85 300TD Turbo (260k):

Hot air is trickling out of the vents and I cannot stop it. Seems as if it is a vacuum or servo issue, a vent isn't closing somewhere. I have read some similar posts in the archive but nothing specific. Can anyone lend any insight to this problem. Thanks for your time.

Alec.

You have water flow if you have heat. The water valve (monovalve)is what controls the water. The valve is controled by the pushbutton control unit in order to achieve the asked for temperature.

The valve has three stages of control. it has power to it all the time and the PBC (pushbutton controller)grounds it to shut the valve. The three stages are: Min = the ground is always engaged the valve is always closed. Max = the valve is not grounded and the valve is always open. The third stage is everywhere else. In this position the valve is pulsed in increasing duty cycle as the called for temp is asked for. The controller compares the temp sensors and the asked for setting and pulses the water control to mix with A/C to get the appropriate temp. The farther the existing conditions are from the asked for state the longer the off or on cycle.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-30-2000).]

I had the same symptoms, lucky for me it was just a dirty fuse. Too bad I didn't find this out after my trip across Alligator alley last summer. boy was it hot!

Larry

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03/83 300D
07/73 280

Unrequested heat on a vehicle equipped with Automatic Climate Control is frequently caused by a defective heater monovalve. Your vehicle's ACC system uses engine coolant as the source of heat for adjusting the vehicle's interior temperature; coolant movement into the ACC's heat exchanger is controlled by the heater monovalve. This valve utilizes a rubber diaphragm attached to an metal plunger to gate coolant entry. Over time, the diaphragm fatigues and ruptures, allowing uncontrolled hot water entry into the heat exchanger. This diaphragm can easily be inspected and replaced: on a cold engine, locate the heater monovalve (on your vehicle - mounted on firewall, approx. inline with exhaust manifold); two engine coolant hoses and a power supply cable are attached to the valve. Detach the power cable and remove the four screws holding the valve's electromagnet plunger assembly in place. Remove the plunger and inspect the rubber diaphragm surrounding the plunger. Any loss of diaphragm integrity warrants diaphragm renewal. A repair kit is available from either Bosch or M-B and is substantially cheaper than a new monovalve. Installation is the opposite of renewal; correct for any coolant loss.

Q: "This valve utilizes a rubber diaphragm attached to an metal plunger to gate coolant entry."

The explanation works so well I hate to be picky, But, the diaphram is just a flexible seal and IS NOT a part of the water control function. The valve seals on the half inch dia. rubber seal at the tip end. I would love for someone to explain the screen function as that also isn't where the water flows.

The main reason I point this out is that one of the most common failures does occur when the diaphram tears. That problem is a lack of heat or more precisely a heater that heats and then quits after a few miles. I have heard some good explanations why, but the one that keeps my memory is that the coolant gets behind the piston and the pulsing in effect jacks the valve closed.

BTW the monovalve capsule kit is MB# 000 835 06 44, Bosch# 1 147 213 007 MB list $35.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-31-2000).]

My 300 SEL has a different problem: not enough heat. My Tech said is was NOT the monovalve. My dealer said it has something to do with the electric push button (around $300).

My heat only works when it is on LOW. When you put the heat on HI, it blows on cold air.

What is the problem and why so expensive to fix?

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)

Simple test, pull they connector off the monovalve and try your heater in all the positions.

If you don't get heat check your aux water pump, power it if need be. If you still don't get heat you probably have the diaphram problem discussed above. With the connector off you should get full heat, if not you've got either a bad valve or a restriction.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

84 300D-T 204K

Like Alec, I am getting major amounts of hot air coming out of the vents, at any temp setting. The heat wasn't great during the winter, but now that summer is around the corner, I truly don't think I can handle the perm-heat situation. Is the mono-valve the problem? Or the control panel? The car previously has been doing this about once a week, but the ECON setting won't cool at all now. Guess I'll be getting lots of fresh, Texas air...
Robert




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R Talley
84 300D-T

The valve has power all the time and is grounded by the pushbutton controller when NO HEAT is desired.

The test is to verify the constant hot (fix if not) and verify the other side is grounded.

If you have these two conditions and you still have heat then you have either a bad coil in the valve or a stuck capsule. Check that the coil isn't open with an ohm meter and replace the capsule if it isn't. Its MB# is 000 835 06 44 and should be available from mercedesshop.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

I took the valve out and inspected the rubber washers (a.o.k.) and the coil. The coil had a fault where the thin copper wire burnt or coroded. I soldered the wire back and it seems to work fine for now. If a new coil isn't rediculously price I'll replace the old. Thanks so much for the info.

TCCBass...let me know how your venture turns out. Thanks.
Alec.

Hi,
Below is my original question, having read the previous answers to the hot air problem, since I cannot find any blown fuses, I am guessing it must be the coil. But where on the car is this coil located.

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300TD Turbodiesal

With sparks flying, I shorted out the circuit that controls the heater control valve, by accidently shorting the outputs of the heater control cable to each other. The cable was disconnected from the heater control valve. It is electrical controlled, not vacuum. Since I cannot find this circuit in the Haynes Repair Manual I would like to ask if anybody can help me understand what circuit I have blown and what I need to do to fix this. Also would it be OK to jury rig the heater control valve to be closed by applying a it directly with 12volts.

With respect to finding replacement parts, does anyone have comments on buying parts from SPI, Santa Rosa California.

thanks Michael

quote:

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Originally posted by Alec:
I took the valve out and inspected the rubber washers (a.o.k.) and the coil. The coil had a fault where the thin copper wire burnt or coroded. I soldered the wire back and it seems to work fine for now. If a new coil isn't rediculously price I'll replace the old. Thanks so much for the info.

TCCBass...let me know how your venture turns out. Thanks.
Alec.

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New 300TB Turbodiesal Owner
Home Mechanic

I am a very happy 300D owner once again. On the way home from work yesterday, my AC turned to heat like I've never known. Today I changed the thermostat, and took apart the coil. It was very corroded. After spraying it with carb cleaner and letting it soak, dried it off, and presto I have cool air again. And, with the new thermo, the car temp is down about 10 degrees. Thanks for everyone's suggestions.
Robert

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R Talley
84 300D-T

[This message has been edited by TCCBass (edited 04-13-2000).]

MBZflyer,

There are pics of the valve (with coil) in the Haynes manual. One of the pictures is printed upside down (this fact is not surprising). If you happen to have the same print it is on page 112 figure 11.2 & 11.4.

Hope this helps.

-Larry

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03/83 300D 184k
07/73 280 160k

I have found the heater control vavle in the Haynes manuel,
but what I cannot find is the control circuit on the wiring diagrams, p 279-281
I don't know if there is a name change in the terminaolgy. I cannot even track down the wiring colors. However from previous posting i now know it is controlled trhough the ground side, but more help would be appreciated.

Also it seems like my aux fan is dead, it tested for elctrical continuity but there is none.

quote:

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Originally posted by Larry Delor:
MBZflyer,

There are pics of the valve (with coil) in the Haynes manual. One of the pictures is printed upside down (this fact is not surprising). If you happen to have the same print it is on page 112 figure 11.2 & 11.4.

Hope this helps.

-Larry

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MBZ, is there anyway that you can take a peek at the wires on the electrical connector that goes to the heater control valve? Maybe you can push the plastic sleeve back a little? Also, what year is your TD?
-Larry

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03/83 300D 184k
07/73 280 160k