Hi all,
As winter looms on the horizon, I'm hoping to get the heat working in my 300CD.

It has the early generation of automatic climate control. Vents and blower respond accordingly to the push buttons. However, a few evenings ago, as temps dipped near 40s, I realized that with the temperature dial set on the warm side (80 / 85 degrees) there was no heat coming from the floor vents or defrost vents.

I have seen on several threads where the monovalve is referenced. Did the early ACC use these? If so where would it be located?

If the monovalve isn't the problem in these models, what should I look for?

Does the **************.com manual conversion kit work on this model?

Thanks!
Drew

Don't think you have a monovalve.

I have the 1980 with ACC servo under hood.

During the several years that mine would not product heat
I bypassed the ACC valve under the hood on passenger side.

It is really easy to do, it leaves the water on all the time and was fine in winter.
If you need to shut it off put a ball valve in the line. Just cut the bypass hose
and add the valve and two more hose clamps.
This really works well.

Then.... start watching Ebay for a replacement valve and amp/controller
(the amp controller is behind the glovebox).

This is a super easy repair and feels good when it works correctly.

The $550 complete replacement is the other real choice for a working system.
Had I not found a aluminum bodied ACC valve for under $300 that would have been my choice. The wife was all for it.

The bypass is one hose only and you disconnect the two hoses on the engine side of the valve, use the bypass hose to connect the fittings that those two hoses connected to.

The two hoses on the fender side stay connected.

One more thing... two vacuum lines that go to the bottom of the valve.
Connect them together. Bypass that too.
Disconnect the two plastic lines and put a piece of rubber line to connect the two.
Those two lines are connected to a switch that only opens when the water is hot. I always worry that if it is faulty it will also cause no heat.

Good Luck.

__________________
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

There are several options, I bypassed the whole system with a valve i pieced together for under $20, but I only had heat from full blast from the defrost vents. Another option would be replace the parts with the orginal design parts, or the updated replacements, which are still prone to failure. The last, and best option is to get the all digital replalcement that sells for $700 on fastlane. It is the best because it retrofits updated technology in with old school technology to make the best climate control system possible on these old cars.

Thank you both. I believe I will go ahead and do the bypass for now. I'm pretty tempted by the ************** kit, which has the valve and a push/pull control for some semblance of control from inside the car.

Time and money permitting, I would like to go the digital conversion route eventually.

At the moment—I just bought the car last month—I'm trying to get things in order for winter: heat, side window seals, seal up a leak around the back window using t_walgamuth's method posted on here and replace the cracked exhaust manifold.

Thanks again for your help. This site has given me a wealth of information to use as I give this car a little TLC.

What happens when you hit the defrost button?

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Full blast, cool air through the vents at the base of the windshield.

Does this car have the Aux water pump? If so, check that as well.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

One More Thing.

Update your profile and tell us where in the world you are.

__________________
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

I have been under the dash today, removing wiring from an old Motorola car phone. While I had the passenger side under-dash panels off, I discovered two things.

1. the hose for the in-car sensor had become disconnected from the underside of the speaker grille and was hiding out behind the glovebox.

2. The hose that detects the outside air temp (via a plastic tube that goes through the body and into the doorjamb cavity) had crumbly ends and was more or less disconnected. I managed to wrap the ends with some Gorilla Tape and the hose is now snugly in place.

Also, I applied power to the electric auxilary water pump that sits just in front of the climate control servo as recommended by rrgrassi . Thankfully, I could hear the little motor whirring away.

It's supposed to be in the 50s again tonight, so hopefully I can see if the discombobulated temp sensor plumbing was the cause of my climate control troubles.

Vents and blower are all working properly. I must note however, that I can't use my A/C for the time being, as the hose between the compressor and condensor is gone.

I'll be back with the results...

If this fixes it, I may just go ahead and start setting some $$$ aside for the digital retrofit in the event the servo fails.

Well, I'm not sure what I was thinking in that last post. Because, as I later realized, if the defrost setting is putting out heat, nothing temperature-dependent is going to put out heat either.

Sure enough, I discovered later, the plastic servo had what appeared to be a small leak. On Friday, I did the bypass recommended by rhodes2010, leaving the fender side hoses connected. I disconnected the hoses on the engine side of the servo and connected them to a 1/2" ball valve fitted with 1/2" galvanized barbed nipples on either end.

Driving around later that day, I lost most of my coolant! It was flowing out the ports I had bypassed, leading me to conclude that the servo itself must have had an internal fracture.

One 1-foot section of hose (free from my friendly local NAPA) and two gallons of coolant later, I have plent of heat blowing through the vents at all settings.

While it was probably redundant at this point, since the servo no longer had coolant flowing through, I went ahead and connected the two vacuum lines that connected to the bottom of the servo, as rhodes2010 recommended.

Thanks to everyone who chimed in with helpful advice on this one. I'll post some pictures of the set up later today.

I came across this fix in a 300CD for sale on eBay recently. I was intrigued by the seller's description and the photo of the switches installed, so I e-mailed him. Below is his description and photo.

From the car's description:
The heater/AC is a special story. Unlike the run-forever engine and transmission reliability characteristic of this vintage Diesel Mercedes, its heater and AC controls are notorious for frequent failure and incredibly expensive repairs. It is a complicated system using extensive vacuum lines and devices, many under the hood and close to the battery. Over time, the normal spray of battery acid eats into the lines and is drawn into the heater control valve.

Being an electrical and mechanical engineer, I decided to install a modern, much more reliable control system. The $400 control valve is replaced by a simple heater water bypass valve, commonly available from local auto parts stores. An integrated circuit electrically cycles the valve-open time, effectively controlling the air temperature. Separate switches directly control the blower speeds and AC compressor. The result is a much simpler, very reliable system. The controls are located in the wood panel above the original heater/AC control buttons – see picture.

Seller's response to my question about the specifics of the fix:
The bypass valve is vacuum operated. At first I tried to modulate vacuum to position the valve but it was too sensitive; it wanted to either open or close. So I put a 12V solenoid controlled vacuum valve (surpluscenter.com, ~ 8$)and built an electronic circuit using a 555 integrated circuit timer. It's kind of a universal device and I set it up to turn a relay on and off, which opens and closes the valve. Radio Shack has a book on 555 timer circuits. I just switched in different combinations of resistors to vary the on-time.

The heater water and coils have a lot of thermal inertia so the valve does not need to cycle very quickly. Mine does about 20 seconds closed and the open time varies from around 10 seconds to a minute or so.

I haven't got organized enough yet to make a diagram but ran across a Diesel service website person who wants me to put the stuff together in a kit he'd sell on his parts site.

Good luck.

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