1980 mercedes 300cd. The heater blows cold. It has a brand new servo unit. The recirculation pump is works when hot wired. All new hoses and clamps.The vacuum heater valve is operational. I am ready to have it crushed

I need more info. When you set the vent control to defrost does it direct the air to the windshield. and when you set it to floor does it go there. if yes the control to divert air is working. before replacing the control unit you need to find the lines that feed hot water to the heater and check to see if line coming in to the heater is hot and the line returning from the heater is warm to hot. sometime the heater gets plug. if so just disconnect the lines to the heater and flow water from your garden hose to the return line of the heater for about a minute or so this will flush dirt from the heater. reattach the lines and check for heat. This is a known problem for all cars as they get old.

try locating the physical tap that connects your heater hoses to the interior of the car. note what position that tap is in before and after you turn up the temp dial inside. whatever connects the dial to the tap may be damaged.

if the tap itself turns then the heater side of the blower may be blocked.

CHECK all the vent hoses, are they connected, no ob strucctions, when the last time you replaced the cabin air filter ? do you have one? look @ your owners manual, also, you may need to have several heater controls, or other control on as well, see owners manual, anything fall into the defroster vents ? replace thermostat @ engine. see how this works for you, good luck

This could be a few things.Try the easiest first.With the engine warmed up,the heater core hoses should both be warm(shows water flowing through core)If you have electronic controls,the cold/hot door is controlled by a servo motor(similar to the servos in a remote controlled car toy)These will not move/function if one wire is broken(saw a few GM products do this)Also remember a neigbor's Jeep that had a stuck motor,careully took the servo apart,and lubed the motor to unstick it).Manual or vacuum controls are pretty basic,just figure out where the cold hot door is.

I assume you actually touched the water hoses to make sure they are gotting hot?

But hot water to the heater hoses, does not mean the heater valve is opening and letting into the heater core. Most valves are operated by a vacuum servo these days, although some still use a cable. Have someone operate the lever back and forth, while you watch to see if the valve is getting movement.
If not, it could be the cable, or it could be lack of vacuum to the servo, or it could be the servo is leaking, or it could be you have a computerized climate control that is trying to regulate temperature and not working. The valve is usually out in the engine compartment, because it still get hot even when closed, and you don't want that inside the car in the summer. So you should be able to see if it is cable or vacuum. And you should be able to manually move the cable, or supply direct vaccum to the servo. If you then get heat, you know what it is then.
If you don't get heat even with the valve open, there could also be a temperature regulator. That would be likely under the dash, and is designed to allow exact temperature modualtion, like when you want one side different from the other. And then you run into computer controls again, where I don't have the model experience to help.

possible low anti freeze, air bound heater core, defective heat blend door control servo. Start with coolant level, if o.k. check hoses feeding heater core. If the lines are both hot suspect blend door servo/control unit. If only one hose hot suspect clogged/air bound core. Special bleed procedure for engine. With engine running, check for bleeder scews at upper hose mounting, open bleeder and watch for steady stream of coolant. Be careful as coolant is hot. If this does not help, disconnect hoses frm heater core when cool and blow out core with compressed air to unclog system. I hope this helps.

hi got an r reg mercedes sprinter van mwb got no heat on the heater just blowing cold?

Hi, the flaps that direct the air for either hot or cold air works with servos situated against the heater box. The grinding noise was the gears on the inside of the servo stripping. After numerous reset processes (switching the car on and off), the servo was reset to its normal state which is in the cold direction. You will have to replace the servo in order for the heater to work again.

hello,good day,seems your recirculating pump need tobe check,ty