Heat problems (please explain what the Blend Door does and how)
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Heat problems (please explain what the Blend Door does and how)
Well I having some problems with the heat in my 98 F150 XLT 4x4 Off-Road.
Monday was very cold and I have been looking at new truck and I don't think my 98 liked sitting at the dealer while I drove a new one for the weekend. So as I left the dealer Monday night the heat just never seamed to kick in. Truck came up to temp but no heat. After driving the past few day very little to no heat I think I figured out the problem. And when I call the mechanic bitching tomorrow I'm hoping I will sound like I know what I'm talking about. So please help.
Back in August I had my heater core go. Hired a shop to do the work, But ever since then I have a squeak in the dash. I though I got a good price so I did not worry about it. Now with winter kicking in and us having our first real cold snap I need heat. By the way Saturday was like mid 50's, this was the last day I drove the truck till Monday night when it was -0 with windchill. My temperature gauge was set on the cool side at about the 11 o'clock mark.
After being cold for the last few days I worked on the truck tonight. I had two problem. I was not getting heat or volume of air through the vents.
This is what brings me to ask what the Blend Door does and how.
The blower works fine. I took the blower resistor out and let it hang, then turned on the blower. I have lots of air moving out that hole but not much out the vents. Started playing with the temp settings and I could here the blend door move with an slamming sound every now and then. As I put my head down in front of the dash by the cup holders I felt air moving. I pull the cup holders out and the air was coming out by the cup holders better than the vents. Now this is the area where the blend door is. This area also has a few pieces of duck work join together here, I found two pieces not aligned correctly making a opening.
I think my Blend door is getting hung up inside the ducting. After playing with the temp. I have heat and air volume again but if I mess with the temp it will be a pain to get back. Took me and hour to get the heat to the right setting. Luckly I have access to a garage with a vent system.
So I know the blend door opens an closes to give you hot or cold air but does this affect the direction and amount of air flow too? When the heat was not working there was low volume of air movement out the vents, like I said above. But, when I turned the Temp **** to cold air, the air came out the vents like normal. Also no matter what the temp setting air came out the miss aligned ducks about the same. I also thinking my dash squeak is the vent ducks that are not aligned properly. The squeak comes for the same area.
Monday was very cold and I have been looking at new truck and I don't think my 98 liked sitting at the dealer while I drove a new one for the weekend. So as I left the dealer Monday night the heat just never seamed to kick in. Truck came up to temp but no heat. After driving the past few day very little to no heat I think I figured out the problem. And when I call the mechanic bitching tomorrow I'm hoping I will sound like I know what I'm talking about. So please help.
Back in August I had my heater core go. Hired a shop to do the work, But ever since then I have a squeak in the dash. I though I got a good price so I did not worry about it. Now with winter kicking in and us having our first real cold snap I need heat. By the way Saturday was like mid 50's, this was the last day I drove the truck till Monday night when it was -0 with windchill. My temperature gauge was set on the cool side at about the 11 o'clock mark.
After being cold for the last few days I worked on the truck tonight. I had two problem. I was not getting heat or volume of air through the vents.
This is what brings me to ask what the Blend Door does and how.
The blower works fine. I took the blower resistor out and let it hang, then turned on the blower. I have lots of air moving out that hole but not much out the vents. Started playing with the temp settings and I could here the blend door move with an slamming sound every now and then. As I put my head down in front of the dash by the cup holders I felt air moving. I pull the cup holders out and the air was coming out by the cup holders better than the vents. Now this is the area where the blend door is. This area also has a few pieces of duck work join together here, I found two pieces not aligned correctly making a opening.
I think my Blend door is getting hung up inside the ducting. After playing with the temp. I have heat and air volume again but if I mess with the temp it will be a pain to get back. Took me and hour to get the heat to the right setting. Luckly I have access to a garage with a vent system.
So I know the blend door opens an closes to give you hot or cold air but does this affect the direction and amount of air flow too? When the heat was not working there was low volume of air movement out the vents, like I said above. But, when I turned the Temp **** to cold air, the air came out the vents like normal. Also no matter what the temp setting air came out the miss aligned ducks about the same. I also thinking my dash squeak is the vent ducks that are not aligned properly. The squeak comes for the same area.
Technical Article Contributor
Joined: Dec 1997
Posts: 9,417
Likes: 11
From: Windsor,Ontario,Canada
The blend door changes the path of the cold air. When set to COLD air is routed to bypass the heater core. When set to HOT air flows through the heater core. The heater core is a restriction to the air flow so it slows down a bit. The only slamming you might (should) hear is when you change from where the air is directed like Vent to floor or defrost. They are controlled by vacuum and can sometimes slam. BTW I have changes the heater core 8 times and my dash does not squeak. The trick is to reuse every screw/bolt. 
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Jean.
The Blend Door on many "modern" units (Post '97) tend to be electronic modules. I've seen Blend Door Modules on ('97-"11) F-150, Expedition trucks, and particularly on Navigator models. I believe that BD modules are also on Explorer units.
I don't want to get into a detailed explanation of exactly how they function, but I had posted some pix of some recent service work on an Expedition system; which I had posted in my Photo's.
I would consider this type of system essentially a "Comparator" based deflection unit. The dash mounted "Temp" control is basically a rheostat or variable voltage divider which establishes a reference voltage from 0 to 12VDC which is supplied to the module which tracks its position against the reference voltage from the Temp Control. It then opens or closes the Blend Door according to the reference voltage.(VDC/angle def.)
For example the typical TEMP control supplies 0.0 VDC to the module or controller at the full "COLD" setting. The module determines that this voltage requires the Blend Door to be in fully closed. When the TEMP control is set to "HOT" the rheostat puts out the full voltage usually +5,+12VDC which is determined to open the Blend door. This results is maximum air-flow across the core; which provides heated air the truck.
There are modules made specifically for the "Manual-Electronic" HVAC module, and a similar unit made for Computer HVAC units. They are similar in function, but use different reference voltages.
I have not seen cable type units in models later than 1996, but there might be some industrial (commercial) units on Police, e.t.c models.
I have some diagnostic tests that you might want to apply to service these units. Thanx.
The Blend Door on many "modern" units (Post '97) tend to be electronic modules. I've seen Blend Door Modules on ('97-"11) F-150, Expedition trucks, and particularly on Navigator models. I believe that BD modules are also on Explorer units.
I don't want to get into a detailed explanation of exactly how they function, but I had posted some pix of some recent service work on an Expedition system; which I had posted in my Photo's.
I would consider this type of system essentially a "Comparator" based deflection unit. The dash mounted "Temp" control is basically a rheostat or variable voltage divider which establishes a reference voltage from 0 to 12VDC which is supplied to the module which tracks its position against the reference voltage from the Temp Control. It then opens or closes the Blend Door according to the reference voltage.(VDC/angle def.)
For example the typical TEMP control supplies 0.0 VDC to the module or controller at the full "COLD" setting. The module determines that this voltage requires the Blend Door to be in fully closed. When the TEMP control is set to "HOT" the rheostat puts out the full voltage usually +5,+12VDC which is determined to open the Blend door. This results is maximum air-flow across the core; which provides heated air the truck.
There are modules made specifically for the "Manual-Electronic" HVAC module, and a similar unit made for Computer HVAC units. They are similar in function, but use different reference voltages.
I have not seen cable type units in models later than 1996, but there might be some industrial (commercial) units on Police, e.t.c models.
I have some diagnostic tests that you might want to apply to service these units. Thanx.
Technical Article Contributor
Joined: Dec 1997
Posts: 9,417
Likes: 11
From: Windsor,Ontario,Canada
98,
I agree with you posted. I was just explaining how a decrease of air flow could occur. I have changed the actuator in my truck before. It is definitely not a cable.
I agree with you posted. I was just explaining how a decrease of air flow could occur. I have changed the actuator in my truck before. It is definitely not a cable.