1981 300TD No heat when driving over 20 mph

We have a 1981 300TD that while sitting in the drive will put out all the heat you could want. Once you start down the street and get over 20mph it blows cold. The thermostate has been replaced.
Need help it's starting to get cool in the mornings here in Alabama.

[280CEBoy]

Check to see if your fan-clutch is locked up....it may be staying engaged and cooling the motor too much...how is the engine temp once you start driving?

[stevebfl]

Monovalve capsule.

[280CEBoy]

*smacks head* Yep! I couldn't remember the name of that....thanks!

[can-do]

Greetings,

I hate to steal the fire from someones post, but I'm getting cold in the mornings to, and I've gotten no responses from my post. I've got an '80 300TD with the ole vertical pushbutton style climate control. I am not getting any heat at all coming out the vent in the heat or defrost mode. Any way to isolate the problem down to whether it may be the actual temp dial on the dash or perhaps the water control valve under the hood? When the engine is hot there is no warm cooling water going up to the heater core. The heating water pump is turning freely and working fine. The blower turns on immediately but blows cold air even in the max heat setting.

I'd appreciate any info you can provide Steve.

Thanks,

Freezing in the mornings

Charles

[stevebfl]

Well Charles, we probably should start a separate thread on your car as its causes are so different from the pattern failure I threw out for the 81 car.

The characteristic symptoms of no heat after the car starts driving in the later Klima system makes the Monovalve capsule a low risk call. On your system the lack of heat is likely to be that the servo/heater valve is not opening. That can occur for a number of reasons. One diagnostic observation is to see if the center duct changes during a change from the coldest setting to the hottest. Airflow should gradually stop as the heat mode is engaged.

If this occurs then you probably have a bad servo as the same action of the servo will normally open the water valve as the internal vacuum distributor moves to change the ducts.

If there is no change, then I would either suspect a locked up servo and/or an inactive controller/amplifier (behind glove box).

The servo has a clockwork mechanism inside that is driven back and forth in an electronic balance by a small motor driven and monitored by the amplifier. The teeth of the gears commonly break cuasing it to jam. With the motor unable to move the amp is overloaded and often is taken out by the first failure.

One of the neatest tools my brother (the electronics nut) built for us years ago works wonders on this system (we should have sold it to MB). It inserts in series electrically into one of the connectors at the servo (one connector is all inputs and activities and the other is blower control). It has a green LED and a red LED showing us the polarity of current through the internal servo motor (this shows whether there is driver motion either in the hot or cold direction), it has an analog meter that shows the feedback potentiometer reading (this tells the position the servo is in - heat being all one way and cold the other). There are a couple other signals, but the best thing it does is allows the operator to over ride the amp signal and manually control the servo.

Since he built it we have only had vacuum problems with that system. Electrical problems aren't.

[can-do]

Greetings Steve,

I know this '80 is a different beast climate control wise. As you stated I do get air flow out of the center vents throughout the entire cycle of the temp wheel. It will only stop flowing air if I push the selector button for defrost. The symptoms of the system failing a few nights ago was that it would not send heat to the side ducts or floor unless I cycled the defrost button first for a few seconds then pushed either the fan hi or low button. At this point the center vents would remain close and I would get heat somewhat as normal but flow seemed somewhat decreased coming out. Now as I have said, I get no heat regardless of setting, ie. defrost or heat mode. Is it at all possible that the temp wheel selector is defective not causing a difference in resistance between it and the sensor? I'm trying to understand the operation of this sytem through the cd info etc. but can't seem to find a starting point at which to start testing from.
Have you got any ideas to start the process of elimination for part failure?

Thanks,

Charles

[stevebfl]

Defrost bypasses all temp evaluations. If the unit doesn't open in defrost it is not a control problem as such. In other words the decision is removed with the defrost setting, all sensors are redundant then.

[can-do]

Greetings Steve,

Are you saying that the defrost position bypasses all related electrical imputs and immediately moves the servo motor to allow coolant flow to the core regardless of any other setting? If that being the case should I look at the servo motor as being defective? Is the input to the servo electrical or vacuum operated to cause it to move? I noticed two seperate vac lines going to it from the bottom, I think one was yellow and the other black. Do these have a direct input on servo motor movement?


Charles

[stevebfl]

Go back and read my first reply.

I will add to it here. The servo is controlled by the amplifier. It does this by powering the small DC motor in the servo. The amplifier changes polarity to move toward heat as opposed to cold.

The amp is a comparitor. It takes the sum of inputs and compares what exists to what is being asked for. The decision is implemented by winding the servo one direction or the other. The various inputs are: incar temp sensor, ambient air temp sensor, evap temp sensor (?), servo feedback potentiometer (tells what position servois in and shows movement to amp) and the temp wheel setting. Vacuum is an output from the servo. The only input to the servo is the two wires that power the internal motor.

The two vacuum lines at the bottom of the servo are important. Unlike the massive group at the top of the servo, the are an input of a sort. They are at the beginning of everything and pass through a temp switch in the water part of the servo. By not passing vacuum at low water temps the master vacuum switch will keep the blower from running. I forget the temp at which it occurs but I don't think that temp exists in Florida as I have never seen it operate. In normal op conditions vacuum will be on both sides. Since you have blower forget about those lines. And, YES the defrost position should over ride everything. The servo should move.

Without a diagram and my neat tool I can't say whether the servo is controlled by the amp during defrost. I think so as I don't know where the power would come from. I'm sorry but that doesn't eliminate the amp from consideration.

I would identify the two wires going to the internal motor and check power. The voltage polarity should reverse by moving the temp wheel from hot to cold. If you can't figure it out from the diagram, take the top off the servo; its easy. There are four screws under daps of black silicone at the four corners on top.

[can-do]

Thanks So Much,

I will start at the servo and check for power and ascertain if movement is taking place, and when. Hopefully at that point I'll be able to determine if there is power present, why the valve isn't moving, ie. broken teeth as you mentioned.

I Appreciate all the help you provide to all us DIY'ers


Charles

Originally posted by 280CEBoy
Check to see if your fan-clutch is locked up....it may be staying engaged and cooling the motor too much...how is the engine temp once you start driving?

The temp sets right at 80 where it should be when driving.