W126 sudden heat loss - PeachParts Mercedes-Benz Forum

LandYaghtLover · #1 02-16-2013, 10:19 PM

Not sure why, but my 91 560 SEL suddenly has no heat. Upper radiator tube is hot, washer tank is heating just fine. But the hoses behind the tank, which I think are the windshield heater, and HVAC heat are cool.

If I adjust the temp, the fan controls speed. Seemingly matching actual air temp so I dont think the cabin temp sensor is out. All fuses look good. If I go full defrost, I just get major air movement that is cool.

I am not familiar with the system. Heat was HOT before. I know there is a modulator thing that can go bad, but typically its more gradual. And there is the aux heat circulation pump. If that goes, wouldn't the main pump still pump coolant?

Any ideas or things I can rule out?

stormyc88 · #2 02-17-2013, 06:13 PM

If the aux coolant pump went bad you'd still have heat. The pump only runs at idle to help flow. Try testing the heater monovalve by unplugging the electrical connector- this causes it to open.

LandYaghtLover · #3 02-17-2013, 07:18 PM

Update: This morning, expecting a cold drive to work, the heat came on strong enough to cook eggs and when set to cooler temps, did as it should. All worked well.

Then on the way home, no heat again. Unless I put the fan on low and recirculate the air. Thats the only way I get some hint of warmth. And its not much at that.

Where is the plug for the monovalve located? (I will try searching too)

NJ300sdl · #4 02-17-2013, 08:26 PM

Plug is inboard of the battery, on top of unit.

LandYaghtLover · #5 02-17-2013, 09:06 PM

Found it. Its under some electronics and a hood switch. I will dig it out in the morning and see whats up with it.

There is one brant, MTC, any good? They do not get a good review from Pelican. But for this part, are they an option? OEM is over $100 for a rebuild kit!

draglink · #6 02-17-2013, 09:35 PM

Quote:

Originally Posted by LandYaghtLover

Found it. Its under some electronics and a hood switch. I will dig it out in the morning and see whats up with it.

There is one brant, MTC, any good? They do not get a good review from Pelican. But for this part, are they an option? OEM is over $100 for a rebuild kit!

All I could find was the MTC. If you buy it from Pelican Parts above its only +$15. It is worth a try...I did

LandYaghtLover · #7 02-18-2013, 08:33 AM

Found a thread, seems there is about a 90% failure rate for MTC in the first couple of months. I will check local sources first. I probably wont even pull the monovalve until I get a new one. At this point it seems it is the issue.

LandYaghtLover · #8 02-18-2013, 05:31 PM

Because the engine was running a little cool (below 80, usually 2 needles above 80) I decided to pull it anyway. The edge is torn all around. I assume this is not good. I was surprised when coolant starting dumping out upon removal!

I put it back in, for now, and left it unconnected. I will keep an eye on the system, but I suspect other issues.

The only recent change I made was a new 20lbs expansion tank cap. Based on how the upper hose feels and the sound when opening the tank I am certain the old was was bad. Maybe the new coolant pressures are stressing the old system.

Gilly · #9 02-18-2013, 06:03 PM

You just need the rubber bellows part of the water valve. I've never heard this described as a rebuild kit, just the insert or bellows.

EDIT-
Like this:

http://www.ebay.com/itm/200851385331?item=200851385331&viewitem=&vxp=mtr

LandYaghtLover · #10 02-20-2013, 02:28 PM

Ordered some parts. I hope the answer to this is "yes" or "likely". If the valve is torn, badly, will the valve electrical connector will I still have no heat?

With mine disconnected little has changed, same heat issues and usually total loss when I get on the highway.

Gilly · #11 02-20-2013, 08:06 PM

Yeah you are on the right track, if the diaphragm is torn you "probably" will have no heat. With a good diaphragm, if you disconnect the electrical from the valve you should have full heat, but not if the diaphragm is junk.

LandYaghtLover · #12 02-20-2013, 08:59 PM

Good. I will update when repairs are complete.

LandYaghtLover · #13 02-22-2013, 10:04 AM

New monovalve, same exact issues. I am now thinking my fairly new t-stat is bad and at times sticking open. When I have no heat the engine runs cool too. Even the exhaust smells harsh. It was installed last fall, but I think its worth a shot. Not sure where else to look. System is full and purged of air.

LandYaghtLover · #14 02-22-2013, 05:35 PM

Well I think I am on my own on this one. I am going to try a new t-stat, maybe a water pump. Today on the way home from work where I am on the highway in just 3 miles the heat was fine. I put it on defrost then eventually dialed the temp to the same. Heat came on quick as it normally does and held for many miles. I even dropped the temp way down, turned the HVAC off for a while and the heat came back both times just fine.

The only odd thing is still the engine temp. It climbed up above 80, where it should be, but then dropped back under 80 and stayed there. Near home on city streets near a stop the exhaust smelled a little rich, like the engine was just fired up on a cold day.

I tend to think the temp gauge it correct. Fuel economy is suffering at about 16mpg where I normally am always over 18. Something with the cooling is up, I just do not know what.