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Old 08-26-2012, 11:27 AM   #1 (permalink)
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Date registered: Sep 2009
Vehicle: 1999 S420; 2007 S600
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No heat - alternatives?

Hiya folks, I've had a problem with no heat coming from the vents in my 500. The 420's heat is fine so I just swapped the duovalve between the two. Makes no difference.

It's worth noting that on a hot day, with the heat on "HI" on both sides, the air coming from the foot well vents and from the center console (for the rear seats) seemed to be as cold as the air when I chilled it down to "LO". Even though the compressor obviously wasn't on.

The heater core could be clogged. COULD be. But this is a '99 with 70k miles. There's no corrosion or sediment evident anywhere in the cooling system, and I've replaced the water pump, and done various other things that drained the coolant. If it was a '94 or '96 and had 150k-200k miles I'd think it's plausible but as it is, I just do not believe that there's a blockage in the heater core. I'll end up ripping out the dashboard, putting in a new heater core, and find out that I still have no heat.

Here's my question: What ELSE could cause no heat whatsoever? I'm assuming there's a cabin thermometer; what if that's faulty?

I pulled trouble codes from the climate control a couple of days ago and came up with Eb1 231 (ECT sensor) and Eb1 234 (Sun Sensor). I have a new coolant temp sensor on order, should arrive in a couple of days. I have not had any of the codes recur in the last few days. Could an engine coolant temperature sensor on the fritz cause this? I'd assume the sun sensor cannot.

Could this be a problem with the vanes in the duct work?

I can believe there's an electrical problem, but I'm just reticent to pursue replacing the heater core until I've eliminated all less invasive possibilities.

Is there any definitive way to determine whether the heater core is clogged without removing the dashboard?

Thanks all!
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[sold] 1999 M-B S500 - 75k miles - silver
1999 M-B S420 - 85k miles - white
2007 M-B S600 - 30k miles - designo mocha

Last edited by andyman32; 08-26-2012 at 03:42 PM.
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Old 08-26-2012, 02:08 PM   #2 (permalink)
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You can try to operate the valves directly, it should be straight forward to check if the heater matrix is blocked. Rather unlikely however
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Old 08-26-2012, 03:20 PM   #3 (permalink)
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Quote:
Originally Posted by andyman32 View Post
Hiya folks, I've had a problem with no heat coming from the vents in my 500. The 420's heat is fine so I just swapped the duovalve between the two. Makes no difference.

It's worth noting that on a hot day, with the heat on "HI" on both sides, the air coming from the foot well vents and from the center console (for the rear seats) seemed to be as cold as the air when I chilled it down to "LO". Even though the compressor obviously wasn't on.

Here's my question: the heater core could be clogged. COULD be. But this is a '99 with 70k miles. There's no corrosion or sediment evident anywhere in the cooling system, and I've replaced the water pump, and done various other things that drained the coolant. If it was a '94 or '96 and had 150k-200k miles I'd think it's plausible but as it is, I just do not believe that there's a blockage in the heater core. I'll end up ripping out the dashboard, putting in a new heater core, and find out that I still have no heat.

What ELSE could cause no heat whatsoever? I'm assuming there's a cabin thermometer; what if that's faulty?

I pulled trouble codes from the climate control a couple of days ago and came up with Eb1 231 (ECT sensor) and Eb1 234 (Sun Sensor). I have a new coolant temp sensor on order, should arrive in a couple of days. I have not had any of the codes recur in the last few days. Could an engine coolant temperature sensor on the fritz cause this? I'd assume the sun sensor cannot.

Could this be a problem with the vanes in the duct work?

I can believe there's an electrical problem, but I'm just reticent to pursue replacing the heater core until I've eliminated all less invasive possibilities.

Is there any definitive way to determine whether the heater core is clogged without removing the dashboard?

Thanks all!
few months ago i thought i had the same problem cuz i changed the mono valve "duovalve" twice so i thought its blocked somewhere, but the problem was the 2 duovalve were bad,lol. so re-install the duovalve back on ur S420 and see if its still working. to find out if u have a clogged or not try to blow air to the hoses that go to the heater core . thats wt i did but i found out the air was comin out from the other hose. that will help to push anything clogging the system out.
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Old 08-26-2012, 03:42 PM   #4 (permalink)
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You can try to operate the valves directly, it should be straight forward to check if the heater matrix is blocked. Rather unlikely however
This is one of the diagnostic modes in the climate control head unit?
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Old 08-26-2012, 06:18 PM   #5 (permalink)
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If you disconnect connector from the duo valve it should open and give you a full heat mode. This test will eliminate all electrical circuits and push button unit.
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Old 08-27-2012, 07:23 AM   #6 (permalink)
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If you disconnect connector from the duo valve it should open and give you a full heat mode. This test will eliminate all electrical circuits and push button unit.
Interesting! I'll give that a shot this evening.
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Old 08-27-2012, 02:59 PM   #7 (permalink)
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I just received a new engine coolant temperature sensor and popped it in. Started the car, warmed it a bit... and boom, the heat is back. Perfect!

That's certainly another culprit to check, in addition to the duovalve!

Thanks all!
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Old 08-27-2012, 04:07 PM   #8 (permalink)
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So educate me. Are there two coolant temp sensors? One feeds the engine management and dash gauge, and the other feeds the climate control? Is that right? So you weren't seeing any issues with coolant temp via the gauge?

Brett
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Old 08-27-2012, 05:38 PM   #9 (permalink)
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Brett - I wondered the same. I do not know if there are two coolant temperature sensors. I did not see any problems on the gauge; actually, I had been using the gauge to observe the previous overheating (or, not overheating, but hot-running) condition before I found that the fan clutch was shot and replaced it. The gauge was reading accurate temperatures which I verified using an infrared thermometer.

I had an error code in the climate control head unit for the ECT Sensor (Eb1231). I cleared that code and it didn't come back after a few drive cycles, but the heat still didn't work.

As I mentioned in the original post the known-good duovalve from the 420 made no difference in the 500.

However, I popped in the new coolant temp sensor today and cranked up the heat and that was that... warm air in the footwells and up towards the windshield!

The one I replaced is at the forward edge of the intake manifold. If there's a "sensor" and a separate "sender", I don't know where the "sender" is located. The old sensor was black on its nose, the submerged part. I didn't test it for resistance before I ditched it. In retrospect it probably would have been informative to do so.
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Old 11-10-2012, 11:46 PM   #10 (permalink)
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Hello there guys my wife just got a 1995 s420 it drives great. I mean great. brakes need to be checked out there is air in the lines or something, they are relly soft, and the fluid smells like mushrooms. But She just got this car Friday 11/9/2012. Now to the point.

There is no heat. I have put the car in rest, and I can hear the driver side monovalve spinning, but no heat. The therm or the main therm. the one that tells what the engine temp is, is right on moves to up to running temp. In the time it should. After that is when I did the rest check. It does blow a little hot air after I turn the rest mode off, but very briefly. The monovalve sound like it is slurping. That would be the best way to describe it. The AC works fine. She bought this car at a used car lot. A place where she has bought cars from forever. The man that owns the lot is sitting on another s420. He said it had a few problems, an oil leak he cant figure out and said he thinks it has a water pump leak. he said something about it having two water pumps. Is that correct? Two water pumos. From what I have told you about the one she bought do you think having him switch out the monovalve from the one he is sitting on, presuming the heat on the other one works, would correct the problem or would you just trade for the other one, and replace the water pump?
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