W126 sudden heat loss
 

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?

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.

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)

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

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

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.

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.

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

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.

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.

Good. I will update when repairs are complete.

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.

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.

Even with a bad t-stat there should be "some" heat. 16mpg is pretty normal for a 560....my 420 only gets 14.5 with the way I drive. :D

Does the return hose from the climate system get warm when the car is warm/running? Could be an air pocket....

you are in the same boat as me....or close. Good luck to you, I havent soved my issue yet....been throwing parts at it since begining of Jan

Before when I had a bad t-stat, yes I had heat just not intense heat and temp was just below 80, as it is now. But now the only way I get heat, when it drops out, its to recirculate air and have the fan on low. Then I get some heat.

Oddly now when its below 80, I get better heat than last year when heat does work. So its not the same "formula" as last time with a bad t-stat. Also before it would take a while to get heat. Now I still get heat quickly. It just eventually goes away, particularly on the highway.

As for milage, all summer I was getting 18 or more easily. My goal was 22mpg highway only. Got that up to 20 at least. But average was always near 19. I know winter will kill economy, but the first half of winter I was doing just above 18 and seldom in the 17s.

As for climate system, the mono valve is sure flooded with coolant, but when heat dies both the lines and windshield heater lines are just warm, not hot like the washer tank lines.

I think my biggest concern is overall engine heat. It has to be a clue. If engine heat was normal, just above 80, then I would suspect something clogged or air trapped. But the cooler running engine has to be a hint at something.


EDIT: note, I do not have a viscus fan install and have not for months now. Its sorta stuck on all the time, so I took it off this winter. I wonder if the new expansion tank cap is the issue. I dont think these issues started with a new one exactly, but when I remove the new one it will launch off the tank! The only one did not vent much air when the engine was hot. So the new one certainly is holding a lot more pressure. The new one is rated 20lbs and it some aftermarket brand. I did order an OEM unit that should be here next week.

I wonder if the cap pressure being too high would make air stay in the cooling system especially the heater system?

Well I put the old cap on. Its hard to say if things changed, the engine did get above 80 but did not quite hold there perfectly. When I left this morning for work, I lost heat after reving up the ramp to the highway and before engine op temp was reached. Had no heat until the exit ramp, then it came back. As it did, engine temp dropped close to 80 but not below. As if the sudden change in HVAC was cooling the engine a bit.

After work heat came on quick, as usual, and I cranked the heat to max and it held on the highway until it hit normal full operating temp, then heat went away quick. Did not come back until exit ramp. One a stretch of road from the ramp to my house I sped up and lost heat. Came to a stop, had heat. Seems getting over 3k rpm is a bad thing for this problem although engine temp always stayed above 80.

So I really have no clue. In my driveway I decided to try something. In park I got the engine up over 3k rpm and held it there. The heat I had was once again gone quick. After holding for a while engine temp climbed slowly. Nothing that I would worry about since I have no viscus fan. If temps did climb quick I would think water pump.

So... still no clue. Hot engine, cool heater core. Especially after getting RPM up. And it seems to be an issue after the t-stat does its initial opening - but not always, as I found this morning.

The cooling system is so simple. Coolant is new, clean and the proper stuff. I guess the only thing next is a water pump and t-stat. I may do the t-stat quick. If I do both and things work fine, I will never know for sure which was the issue.

This is no fun!

Keep us posted....Im watching your progress! T-stat and brand new radiator didnt help me at all. I have not replaced water pump

It's sounding more like the heater system then the cooling system itself, unless you mean really drastic changes in the engine temp display. Almost like you are really scrutinizing the engine temp. I would disconnect the monovalve connector and see if it goes to ful heat once the engine warms up and report back, if the heater temp is still going up and down after that I would almost have to think it's the monovalve itself,unless others have ideas. But as a test, i would say next step is now to disconnect the monovalve electrically and report back.

Second thought would be a plugged (or partially blocked/gunked up) heater core.

Gilly. The engine temp has always been rock solid at above 80. Needle goes through the upper part of the 8. That has not changed since I replaced the original bad t-stat last spring or summer. I forget. But it never would drop below 80. And that is what often happens now.

If it was not for engine temp being wonky I would assume the same, a clogged core, etc. Maybe its multiple issues.

I will try unplugging the new mono valve again and see what happens though. I will also keep the old cap on until the new one comes in.

Just wish the symptoms were more repeatable, reliable.

I was doing some more thinking. I wonder if I blew a head gasket. Exhaust getting into the coolant would explain a few things. Fluctuating temp as air is trapped by the sensor housing, air blockage for heat, etc. I hope not, but it might make sense. Higher rpm forces exhaust into the system and only reaching lower rpm lets the water pump properly expell the gasses. Also lower rpm means less exhaust pressure anyway.

Replaced the fairly new t-stat, no change. Compression test will be next.

There could be something going on with one of the vacuum actuators that lets cold air in at speed. Engine vacuum drops with throttle, so the vacuum could drop to a point where the flap shifts. At idle you have high enough vacuum to shut the flap. Just a theory, but I have read that those vac elements fail after a decade or two.

Something along those lines was brought up on another site. I know they are a PAIN to service. Since this is clearly an RPM related issue, there is a very good chance. Im going to dig up the diagrams and see what I can disconnect to test the theory.

No heat at highway speeds
 

I also have a 1985 300SD with same problem, replaced mono valve insert, thermostat, pushbuttons, temp control unit and aspirator motor. still same problem.
Last week when the aspirator motor seized and popped the fuse I had plenty of heat. Whenever I buy one of these old german cars I make it a point to clean all the fuse contacts and replace the fuses. having a number of these cars in the past I'm pretty well versed but this one has me for a loop. Last night I was driving stop and go, had plenty of heat but once I hit 65mph on the highway again no heat. Pulled off at a rest area and unplugged the mono valve and had so much heat I had to cycle the off and EC buttons to keep from getting cooked.
Has anybody had an experience with a mono valve magnet getting weak, I'm almost thinking that the plunger is getting sucked down at higher speeds and shutting off water flow.

Since the valve is capable of working, it would sort of point to the controller in the dash.
Interior temp sensor; maybe. Those seem to last the life of the car though, not much can go wrong with them.

I re soldered my HVAC controls. No change. I did not have time to pull the plug on the mono valve since installing a new one. I need to try that.

I re-soldered the HVAC controls beacause a few days ago I had a hard fault on the controls. The air recirculation switch was steadily flashing and I had no control over the system. Not even able to turn it off. But get this, I had plenty of hot heat coming out.

Odd, huh? So thats why I need to try pulling the valve next.

But so far a new t-stat, new valve and re-soldering the controls has not helped. All vacuum lines look good. Idle is normal. I just need to keep tinkering when I have time, which is little.

Just to say the obvious, ( somebody has to) Is the coolant level full?

Yup. Burped, purged, etc. And its newer coolant. And even more new added when I replaced the t-stat in an attempt to resolve this. G-05 stuff.

Remember the emergency default (ie NO power to the valve) is full heat.

Right. So since it went default and I got tons of heat, be at low speed, I should have full heat with it unplugged. if I do, its one more part of the puzzle.

edit: before, I did not get any heat unplugged.

I am curious. What does that switch do? I have been searching to find out but to no avail.

I know you replaced the monovalve but it still has the symptoms of a bad monovalve. My car did the same thing; some heat at low speeds and no heat at highway speed. The diaphram was damaged. I installed a MTC monovalve and it worked correctlly. It has been about 6 months now and it is still working fine. Maybe try another cheapie MTC monovoale, seems they might be a little hit&miss as far as working right.

Pretty sure the switch is part of the alarm system.

I have full egg-cooking heat though at low speeds. And when the system went into default on the highway I had full heat at low speed (when default, low speed fan is only option).

I am going to pull the plug on it tonight now that I am home early. I will see how the heat works over the next couple of days.

Disconnected the new mono valve and have tons of uncontrollable heat. Figures. That rules out air pockets, etc.

did you find the problem...?

i am having the same problems....

did you repair your monovalve with the MTC repair kit or did you buy and OEM monovalve...?

I did not find the issue for certain yet. I did replace the parts with MTC. No change. So I took it apart again. This time I baked the solenoid on the over.

What I found is that the valve was leaking a bit. I think this flooded the solenoid. From what I can see, the solenoid was causing some electrolysis of the coolant by being exposed to it!!!

Anyway, after expensive cleaning and "drying out" I now have 100% heat at all times. Given that winter is here, I am okay with that for now. But when I have time I plan to remove the solenoid again and see how it moves when I adjust the temp. The plunger should move in and out. Voltage at the connector is fine and working with controls.

If I had a spare solenoid, I would just swap it out to test.

Does anyone here know if a monovalve from a w124 will work on the w126? They physically appear different but maybe an updated version as 124 is a 94 and 126 is a 90. I have a spare from the 124 and I think my 126 needs one.

I have a 91 560SEL and when I got it 6 years ago the monovalve was bad, replaced it and haven't had a problem until recently. Same issue, around town heat works GREAT, once on hwy it turns off. Just bought a genuine MB monovalve today from Pelican Parts, I've learned the hard way that when it comes to important stuff like heat or engine gaskets I always go genuine; cosmetic stuff I buy aftermarket - and am still disappointed by the low quality.

Sounds like your issue is still the MV.

Ya I am sure it is the M valve. I bought the "kit" from aftermarket and it will not fit in the electromagnet (waaay too tight) so I removed the circlips and swapped out the other parts. Put it in and had heat! till I got to highway :mad:

Anyway now not sure what to do with it. Need heat I live in Nova Scotia. Not sure why it wont work now tho, It really should I think.

Does your engine temp stay constant? Next just disconnect the valve. With it disconnected you should have full heat at all times.

Temp is a constant 80. I will try unplugging the darn thing and see what's up. If I get out and squeeze the heater hoses I can get heat.

well unplugging it didn't work, Not sure where to go from here. I don't think the core is plugged the car doesn't have very many miles on it and has always been serviced with new coolant and such. The old M valve did have a torn bellows so pretty sure that must be it. Is there any way to test it on the bench? I'm not even sure what it is supposed to do. Like I said I had to take the old one apart and put the new ones parts on it as the new one didn't fit very well. Maybe it is just crap to start with. When I had it apart the aux pump was working as I checked it and could see the coolant flowing in the valve. The hoses are not hot so safe to say no coolant getting in the core. If I squeeze them I have heat for a bit. Nothing is ever straight forward it seems.

My 420 has the same issue, higher speeds I get weak heat, low speeds ultra heat. I did confirm my monovalve is weak/dead. I replaced the torn diaphragm unit with an MTC and that got me my low speed heat back (used to have almost nothing). I pulled my valve and magnet and used a 12 power supply to test. The magnet is very weak compared to others I've tested. Its not strong enough to do the initial "shove" it needs to move the valve. I'll probably swap the magnet with another at some point. I drive 90% city and don't use the car in the coldest months....so my unplugged valve gives me plenty of heat for now.

I was wondering if the magnet could be weak. I'll test it and see. These things are crazy expensive.

The good ones I have tested will go "THUNK" and slam the valve down with good force. When I tested mine last week you could feel a weak magnetic field but it wasn't strong enough to push the valve against its little spring and move it, if you helped it it would stay down....but thats it.

Good to know, I will check it out for sure. I will post my findings just for FYI purposes. Thanks.

The magnetic coil is what I am pretty sure it bad on mine, because the valve leaked the could got saturated. Then good old electrolysis kicked in and its bad. You can test it simply by removing the entire solenoid and hooking it up to the harness. Then just change the dial for temperature. The valve should react. But if you can bench test, all the better.