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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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The heaters on my 2004 s class are warm but 't get hot

Customer Question

Customer: the heaters on my 2004 s class are warm but don't get hot
JA: Thanks. Can you give me any more details about your issue?
Customer: my s class mercs heaters are not letting out any heat .you can have the control set on hi it is only slightly warm
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Submitted: 1 year ago.
Category: Mercedes
Expert:  Juan Crespo replied 1 year ago.

Hi there. Welcome to our site!


I’m a retired master technician and tech instructor with over 30 years experience. However, if you’re thinking on working on the vehicle yourself, all my knowledge and resources won’t help if you…

… don’t have tools or the skills to use them

… don’t know there will probably be some special tools and equipment needed for a proper diagnosis

… don’t realize it may take a few back and forth replies to solve the issue

… don’t understand that I will need you to provide specific information

Poor heat out put from the HVAC unit on the S class is usually due to a faulty auxiliary coolant pump or its control and circuits. The quantity of coolant circulating through the heater core when the engine is running depends on the coolant pump. The coolant circulation pump ensures that a uniform flow of bubble-free water is present in the heater core even at low engine speeds. Actuation of the coolant circulation pump is controlled by the automatic A/C control module CAN bus and right signal-acquisition and actuation module (SAM).

Please let me know if you have any other questions.

Best Regards.

Customer: replied 1 year ago.
can you explain what the CAN and SAM are and where are these buttons. can you send me a diagram showing how to fix it
Expert:  Juan Crespo replied 1 year ago.

I'm sorry, but there's no such diagram that I could send you even if my life depended on doing so. I only wish it was that simple :-(

Let's start with the acronyms:

CAN stands for controlled area network which is the communications protocol the computers/modules in your car use to talk to one another.

SAM stands for signal-acquisition and actuation module which is the module that in this case receives the signal from the A/C control module to activate the auxiliary coolant pump.

Basically, your Mercedes is full of computers or modules; some you interact directly with (EX: HVAC control module, power window control module, etc.) and some you don't (EX: SAM, etc.)

To repair the issue with your heater, a dedicated computer/scan tool must be plugged in so it can communicate through the CAN network with the HVAC and SAM modules involved in activating the auxiliary coolant pump to see which command signal or actuator is not working as it should.

In other words, as I stated in my introduction, I have and I'm willing to provide the technical knowledge you'd need to handle this issue yourself, but to complete the job you'd need to provide the tools and skills to use them.

Now, if I were to give an educated guess, I'd say the most likey cause of the symptoms you describe would be between the auxiliary pump and the right front SAM - all you would have to do is use a high impedance voltmeter to check for voltage at the aux pump when you select heater on the A/C control panel. If there is voltage, the SAM is OK and the pump is the issue. However, if there is no voltage, chances are the aux pump motor failed and when it did, it drew more current than the SAM could handle and damaged it as well.

I don't mean to confuse you with any of this stuff; it's just what wsome of us techs face every working day.

Please let me know if you have any other questions.

Best Regards.

Customer: replied 1 year ago.
where is the auxiliary pump, and how do I get to it
Expert:  Juan Crespo replied 1 year ago.

Here are instructions (also see attached graphic)

- Danger of severe burns to skin and eyes from hot coolant spewing out. Coolant is
toxic when swallowed
- Open cooling system only when coolant temperature is below 90 degrees C. Open
cover slowly and release pressure. Do not fill beverage containers with coolant.
Wear protective gloves, protective clothing and protective goggles.

1 Release pressure in cooling system

NOTE: For this purpose, turn radiator cap to catch "1" and close again.

2 Remove wiper system
3 Disconnect connector (5) from duovalve
4 Disconnect connector (M13x1) from coolant circulation pump
5 Slide rubber mount (1) out of retaining plate (2)
6 Attach clamps to coolant hoses (3)

7 Release hose clamps (4), pull off coolant hoses
8 Remove heater supply unit (A31)
9 Install in opposite order.

NOTE: Installation: After completing work correct coolant level and check for

Please let me know if you have any other questions.

Best Regards.