I have a 82 300SD that provides heat sporadically. On cold days, it does not heat constantly. On those days the temperature in the car never reaches a point at which the fan shuts down to a low speed. If I turn the temp control button to cooling and then reverse it to heating, the heat comes through warm and cozy for several minutes and gradually feeds in cool air. Turning the temp control switch further in the red zone does not help. On warmer days the system works just fine.

Any ideas concerning what I should be looking for. I thought about the thermostat but the car seems to maintain the proper engine temperature. And being able to get some short term results by turning the switch from cool to heat make me believe the thermostat is not the problem.

Any Suggestions!!!

Richclay

Kinda sounds like the "monovalve". Plenty of good reading if you search on that word.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Sounds to me like the foam tube to the temperature sensor has disintegrated. Thus, the sensor is seeing warmer underdash temps. You could test by turning the dial to max heat until it clicks. That is supposed to override to sensor.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Does the 300SD have the same kind of foam tube inside the dash as the 300D?

I ask this because it looks like the temperature sensor on the 300SD is not on the dash top, but up near the dome light.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Oops, I may have spoken too soon. Never owned a W126.

However, my W210 has a fan that draws air into the temp sensor and I remember it acting pretty funky when that fan quit. The W126 could have a similar setup.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

I experienced similar symptoms on my 300SD. Every time I started it up from cold - no heat. I'd drive until the temp gauge would reach 50C, pull over, shut off engine, set heat to MAX, restart and get full heat (no control). Lived with this way too long, then replaced the monovalve. Problem solved.

The 126 sensor was in the dash, similar to the 123's, until about '85 or '86 when it moved to the dome light. When the sensor moved to the dome light, the tube changed from foam to something similar to agricultural drip line.

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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

Richclay, where is the temp. sensor in an 82 300SD. I had an 81 that had it in the middle rear of the dash. The 83 I have now has it by the glove box.

I replaced the foam tubing connecting the sensor to the plastic line that comes from the vent fan that draws air past the sensor with 3/4 inch pipe insulation from Home Depot on the 81. Not a difficult job but time consuming as I had to remove the glovebox, the center vents, disconnect one of the vaccum pods and remove assorted venting. Haven't had to replace the tubing yet in the 82 but should be a much easier job.

Replacing the tubing on the 81 fixed the same problem I was having as the sensor was reading the temp. behind the dash and not from the cabin. MB uses foam tubing so as not to have any vibration.

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Fred Johnson
1977 240D
1983 300SD

temp sensor on my 82 300SD is next to the dome light