According to the dealer diagnostic, saying my Climate Control Unit was malfunctioning, I replaced with a Programa rebuilt unit yesterday. It's still the same problem. The heat on the passenger side still can not be regulated. I have to turn the (passenger side) dial to HI in order to have heat or else it's cold air.

How do you diagnostic the heater control duovalve? If it's not working properly, how do I remove the unit?

Thanks

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I can't help much with the diagnosis, being a W140 newbie. But if you get heat on High, then the valve is opening and is probably OK.

Here is the functional description from Alldata:

Duovalve

The duovalve regulates the quantity of coolant flowing through the heater core.

When the heat requirement is low the pushbutton control module switches the duovalve solenoids to ground, the solenoids pull in and block the coolant flow to the right and left sides of the heater core separately.

At the maximum temperature setting the pushbutton control module interrupts the ground connection, current no longer flows through the solenoids, and the coolant flow through the heater core is at its maximum.

During normal regulation the pushbutton control module completes and interrupts the circuit to ground intermittently. The closing and opening times for the duovalves depend on the deviation between the temperature present on the interior sensor and that set on the pushbutton control module.

So I would look at the sensor or the temp wheel.

But here is the procedure:

Coolant
Drain.

2 electrical connectors
Disconnect.

5 hose straps
Release and pull off hoses.

2 rubber mounts for heating system delivery unit (arrows)
Press off and remove heating system delivery unit

Install
In the reverse sequence, top up coolant and bleed heating system by turning both temperature selectors to the red segments and running the engine at alternating speeds.

Function check

NOTE: After completing the work erase fault memory in control unit.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks for your help. I would like to have the temperature regulation during the winter season. Too hot is not pleasure inside the cabin. No warm air my passenger will be freezing to dead.

I was disappointed my local MB dealer. They charged me the diagnostic fee and gave me the wrong information. I was lucky that I did not let them replace my CC unit. The dealer repair cost is over two grants. I will have to eat the wrong repair money. Hopefully, I can find out the problem soon.

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I strongly recommend investing a few bucks in an Alldatadiy subscription for your car and getting some sort of OBD tool so you can read codes. Some of these systems are so complex and expensive that you can't figure them out without information, and even then it's not easy.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

go to V12 uber alles its free and filled with step by step description on the repair of the duo valve, moderate skill needed. but will save you $

Excellent photo-documentary of a duo-valve repair! Best I've ever seen.

Another point: If there was ever a reason for the use of regular antifreeze changes and the use of distilled water, the photos in this article show what happens when you don't!

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Regards . . . . JimF
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'94 S500 Cpe

Visit my Mercedes Web Page

I would take a look at JimF's page and run diagnostics on your climate control unit: http://pages.prodigy.net/jforgione/MB_AAC.html.

Also, as an alternative to Alldata, I would consider the W140 Tech DVD from MBUSA. It's a little more expensive, but if you were to compare the two I think you would find it well worth the extra money.

I would like to purchase the W140 DVD from the MBUSA. I have tried to call them a numbers of times but unsuccessful. I definitely need the Service & Repair DVD.

I can not find the link on the MBUSA web site for the W140 model. Is there any other ways that I can buy the W140 DVD for my car? Please help. Thanks

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I have the OBDII for my BMW but can't use on the 95-S500 OBDI. I have used many times the OBDII when the ASA supercharge was first installed on my BMW because the CEL was on and needed to be reset. I definitely well spent the money on the OBDII scanner. What kind of OBDI tool would you recommend?

Thanks

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I would use an LED from Radio Shack. It is pictured and its use is described in this thread: Mercedes fault code reader?. The part number is 276-209. I prefer this to other LEDs because its built-in resistor allows it to operate with 12-volts. You'll need to attach 2 wires to the cathode (-) lead of the LED and one to the anode (+) lead to make connections to your car's diagnostic socket and chassis ground.

If you continue to have trouble getting through to M-B, I can refer you to someone who will make you a copy of the DVD and mail it to you for $40. If interested, send me an email.

Jims page shows how to read codes using your control panel,
The mb page is
http://www.startekinfo.com/StarTek/outside/7594/?requestedDocId=7594
continental imports also shows the diagnostic
http://www.continentalimports.com/
The MB DVD is copy protected, results can be questionable