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!

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David
1986 300E Anthracite + ECodes + MB Mileage Award

Bad contacts on the Temp Control dial

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Zafar
94 E320 58000 Miles

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?

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David
1986 300E Anthracite + ECodes + MB Mileage Award

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


temp dial gauge

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Zafar
94 E320 58000 Miles

Anyone solve the problem of hot air fromside vents ona W124

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?

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David
1986 300E Anthracite + ECodes + MB Mileage Award

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?)

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David
1986 300E Anthracite + ECodes + MB Mileage Award

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.

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David
1986 300E Anthracite + ECodes + MB Mileage Award

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

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1) 84 300SD 285K+ Miles (Ver Calif)
2) 84 300SD 175K+ Miles (Ver Federal)

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