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Old 03-31-2000, 06:06 PM
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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.

It really bites on a cold morning to get the frost off.

What is the problem and why so expensive to fix?

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

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Old 03-31-2000, 10:08 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
You could have a faulty auxillary water pump...When they fail, though, they will burn a trace in the pushbutton. I have replaced the aux pump and taken the pushbutton out and used a small wire to jump the burned trace...This is a very advanced repair, if not comfortable with this, take to a tech.

1981 280GE SWB
1987 16V

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Old 04-01-2000, 09:27 AM
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Could that explain why on some very, very rare ocassions it smells like there is something burning when I put the heat ON. If I simply turn the heat OFF and then back ON again, the smell goes away - this does not happen often maybe 3-5 times a year. Since winters in NC are short lived - have not worried about it. On one occassion a year ago one winter, I saw white smoke coming out of the windshield ducts. I quickly shut the car off and pulled over. When I pulled the hood open, NO SMOKE, could not find where it came from and it the smoke was never visible again.

Is it OK to leave it the way it is, or can leaving it alone cause greater problems. What does the aux. pump do and how can that be the problem? My common sense tells me if the the heat works in LOW it should work in HI, just the fan blowing faster over some coils. Please straigten me out. Is this something that I can see on the CD Rom. Is this a typcial malfunction in the 300?

'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)
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Old 04-01-2000, 10:08 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I think I answered this in a reply you made on another thread. It's good my memory for technical issues isn't as bad as my moment to moment memory.

The logical answer is the monovalve capsule, but as Benzmac points out a defective aux water pump can also cause reduced heat.

The pump is used to keep the water flow consistant through various engine speeds. Many of the engines including the 103 motor do not produce much flow at idle. The problem with this is that the temp controller opens the water valve to compensate and then you rush off from the light and all of a sudden you get a blast of hot air.

This conditioning of the flow by use of the aux water pump is much more important in the 124 chassis. I have not seen reduced heat caused by pump problems in the 126 chassis. (then again who needs heat in Florida).

As to your smells and smoke, I doubt that the pushbutton controller would smoke more than once, and then it would do very little. I would presume you have had a pre-failure event with your blower motor or leave debris burning in the motor or resistor pack (I think the 126 still had blower resistors)

We use the heat so infrequently in Florida that some burning type smells occur the first time each season.

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

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