My 97 S420 blows no heat. Air-cond works fine.
Can someone give me an advice on what to look for ?
Appreciate the help

 

Could be any number of things, the heater cores, valves, thermostats etc.

The first and easy thing to do is check if it is the valves which mix the hot and cold air, they can often stick especially after a long summer and is good to cycle them to keep them free moving.

To do this first start engine and wait until engine temp is maybe 60degrees cent+, that should heat up the heater cores nicely.

Now set the temperature on LOW on both sides at the front. Put the speed of the fans up to max, select windscreen only on both sides. Now raise the temperature on the front, on both sides by one degree at a time, until you get to MAX. Then take it down a few degrees. Maybe to down again to 28cent. That will fully open the hot vents. If you have sticky mixing vents that almost always works and unsticks them.

If that doesn't work the next thing to do is check the heater cores, and their temperature. The heater gets its heat from the engine coolant, and not an electric element.

To do this collect sensor data, with engine going, and aircon on AUTO both sides, press and hold the REST button for 5 seconds or so, but make sure the EC light is off. The display will change from your temperature settings to the diagnostic mode. Number on the left is the sensor number and the one on the right is the temperature value. Use the left and right "Auto" buttons to cycle through the sensors. Press REST again to exit the diagnostic mode.

Key ones are

3 - Left heater core temperature
4 - Right heater core temperature
5 - Evaporator temperature

Note the Evaporator temperature and the heater core temperature. The Evaporator temperature should eventually cool down to 4 centigrade and then cycle between 3 and 7 degrees centigrade as the compressor cycles on and off. If it doesn't get cold then you are most likely low on refrigerant or your compressor isn't kicking in...

If the heater cores are similar to your engine temperature then it means your duo valve is probably going bad. If they are cold, then the cores are broken (maybe blocked, air bubble in there, pump not working, valve not working etc) and that is most likely your problem.

Hope that helps.

 

Thansk JRHILTON for the advice. I did follow the steps above and that didn't work. I then collected the sensor data as adviced. Sensor 3= 56, 4= 42 and 5= 39 ( I believe these are Faraheigh temp values ?). Actually I did open up the duo valve for inspection. I don't see much of debris or corrosion. The pump works fine (i can feel the rotation). I checked the continuity and input voltate (12v) at the 2 valves, ok there. I then reconnected back and test run for about 1/2 hour. I noticed the 2 out of 4 hoses that weren't hot at all ?? Wonder if the cores are bad ? Base on the sensor readings. It's kinda pointed to that direction .. Your advice on this please ?.
Appreciated your help so far.

 

Hi, assuming the temperatures you have quoted are in fahrenheit, they look to be around 6-13 degrees centigrade (sorry I only know centigrade really so have to keep on converting). Was it around this temperature outside? If so then yes I would guess the heater cores are either blocked, have an airlock in them or are no longer opening/closing etc or something is blocked, stopping the warm engine coolant getting there.

On a side note, it is worth checking on your service book to see when the engine coolant was last properly and fully flushed in case there is some dirt in there somewhere...

I have not had to fix heater core issues myself; I only sometimes have sticky mixing vents, so am unable to say any more than the above steps to diagnose what the issue could be.
Maybe someone else on here can help? It is the sort of thing that a good independent will be able to nail down very quickly, especially with the info you have already found out. I wouldn’t take it to a main dealer...

 

the heat also controlled by a double valve system which is located under the hood driver side remove the plastic cover and you will see electric pump and about 6 black rubber hoses together with an electric pump thats were the hot water enters the heater core . junk yard 45 dollars.

 

and even with your pump not working you still should have some heat

 

and you said 2 of the hoses are cold? the two lower ones are going in to the heating core if they cold that means the double valves stay shut so there is no hot water goes in to the heating core,

 

Thanks for the respond SZABI, yes., i didn't feel any warm hose out of the 3 (which i believe 2 are the inputs to the heat exchange and 1 is the return). Only the small hose at the middle and the one right below the pump were warm. As mentioned in my early post, I did remove the duo valve , dissect it and only see a few minor debris in the system . I did clean out and put it back. I did measure input voltage and continuity check. They are both ok. One thing I didn't do was to apply the volage to it while the valve was out to see if the valves were acutated ? I supposed if the heat exchange does clogged up then there would be no flow. I'll find a chance to flush the water thru it to see if the issue is clogging ? I'm torn between spending big money on the duo valve and heat exchange.
Any how.. thanks for the advices folks.

 

junk yard 45 dollars with the pump the whole thing

 

if you dont wana spend any money take the top of those walves open it up and put something under those walves so they stay open like a nut . that way you will always have heat .