*It was MonoValve* MonoValve or Panel? W124
 

Hello All,

It was the Monovalve. Although the old one tests within range (12.6 Ohms), and seems to work (can feel the click), and looks OK upon internal inspection, it must have been sticking in the open position. The new one is in and this problem has ceased. Thanks for all the consternation on this one!

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Hello, I am intermittently getting heat (hot hot heat) out of all dash vents or just the outer vents with cold in the middle. Fan speed does not change. I can sometimes reset to normal by turning panel to "0" and back on, sometimes not. This happens on hot days while air is working (and it blows nice and cold when it does work). Seems to happen after an hour or more of driving.

Question: Is MonoValve an intermittent kind of thing or does it usually fail whole hog. (If it commonly fails all at once, I will start with other things.)

Car is my '86 300E.

Thanks!

Bad contacts on the Temp Control dial

My '87 has been doing this for a while now. Haven't bothered to troubleshoot it yet. It seems to happen when the a/c has completely cooled the car, and it is starting to get too cold inside. The PBU calls for a little bit of heat to be mixed in to prevent the temperature from falling below the setting on the dial, and BAM! full heat.

I've waited a bit (not long, it gets hot fast!) to see if the heat shuts off, and it never does. Switching the PBU off for a few seconds, then back to a/c mode always cures the problem.

The first thing I'd try is unplugging the auxilliary electric coolant pump. If it has shorted out you will get some wacky behavior out of the PBU. Unfortunately I don't know where the pump lives on your car - it's somewhere under the hood, connected to coolant hoses :) On my diesel it's near the firewall, on the drivers side of the engine. I think the location is different on gasoline cars, though.

Good luck,

- JimY

Upon reading some more posts on this subject, this sounds like a vacuum switchover valve. It is still intermittent, but more and more frequent. I get a blast of hot out all vents, then hot on the sides and either cold or cold mixed with some heat out of the middle. Anyone agree?

Same thing for me
 

I seem to have just about exactly the same situation. A/C works very well, plenty of cold air. Auto temp control works fine. Leave it set at about 76, cools when the car is hot, and heats when the car is cold.

After about 1 hour of driving now in the summer, many times the car will just switch to blowing hot hot hot air out of the vents, and it seems unclear if the side vents are blowing the same temp as the center ones.

I have to turn everything off, and sometimes that will fix it, or, if I let it sit overnight, it seems fine in the morning.

The fan speed initially doesn't change when the hot air starts, but after a few moments it does start to increase. I attributed this to the fact that the temp. sensor is now seeing that the interior air temp is increasing, and it puts on more fan to cool it down, but of course since it's blowing hot, it increases the problem. It doesn't know that it's blowing hot air. So to speak!

This says to me that the temp. sensing portion of the A/C is working OK.

On my commute home, it's a little over an hour, and it seems to happen pretty close to after about an hour of running. But not every day.

I'd really like to know what's up!

Rgds,
Chris W.
'95 E300D, 143K

This may solve your problem


http://www.peachparts.com/shopforum/tech-help/45378-temp-dial-gauge.html?highlight=cracked+pin

Here could be the answer
 

Fixed mine!

I tried resoldering the pots in the PBU over Labor Day weekend. They appeared to be a bit loose, but alas, no change in behavior.

One night last week I finally got around to rummaging under the hood and disconnecting the aux water pump. It conveniently has an electric plug which can just be undone.

Result - perfect! The temperature is correctly regulated, no blasts of hot air. Ran the a/c on a 60F morning. Compressor was cycling on and off, discharge air was about 55F, car was comfortable.

Now, I may have to replace the pump later, we'll see how the heat works this winter. But for now it's all copacetic. Total cost: $0.00. My kind of repair.

- JimY

But Wait. I like your fix, but have question about diagnosis.

If the pump is disabled, it does not move hot h2o to the heater core, so no heat. If this pump is failed this would indicate not enough heat -- not too much heat like we have been getting.
If the monovalve is sticking OPEN, one would get too much heat, or heat at erratic times. If the monovalve is stuck CLOSED, (even with an aux pump that worked all the time), one would get no heat. Disabling the pump effectively bypasses the hot h2o flow to heater core plumbing and the faulty monovalve at the same time. I am thinking this is still monovalve.

Can anyone confirm?

No, no, the regular engine driven water pump supplies plenty of hot water to the heater core. The aux water pump is there to help out while the engine is at idle speed - the flow of hot water can get a bit weak then. But when moving down the road, it's quite unnecessary. I had the aux water pump unplugged on my 123 for years and the heater worked just fine.

I understand not having an aux water pump can cause some hot-air-flashes during the winter. When sitting at at traffic light, the heater core cools off due to low water flow. The PBU holds the monovalve open in a (failed) attempt to raise the heater core temperature. Then you put your foot on the gas and move away from the light. A blast of hot water hits the heater core. It's now too hot, and the PBU closes the monovalve. But it's too late, there's now too much heat, and nothing to be done but wait for the heater core to cool down. It's this latter scenario the aux water pump is meant to help avoid.

If there's any consistency to when you get hot air (i.e. after driving 30 minutes on a moderate day), I don't think you have monovalve problems. The monovalve has two positions - fully open and fully closed. (The PBU pulses it between the two to control water flow.) If the monovalve is broken, you're gonna get full heat, and it's going to stay that way regardless of which PBU button you push.

BTW, the PBU on the 124 has built in protection against a shorted out aux water pump, unlike the 123. On the 123 a failed aux water pump destroys the PBU. On the 124 the PBU just kind of shuts down - resulting in the blasts of hot air we all know and (don't) love.

You may question the diagnosis, but I'm about 90% certain you have a shorted aux water pump.

- JimY

Thank you very much! I now actually understand your diag. I will unplug my pump soon and test.

The Monovalve does still work with the aux pump disabled? (The circuit is not interrelated?)

Yeah, the monovalve and the aux water pump are two different circuits. The logic appears to be that if the PBU decides it needs a bit of heat, it turns on the aux water pump. It then pulses the monovalve to raise the heater core temperature to whatever point it mysteriously decides is appropriate. Aux water pump runs whether the monovalve is open or closed, so long as the PBU is in "heating" mode.

Give it a shot. It takes all of five minute to unplug the pump, no tools or disassembly required on my diesel. What's to lose?

- JimY

Pump disabled. Problem seems to be solved. So far. Thought it is a cool, rainy day here, which may affect operation.

I’d buy you a beer if the replacement pump wasn’t so expensive!!

Final question, are we sure this diagnoses the pump and not another circuit somewhere else? I am leery of ‘ye old timee’ computer controlling things.

Also, you seem to be one of the few to diagnose this properly. I think we need to get the word out.

I'm having EXACTLY the problem described....will it apply to to a 126 system? Sudden inconsistent bursts of hot air when AC while at other times everythings cool (AC)

The Monovalve appears to be working (can feel movement in capsule when removing/reconnecting connctor on it) so I am thinking that the aux water pump might be the problem...heat is rarely in SoCal so I think disconnectng it would be no major problem.

Someone said that a failed aux pump will short out the PBU? Is this really the case even on 126's?

I would like to go try this solution, soon as I get my major oil leak problem fixed (completely unrelated problem)

How do I disconnect it on a 126--it looks like its deep behind the firewall



:p

I am at the same place as you, does a bad aux pump short the control unit out. I spent 2 hours last night trying to figure out what to do next. On my 126. the first wall behind engine seems to have 4 screws that will alow me to remove a panel, that will give me room to replace aux pump. that is my next move and repair kit for mono valve, I could not do this with out the help here, thanks everyone steve

I'm pretty certain this test convicts the pump. If you have DVM, check the ohms on the pump. I forget the correct value, but if you get a small number, perhaps 1-2 ohms, that's your problem for sure.

I've always thought that pump was pretty darn steep for a small electric motor and some plastic. Never seen it for less than $99. Since it does not get too cold down here in Dallas I just let it go, never really missed it.

I can't comment on the 126 chassis, so far I've only suffered through ownership of 123, 124, and 210 chassis cars :) If I was a betting man, I'd say a shorted aux water pump damages the PBU on a 126. Both the 126 and 123 were designed in about the same timeframe, so I'd expect similar behavior. When they did the 124 ~10 years later the designers had learned the PBU needed protection from the water pump.

Where did they hide the pump on the 126? On 123 & 124 cars it's pretty much open the hood and unplug it. One minute of work, one minute to wash your hands...

Feel free to mail down some Pacific Northwest micro-brew!

- JimY

JimY, the aux pump is between the wall of the engine conpartment and the fire wall, right beside the mono valve. You can just bearly see it under two hoses. The wall towards the motor has 4 screws that will alow you to gain enough run to remove the aux water pump. I can not see the wiring for the pump, as I would like to test it, but It looks very old, so I will have to remove it to test it. steve

On the 126, can you pull the fuse for the AUX water pump instead of unplugging it? May be easier if the wires are buried. May also disable other components too though. If they are Climate Control related components, this may or may not confuse this as a t-shoot step.

Pump is expensive, yes, but all in all, $100 to $140 is not bad for an electric hot h2o pump which has lasted me (eruh, the car -- I bought it used) 167k miles and 17 years.

d2bernhard. What does T-shoot step mean? steve

Sorry! T-shoot = Troubleshooting

thanx, i'm still learning steve

Disconnected a 2 piece "connector" on a harness that is located right next to the monovalve but is headed towards that aux water pump...it looks pretty easy to find... ( looking at the picture was helpful --see below -- seems to be the harness that I disconnected!)

Needless to say that did NOT solve the problem, but what did was a monovalve repair kit. This on teh higher mileage 300SD. In all fairness, this car had the problem of "continous heat" and not the intermittent heat.

The other car has more of that intermittent problem so I will try to disconnect the aux pump on that one and report back my findings.

I should have disconnect this thing a long time ago as we barely need heat in California. Here's a good picture from fastlane:

http://img.eautopartscatalog.com/live/G305012429BOS.JPG

300sd2000 what a great picture, have you figured out what turns this pump on and off. I still can't seem to find the connector on mine. I am sure I will be doing a mono valve repair this week just to start, see if I can get sum heat out of Big Blue, my 300sd steve

thank fastlane for the picture...I just linked to their existing picture.....as for the location, I found mine buried right next to the monovalve. It is a detachable connector just hanging loose in between battery and monovalve on my car.

Here is my update:
Weather is cooler and heat is on all the time in the morning on the way to work. I am experiencing the hot air blasts -- even when moving steady. Not just after sitting at a light. The system then overcompensates with cool air and higher fan speed. It seems to need the pump in conjunction with the monovalve to keep the heater core temp happy.

On a side note, my AC seems to have grenaded. It stalled the car at idle trying to engage, and now makes a horrendous noise. I keep it on EC until I can get it looked at.

"If you have DVM, check the ohms on the pump. I forget the correct value, but if you get a small number, perhaps 1-2 ohms, that's your problem for sure."

I am a novice at electrical testing. Could someone please explain what the above means? Waht settings do I use on my multimeter, etc.?

Just to add a little confusion, I went through an intermittent stage of getting unwanted hot air sporadically; had one episode last spring, then a few since, and finally full-time unwanted hot air last week. Replacing the monovalve fixed it nicely.
The solenoid will indeed move even if the valve has failed; what goes is the rubber diaphragm that controls water flow. When it's torn, there's flow through to the heater core regardless of solenoid position. It's a cheap and easy job.

Thanks! I was kinda thinking this too, but have 0 experience with these.

This is not the same monovalve as on the W124 (easy but not as cheap). Can anyone corroborate this diagnosis?

On the 124 there is no rubber diaphragm. The solenoid works direct over the metal valve that blocks the hot water flow. Due the very high pressure inside the system, a rubber diaphragm seems to be not strong enough to block hot water flow. Wich diagnose are you asking for?

Could my problem be a monovalve problem? Or do I need to look at the panel at this point?

(the aux pump runs, and seems to be fine.)

Do you get hot air through the center vent in EC mode and max cold temperature settings, and the fastest speed from the blower motor? If so, it's the MV.

by in-large the problem with heat tends to be with the monovalve. I just replaced the sleeve about 2yr ago and I think once before that....

I was having the same symptoms before, that is of inappropiate heat.

Replaced with the monovalve repair kit and everything is working fine again...I think they just wear out fairly fast. Couldn't really tell anything was wrong with the old one visibly, but the repair kit is fairly cheap so I just decided to replace it.

I have the aux water pump disconnected anyway, but it didn't really solve the problem for me...The monovalve was the culprit in this case.

If you haven't done the monovalve repair kit, its certainly worth a try and its easy to replace and much cheaper than the PBU etc.

Make sure you are getting a 12v signal (connect multimeter to connector on monovalve) and make sure you are getting 12 v when the A/C is on and cold air is supposed to be blowing...if you aren't you've probably got a bad PBU. If you are getting 12v, probably bad monovalve.

Even if you aren't getting a 12v signal and suspect the PBU is bad and don't want to spend the money, you could run a wire over from the fusebox over to the monovalve and splice it in...this is what I did before I replaced the PBU a long time ago (when I couldn't afford the PBU) and it actually worked (except couldn't automatically get heat anymore). In the winter, disconnect the connector and you always get heat.

After I bought a replacement PBU, I removed the temporary "splice" and everything is back to normal.

I do get heat out of all vents, but it is intermittent and temporary. It eventually goes back to a semi normal temperature. Sometimes, though, it gets stuck on full fan and full hot, no matter what temp setting.

If you get an intermittent and/or constant behavior, could be due some rust inside the Monovalve (solenoid exactly). Sandpaper (500 or thinner) the inside of the solenoid and the piston, could help to restore its normal work. Please do this with a cool engine.

It is a good idea to get the repair kit for the MV first. Due passage of time , o-rings and rubber parts inside could be fixed to the metal part, and get damaged/cracked when disassembling it, producing coolant spilling out.

The little spring could be broken too. I don't know if the repair kit includes one. If not, you can get one taken the damaged sample. I don't have yet the lengh of this spring but, completely compressed, it must be fit exactly inside the piston.

Being this solenoid a one way device, when it reaches a 12V signal, it blocks water flow (going downwards in this case). When the 12V signal stops, the spring pushes the piston upwards to allow hot water come inside the heatercore, sucked in the aux pump direction again. This may take place several times per minute to reach the asked temp inside the cabin.

Hope this will help you.

Thank You. This helps immensely, but, I thought the W124 MonoValve was not repairable (no kit avaailable).

Don't worry. Here you can find more pictures that explain all what you need to know.

I have had to completely fix the solenoid because its coilcore was broken, filtering coolant.

In picture "1" is the solenoid completely disassembled, with its two new o-rings, new coil, new spring and new heavy duty plastic coilcore.
O-rings and steinless steel spring were obtained at homestores.
Plastic coilcore was made with a lathe (taking the broken sample), and coiled in a electrical motor repair shop.
To calculate the amount of wire in the coil, it is necessary to know the wire diameter and its weight. So, if this should be the problem, take all the damaged/unrolled wire with you. Maybe you'll have to unsolder the coilwire extremes from the plug (pic "4B")

In pic "3" you can see the broken coilcore near the new one.
You can use (well, the lathe operator) any hard plastic capable to handle the hot coolant temp to make it. And there are a lot of plastic types.

In pict "2A" is the "hard to find" rubber sealant. If after reassembling your unit you get coolant through it, make your own soft o-ring putting silicone sealant over it, allowing to get dry before reassembling the MV.

Why all this work? (it's not too much). In Chile, where i'm from, a new MV costs about US$ 300, and i did all this for less than US$ 60.

What else?

Hope this will be useful for you guys.

Hello All,

It was the Monovalve. Although the old one tests within range (12.6 Ohms), and seems to work (can feel the click), and looks OK upon internal inspection, it must have been sticking in the open position. The new one is in and this problem has ceased. Thanks for all the consternation on this one!