pin must have constant positive with ignition on. Side pins must have ground supply when maximum cold is selected on both sides, and no ground when maximum heat is selected. In the intermediate position, the valves are supplied with pulses of ground.
If the valves are not controlled in such a way, either the heater panel or the wiring must be repaired.

[YOUTUBE]FQL5PcuPMLU[/YOUTUBE]

 

[television]

Yes I did get it back to front and I have been down and checked my 140 duo valve, but the fact still remains that if you have 12 volts from 1 to 2 and 2 to 3 then the valve should be closed and no hot water passing through.

On both valves on my car I have 12v in the center pin 2 at all times, 3.4 volt either side on pins 1 and 3

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1994/S350/SYSTEM WIRING DIAGRAMS/5120.pdf

 

[grober]

I would agree 12 volts across the socket pins should mean everything is OK however the situation might change with current draw with the solenoid activated if there was resistance in the control part of the circuit. Might be interesting to set both valves to cold and see if the solenoid coils develop different temperatures indicating different current draws-- might be confused by the temperature of the hot water flow of course.

 

[television]

If there is 12 volt on the said pins then there is no more to it, not much use measuring off load, there is what looks like a little flap over pin each connection, this flap can be lifted with a spike and the meter leads placed directly on the pins when working, any wrong resistance in a wire would see the voltage not reaching the 12 volt needed to close the solenoid.

So if both valves are set to cold one should have the same voltage on pins 1&2, and 2&3 and that should be 12 volt near enough

 

[teabag]

Thank you to Television and grober for your inputs on 19/6/2013, this explains why i was getting a negative reading on the multimeter.
Unfortunately I'm unable to check anything until the weekend as the car is now in the bodyshop having some work done to the front end.

I'll keep you updated when I get the car back , hopefully on Friday .

 

[television]

Thank you for posting back, I was worried

 

[teabag]

Thank you for posting back, I was worried

Thanks for the concern Malcolm, I've been busy all weekend doing the prep work for my front end repaint so I haven't had the time or the energy to look at anything else.

 

[teabag]

Just a note to close off this thread.

After replacing the duo valve twice, first time with a second hand valve from Ebay then a brand new one from MB, the problem still persisted with hot air constantly blowing from the drivers side vents and intermittant hot air from the passenger side with both selectors set to cold.

I then replaced the internal temperature sensor but this did not have any effect.

After weeks of searching I found a replacement "Automatic Heater Control Panel" with the correct part number A129 830 00 85 on Ebay, this arrived today and took 20 minutes to fit.

Bingo!!! everthing now works as it should just as the weather is starting to cool down.

Thanks to everyone for your help.