tobyd

Hi all,

Can anyone help desipher what is wrong with my 500E's heater.

The heater only ever blows hot (not warm) air into the cabin until the temp selector is set to minimum and then we can get aircon cold air.

The other symptom is while the engine turns over at less than 1500rpm or while idling at lights, the warm air stops after a few seconds and then cool air until i get moving or rev the engine, then warm air arrives again.

This is a pain at this time of year so I'd be grateful to anyone who wishes to share their knowledge with me.

Many thanks



Toby.

Jackd

I hope you'll get an answer to your problem as I have the exact same problem with my 260e

Lexino

its the blower, needs to be replaced.

Jackd

The blower works fine.
It has to be something else.

mhorn

You may want to check the temperature sensor in the headliner near the overhead light. This is the piece that acts as your cars thermostat for interior temperature.

jay300te24v

Hi,

I have a 300TE 24v and have recently received this problem. In my case the cool air arrives correctly from the center vents. However, I get hot air coming from the side vents no matter what temperature I set on the dials for both left and right sides.

Someone mentioned that it is the duo value. I don't know or have any knowledge of where this is and how to check or replace it.

However, I recal this problem started when I was checking the engine ignition leads in the rain! Didn't think much of it then, but I now wish'd I had closed the bonnet and waited for the rain to stop.

If you or anyone with a similar problem has successfully resolved this problem please, please let us know.

Regards

jay300te24v

By the way, I have looked in the overhead lighting and checked the temperature sensor wiring, but don't know exactly how to test it or check it properly. Also, once opened it's a pain to get the fitting back to the original quality.

I would be glad to hear from you about this.

Regards

cap'n jasper

Jay,
Yours sounds like a duovalve problem. It's located just in front of the bulkhead (you don't say where you are or if it's left or right hand drive). It has two heater (water) pipes and an electrical plug. Maybe you got the plug wet in the rain? If it's the duovalve it's not cheap (£160 sterling) and there's no way of repairing it.

RunInColorado

I had a similar problem on a 86 190D. The dealer and 2 other shops said they checked it and told nothing was wrong. Although I hated that even when it was summer here I would still get heat unless I turned the air on. Maybe this is a problem that alot of older Mercedes have? Good luck to everyone figuring it out though!

jay300te24v

Iam in UK and its right hand drive. Do you know the technique of removing/replacing it. I need all the help possible.

Regards

mhorn

I'm pretty sure the center vents will only blow outside air. So if it's cold outside, no matter what you have the temp set to, the center vents will blow cold air.

Jer

The center vents only function for A/C.

ozibenz

G'day from downunder.
Does anyone know where the Electronic Control Unit for the heater /air con is?
I have a 1990 300TE RHD with the same heater problem. It blows cooled air out of the centre vents and hot air out of the side vents, regardless of the setting on the temperature dials.
I've have a few more clues to help me track down the problem. It started with a sound like a relay clicking on and off repeatedly. The clicking stopped, but the hot air didn't.
I've found the duo valves mentioned in other replies. In my RHD 300TE they are in the bulkhead just in front of the brake master cylinder. Because I am getting hot air from left and right vents I think my problem is with the control unit. It seems unlikely that both of the duo valves would go at the same time.
I have a Technibooks W124 owners workshop manual which explains how the heater system works and has a decent wiring diagram, but doesn't tell me where the ECU is.
This must be a common problem because I looked at another 300TE before I bought mine and it too was blowing hot air regardless of the setting, but that was only out of the left hand vent. That one may have been one of the duo valves.
In the middle of the the Aussie summer, this problem makes the car almost undriveable! So, please let me know if any of you have any luck solving this problem.
Thanks

mhorn

I'm telling you the center vents will NEVER blow out hot air. In the winter time close them up. They take outside air directly from the vents under the window wiper arm and blow it past the A/C evaporater coil and into the cabin. The air from the center vents doesn't go through the heater. So if you want warm air, keep the center vents closed. The only reason to have them open in the winter time is to get more fresh air into the cabin if you are recycling the warm air inside. If somebody knows something I don't please correct me.

Jer

The damper for the center vents should be CLOSED in the heating mode. Therefore no cold, fresh air should be entering the car through the center vents. If they do not close, the damper actuator might be leaking; it operates on vacuum.

cap'n jasper

Hello all,
I think there's some confusion here because of the different types of aircon fitted to the W124.
The UK/Europe versions have a manual, dual zone system and the US ones have an automatic, single zone system.
The later dual zone systems WILL blow hot air from the centre vents with the aircon on and fresh air with it off (not sure about the early systems).
The US system has a mono valve which, from what I've read, has a repair kit available. The UK version has a duo valve which cannot be repaired.
The 'ECU' for the aircon is attached to the dials behind the dash. As far as I remember there are no relays on it.

So, to sum up; a dual zone (eg. UK) system that only blows hot air from the sides either has a faulty 'ECU' or (more likely) a knackered duo valve.

I guess you could check the ECU easily by swapping it with someone else's or if you have access to an oscilloscope you could measure the voltage at the duo valve. The frequency should change as you move the heater dials.
Duo valves often fail because the rubber inside perishes allowing hot water to flow regardless of the valve position. There is also an electric pump on the duo valve. With the ignition and heating on and the bonnet open you should hear the motor whizzing.
On a UK car the duo valve is on the driver's side opposite to where the battery is. Changing it is a simple bolt off, bolt on affair. Just remember to catch the coolant as it comes out and top up again with MB coolant/water and run it on the hot setting to get rid of any air bubbles.

Hope this is of some help.

ozibenz

I found my heater problem. it was the duo valve.
Thanks cap'n jasper, your diagnosis was spot on and the differences between
US & UK/Europe helped dispel all the confusion.
So here is a summary for UK/Europe/Australia
Dual temperature controls in cabin
When system is working correctly, centre vents always blow unheated air. Remaing vents blow air according to temperature dial setting on each side.
ECU is behind heater control switches
duo valve on RHD cars is in bulk head immediately in front of brake master cylinder.
My problem was the rubber diaphram inside the duo valve had ruptured alowing water to leak into one of the two solenoids. the noise I described as a relay clicking on and off was probable the solenoid engaging and disengaging before it karked it.
The duo valve has a 3 pin electrical connection. The centre pin is earth. The 2 outer pins supply power to the 2 solenoids. Power is supplied from the ECU according to the temperature settings. Power to the solenoid closes the valve, preventing hot water from flowing through. If the solenoid is faulty hot water flows constantly through the heater, causing hot air through the heater vents regardless of the temperature dial settings.
You can test the duo valve by earthing the centre pin and applying 12v to either outside pin. You should hear a solenoid clicking on. One of my solenoids was faulty due to the water leaking through the diaphram. It still clicked on, but not with such a firm click as the good one.
Duo valves cannot be repaired, they have to be replaced
Second hand ones in the UK cost 40 pounds. In Australia they cost $200!

cap'n jasper

I think I would have used a new one, but, at least it's fixed.