Need help for 77 300D, Climate Control moves all gates correctly, AC works with compressor switch, just no heat. Been told may need to replace Mono Valve, can't find it on the 77, have seen them on later models. Is it on top of the Aux water pump??? Any help? Have 230K, backed out of showroom in Knoxville TN May of 77, it is my wife's baby, unfortunately no one at Knox MB can work on these old cars, it is older than they are?

I am pretty sure your 77 has no monovalve.
It should have the Auto Temp II Servo under hood.
Look on the passenger side fenderwell, in front of the wheelwell.
Large rectangular black object with 4 heater hoses on bottom and cluster of vacuum lines connected at top.

If you find this item there, then you have three choices.
Do search this forum for servo and look at all that has been written.
It is really helpful.

Choices:
1. Short term quick heat - bypass the servo and get nice warm air full-time.
Or on-off with a cabin operated mechanical valve or,
(my choice is a simple ball valve in-line on the bypass hose).

2. Acquire a replacement servo and servo amp (couple hundred on the low end), make it work like new (I did this and am very happy).

3. Purchase and install the newly engineered replacement for the servo
for the $550 to $750 range. Excellent alternative and car works like new.
Don't have any experience with this.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

thx for input, what I was afraid of.