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1994 Ford F150:XXXXXwith A/C...heater core

Customer Question

1994 Ford F150XXXXXwith A/C. Still having a problem with heat after heater core install. Can you tell me somethings to look for. Still blowing out cold. Also, the heater core goes into a box with a (blend) door right on the side of it. Is that door suppose to be open or closed while heat is on?
Submitted: 4 years ago.
Category: Ford
Expert:  Mat replied 4 years ago.
Hi I would like to help you out, First check the inlet and outlet heater core hoses to verify they are hot. If they are then the cooling system is operating normally and the heater core is ok. The blend door should be open so when the air from the blower motor blows through the heater core the hot air continues out the dash. If you can see the door check to make sure it moves with the temp control knob. These are cable controlled doors, sounds like you have a broken blend door, or an issue with the cable or knob, sometimes the knob breaks on the inside and doesn't spins on the shaft so you think you are turning it to heat but only the knob move, not the cable or shaft. The know should pull straight off so you can check it for cracks or breaks around the center shaft area. If you need further assistance please ask. thanks you.
Customer: replied 4 years ago.

I am not home yet but playing devil's advocate what could be the problem if inlet and outlet are not hot? I know it can't be heater core that is brand new. Could it be a thermostat even though the truck isn't overheating?

Expert:  Mat replied 4 years ago.
Could be thermostat if the themostat failed and is stuck open. If stuck open the truck will never get up to tempature and the inlet and outlet hoses at the heater cooler will feel cool or warm only. A thermostat will only cause an overheating concern if it fails by sticking shut, this will not allow it to open and fail to send coolant flow to the radiator. So if your hoses are warm your thermostat may be stuck open.
Customer: replied 4 years ago.
ok changed the thermostat and it was definitely stuck open. Now I have a hot top radiator and inlet and outlet hoses are hot. but the air is only blowing out the bottom of the heater box which isnt even a place its suppose to blow out. I checked the the cold and hot temperatur dial and the door does open and close by the heater box.
Expert:  Mat replied 4 years ago.
ok great so it looks like the heat is fixed. The mode door controls the airflow, whether it be floor, defrost, vent. since the heater core was replaced check all vacuum lines on the heaterbox as one may have been easily disconnected and see if the mode door moves while you are turning the knob. Let me know your results.
Customer: replied 4 years ago.
where is the mode door located? That isn't the same door that opens and closed during temperature control knob?
Expert:  Mat replied 4 years ago.

No different door. this model actually has two a panel/floor door, and a floor defrost door, they are located after the blend door. They are vacuum operated so make sure no vacuum line are undone.

Here is a picture of the vacuum actuator and you can see parts 3 ad 4 are connected to the door that is onside the box, you most likely wont see the door itself, so locate the vacuum actuators and the doors are in that area of the box.


Customer: replied 4 years ago.
Matt, I haven't forgotten about you. I just don't have that vehicle in front of me right now. It was the person's only vehicle so they are driving it right now.
Expert:  Mat replied 4 years ago.
No problem, thanks for the update, let me know what you find. thanks
Customer: replied 4 years ago.

Matt, I have vehicle again now. Do you have a diaghram of the complete system with all the blend doors? I am getting heat only right underneath the heater core. I do know that the temperature (hot /cold door is working properly when I rotate the dial. there is another switch right under the control unit that I can manually move but it still doesn't give me heat through the vents.


Expert:  Mat replied 4 years ago.

Hi here is a picture that shows the switches and all vacuum lines, and colors going to each door.

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Experience:  11 yrs at FLM Dealer, Senior Master Tech 05-present, ASE Master tech, Auto trans & Diesel
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