Greetings,
I'm new to forum and new to classic MB's. I bought a 1981 W123/300d three months ago and am trying to fix all the little things that are apparently common to these models. Currently it's a lack of heat at highway speeds. I bought a rebuild kit for mono valve and found the innards to be in good condition but replaced them anyway.
Sitting in the driveway idling, I have complete control of heat and auto fan works fine.
What would be the next thing to check? Again, heat is controllable at idle UP/DN. One more thing, I disconnected the mono valve and get scorching heat at any speed.
I know I'm going to love this car eventually.

Thanks
Pat

Change the thermostat to a stock spec one, maybe on the low end of the allowable temperature scale.
Sounds like you've got too much cooling going on which isn't allowing the thermostat to fully open at highway speeds. I had this problem on an old Jeep Cherokee of mine after I put an air vent in the hood and added a big engine oil cooler. The engine wasn't getting up to temperature when driving so I had to get a thermostat with a lower temperature rating.

Be careful though, if you're not getting good flow of coolant through the engine, this can also restrict hot water to the thermostat and heater. You can get overheating pockets in the engine. It may benefit from a flush, then again, a citrus flush may cause leaks. It's always a gamble.

Good luck!

Phil Forrest

__________________
1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

if disconnecting the monovalve wire gets you max heat at speed... you have a push button unit problem.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Before you condemn the pushbutton unit, verify the foam sensor tube under the dash is intact. The PBU may be doing what it's told based on bad information from the ambient temperature sensor with a crumbled sensing tube.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Hmmm. when the foam sensor tube on my car failed, it would not read the cabin temps well, and if cold would feed MAX heat most of the time... I suppose if it's broken, and drawing in heated air, it could think the cabin is too warm.

Good call.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

THanks guys,
Now tell me where this tube is and how to check it.

it depends, is there a little square in the center of your dash?

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

yes, the temp sensor?

under the dash below that little grill is the tube, look at it and see if it's damaged.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

will do, will let you know.
THNX

I was able to get to the foam tube on my car by removing the glovebox.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

WhooHoo,
I checked the foam hose behind the panel and found it to be basically gone from age. Now the big question, how/what can I use to fix it? I assume you can't get replacement part.

THNX
Pat

I used a chunk of heater hose thay was laying around the garage. It was 5/8 if I remember correctly and I flanged the ends out a bit by pushing them on the end of a funnel in order to make it easier to install.

__________________
'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."

I replaced mine with a scrap of pex plumbing. I'm not sure whether the original was foam due to the desire for accurate temperature or to isolate the sensor from vibration. The more rigid tubing seems to work fine.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

WhooHoo,
It worked, I have controllable heat at all speeds. On to the next thing.
Thank you

Pat