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Old 07-02-2002, 11:53 AM
Posts: n/a
Question 300E Windshield Fluid Heating System

Can anyone advise how is the heating system of the windshield fluid in 89 300E regulated? Mine seems to be heating
non-stop what could be the problem?
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Old 07-02-2002, 04:04 PM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
It is heated by the engine coolant.
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record
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Old 07-03-2002, 12:05 AM
Posts: n/a

How do I turn off the heating of the windshield fluid?
Can I bypass the thermostat and shortcut the wires?
What is the tole of the "pump" that is connected to the thermostat of the windshield fluid container-what is its role?
Does this pump carry the heat into the heating element
I only want to switch this off since it is heating all the time
Please advise a bit more.
I appreciate the input
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Old 07-03-2002, 02:51 AM
Registered User
Join Date: Jan 2002
Posts: 352
The coolant coil in the washer fluid reservoir acts as a mini radiator. The hot coolant is cooled by the fluid. Why would you want to bypass this ingenious MB design. I ber your car will run hotter with this mod. One of my friends told me he runs only water in the reservoir for maximum heat absorption. What would be great would be if a 15 lb pressure cap could be put on the reservoir.
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Old 07-03-2002, 04:28 AM
Posts: n/a
Question Note to question/answer +question

The answer "the hot coolant is cooled by the fluid"
a good answer but thats not what I asked.
I believe that it is the primary purpose of the radiator to cool down the engine.

I must ask again: how to turn off if possible the heating of the
windshield fluid?Can I bypass the thermostat does the thermostat turn on when the temp.= freezing point?
Fumes of methanol are not very healthy since it is
a "wood alcohol"-if it is overheating the fluid so
that it cannot be drained to the windshield because it is
boiling than how do you fix this?
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Old 07-03-2002, 07:15 AM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Lightbulb How about this!?

On a126 chassis there is a system to heat the lower portion of the windshield. A flat radiator panel is under the lower windshield where it receives hot coolant from a small dia. hose fitting on the bottom of the Monovalve. There is an inline thermostatic valve that regulates flow to the radiator panel, perhaps this same flow of hot coolant is used to heat the windshield washer fluid reservoir!

It would stand to reason that the heated lower windshield and heated coolant would be useful at the same time and under the same conditions, it would be very possible that both are connected and possibly share the same controller (the thermo valve)

On my 126 the valve is in the same compartment as the windshield wiper transmission and motor. The compartment is a space between the engine compartment interior firewall and the passenger compartment firewall accessed by removing the black plastic grill covering it.

So you can fix the thermo-valve if it is found to be malfunctioning.

You could always simply add an inline ball valve to the coolant circuit and manually control the heating of windshield wiper fluid.

Or you might check the monovalve itself, if you have not already done so. I saw a couple of other posts you have made concerning unwanted heat in the cabin, this is the default failure mode for monovalves, constant heat , much at idle, less at speed.

Continuously heated wiper fluid does not sound like a German design, so the control mechanisium is the logical fault. Good Luck!

There is always something to learn!

126 Windshield Defroster Heat Exchanger ?

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Old 07-03-2002, 09:35 AM
Posts: n/a
Smile Reply

Well the model that I have is a 124 300E 1989
so it is not exactly a 126. Also there are no heater problems inside the cars air system as I thought. Everything turns
exactly on time. If there are other possible functions
of the heating line that goes into the reservoir adding a
mechanical valve and regulating it manually might
cause problems elswhere.

The inline thermovalve well yes it is more of a thermoelectrical
valve as itself it is connected only to wires......
I dont believe that it is responsible for maintaining the
engines temperature though. The engine has a thermostat of its own. This heating circuit that I am trying to discuss is somehow separate.The best thing I guess is to equip myslf with a good manual.
I dad similiar problems however with the radiator in a
Fiat while in europe...however it was the thermostat which
was directly on the radiator and was not turning on the electric fans circuit so the radiator would overheat. I believe that in
cars such as mercedes the thermostat is located directly
somewhere on the engine so it measures temp directly.
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Old 07-03-2002, 11:14 AM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Lightbulb Well?

Knowing that you car was a 124, I purposely included the qualification that my direct experience was with a 126 chassis, and because the 124 chassis is not a virgin design you will be happy to know that many proven design elements of earlier chassis’s where incorporated into it.

I am fortunate to have a 124 ETM and I have taken the opportunity to investigate and you can be assured that there does not exist an electrical nor electronic control mechanism regulating the washer fluid temp. There is a washer fluid level switch attached to a grommeted sending unit in the washer fluid tank. And there are two electric pumps, one for the windshield nozzles and the other for the headlight wiper nozzles of course.

From a 1989 300 SE 126
Heating of tank by coolant similar to the windshield heater, the heat exchanger (tubular coil) in the washing agent tank of the windshield washer system is supplied with warm coolant from the bypass flow to the heating circuit. A thermo-valve with a wax pill element located in the washing agent tank ensures uniform temperature of the windshield washing agent of +20C to +30C”

Do you think that in the same model year MB might have used the same system and components in two different but related chassis? Your over-heating would appear to be possibly related to a failure of this in-tank mechanical thermovalve.

The thermo-valve that I spoke of is a mechanical valve that is designed and functions to open at +15C and is completely open by +4C outside ambient temperature and below allowing warmed coolant to the windshield glass heater. Again it would not surprise me if the warmed coolant that flows through the heat exchanger/thermo-valve inside the washer fluid tank is provided from the ambient air temp thermo-valve that controls flow to the windshield glass heater! Since an ambient thermo controller and flow is already engineered and installed why wouldn’t those wild a crazy Germans expand and utilize it? It would be much less likely for such engineers to design and install a much more failure prone electric/electronic system when a hardy mechanical system would suffice, don’t you think?

The heat exchanger/valve in the washer tank receives its flow in parallel to other devices and functions I would bet rather than a series arrangement, so the mimimal partial obstruction of its flow would most likely be without adverse consequence. You could always simply pinch it off temporarily to experiment. At low engine speed the electric auxiliary coolant pump directs coolant flow through the monovalve/heatercore/bypass flow circuit and stopping the small flow through the washer tank heat exchanger would have negligible effect.

Perhaps some other 89 300E owner will be able to comment whether or not the washer fluid is always heated rather than only those times when the outside ambient temps would make it useful?

The cabin heat problems I metioned where from other previous threads you had ether posted or replied to. And yes, the relatively small investment in the documentation for your car would help avoid circumstances such as these when no one can provide the exact and specific answer for your particular problem of the moment. Good Luck!
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Old 07-03-2002, 11:48 AM
Posts: n/a
Smile reply

I will inspect the thermovalve in the reservoir and
check its operating characteristic-I might need to replace the heating element then along with the thermovalve if that is
how the Germans designed this car. I noticed that
in the trunk there is an old cylindrical pump which is denoted as
"recirculator" which was replaced in this car.The old one is pretty
rusty........recirculator for the heated engine coolant?? it normally is located about foot downwards from the windshield reservoir.
This is activated electrically with wires as I see the connections....

300TME?-kombi or turbocharged?
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Old 07-03-2002, 12:22 PM
Join Date: Nov 2001
Location: Cape Cod Massachusetts
Posts: 1,427
Thumbs up auxiliary coolant pump = recirculating

"At low engine speed the electric auxiliary coolant pump directs coolant flow through the monovalve/heatercore/bypass flow circuit"

This is the coolant recirculating pump that is used by the ACC to ensure that enough heated coolant is continuously circulating through the heatercore. Apparently the engineers discovered that at idle speed less heated coolant was flowing through the heater core, then as soon as the engine RPMs increased much more flow occurred, this rapidly changing heater core flow condition caused the ACC control to overreact in some way or not react quick enough. The solution was to install the electric auxiliary coolant pump that is controlled ye the ACC unit turning the pump on when insufficient coolant was flowing thereby moderating the flow fluctuations and allowing the ACC to operate with the desired heat flow into the cabin.

A cheap, easy, and valuable DIY project is to install an inline fuse for this pump. The pump failing it the "lock-up" mode will cause excessive current flow through the controlling circuitry and the PC board traces in the ACC pushbutton unit, at $200 for a rebuilt unit, $5 and 30 minutes is cheap insurance! Obviously your car's pump failed once already and you appear to have luckily avoided the failed controller syndrome!! Check here!

140 Phones from MBNA

Good Luck!

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Old 07-03-2002, 07:06 PM
Posts: n/a
On a w124, there is no heat control/thermostat/valves/electrical circuit to control the heat of the windshield washer fluid. Engine coolant passes through the element without any control. Eventually, the windshield washer fluid is at the same temperature (or close to) as the engine coolant.
Although this seems to be a well thought system, it has it,s disadvantages.
Normally, windshield washer fluid is a mixture of alcool (wood) and water. Up north (Canada), windshild washer fluid is designed (mixed) to sustain a -40degF temperature, which implies a larger proportion (%) of alcool.
Since alcool vaporises at a lower temperature than water, in the middle of winter, most of the alcool is vaporised at normal engine operating temperature. With the heating system, the water do not freeze but as soon as you spray the liquid unto the winshield, it freezes instantly.since it is almost pure water.
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Old 07-04-2002, 10:13 AM
Registered User
Join Date: Jan 2000
Location: New Bedford, MA USA
Posts: 1,583
You can bypass the heater in the resevoir by removing the hoses (one or the other) and shunting the engine coolant from the inflow to the outflow. I had to do this once when the thermo valve sprung a leak.
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.
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