At the end of last winter the heater in my 2002 F150 (5.4triton) stopped getting hot. I did sone reading and figured out the problem was almost certainly a clogged heater core. However, as it was pretty much the end of heater season I didn’t deal with it until yesterday. I pulled some hoses and flushed it out. First I tried to flush in the same direction as typical flow and a little got out but then flow just stopped. None. Zero. Zilch. So I reversed the flow and ended up with several gallons worth of coffee looking water with tons of solids before the water started running clean. So I’m pretty sure that was the problem. Fired it back up and sure enough I had lots of heat coming out of the front dash vents so I called it a day.

This morning I climb in to head to work and much to my surprise I’m getting no heat at all through the defroster vents. It’s cold. Ambiebt temp, it seems. Air from the floor vents seems slightly warmer than ambiebt, but not much. Air from the vents is hot.

So I try all the settings. Reference this image:


In the picture you see how off is straight up...everything left of off works fine. AC cranks along and using the “Panel” setting I can get cool or hot or anywhere in between. For the settings right of off I only get ambiebt temp. Air comes from the right location, it’s just not hot.

So, we know the AC is fine.
The heater core is fine.
The blend door is fine.

What’s the problem? Why isn’t hot aiir getting to my defroster or feet?

It’s gotta be a vacuum problem, right? The blend door is electric but the doors that control flow direction are vac operated?

The truck was just in the shop and got a rebuilt engine put in. Did they miss a vac line somewhere?

I hope they are not still using vacuum motors on a 2002. Although my 2008 express van still uses them.

When you say settings right off off what do you mean? Are you saying at the lower blower motor settings you get no airflow?

You might have the same problem I have. A broken blend door. I haven't fixed mine yet. I have a 1999 F-150

Blend Door fix

You flushed the core, did you flush the whole system. Are you sure you don't have air in the system?

quote:

Originally posted by SgtGold:
When you say settings right off off what do you mean? Are you saying at the lower blower motor settings you get no airflow?

Ok, what I mean by “right of off”... if you look at the selector knob on the right side of the control panel the Off position is straight up. Turning the knob left will get you max AC, ac, or panel. Those work fine. Both AC positions are good and if I select panel I can get cool OR hot air. This tells me a couple things:
A) the heater core is clear of obstructions and air as I get good heat from the panel vents
B) the blend door is not broken as the temperature adjustment knob works fine

Now if you look at the air selector knob again you see 4 positions to the right of the off position: Panel/Floor, Floor, Defrost/Floor, Defrost. If I select any of these positions the air that comes out is ambiebt temperature. So, the air flow shows up at the right vent location but it’s kust not the right temperature.

What I’ve found is this:
1) the blend door is controlled by an electric actuator obviously controlled by the temp selector
2) the doors that control airflow location are controlled by vacuum

So, to recap:
1) the blend door must be intact because it works fine with the ac and panel selections.
2) I must have good vacuum as the airflow shows up to the right locations
3) heater core is fine

The only logical conclusion I can draw is that something is causing the blend door to close when the vacuum control knob is turned right of Off... nothing else makes sense but then that doesn’t make sense either as it is a different control mechanism.

quote:

Originally posted by Shaql:
You flushed the core, did you flush the whole system. Are you sure you don't have air in the system?

Yes, it’s flushed. It’s an entire new engine. All new coolant. The only thing that wasn’t flushed with the new engine was the heater core and I did that myself. No overheating issues with the engine and I get good consistent hot air front the Panel selection so the air seems to be out of the system.

quote:

Originally posted by thumperfbc:

quote:

Originally posted by Shaql:
You flushed the core, did you flush the whole system. Are you sure you don't have air in the system?

Yes, it’s flushed. It’s an entire new engine. All new coolant. The only thing that wasn’t flushed with the new engine was the heater core and I did that myself. No overheating issues with the engine and I get good consistent hot air front the Panel selection so the air seems to be out of the system.

If you can get hot AC then that means the heater core is responding to the controls. If you are getting no hot air from the non AC settings my guess is something is disconnected. It all worked before the engine swap, correct?

I was dicking around with the system on my way to work and I realized that there is a difference in air temp at all times... as such I have a new theory.

My theory is this: the AC system is getting turned on at inappropriate times. Let me explain why I think this.

It seems, according to diagrams online, that ALL air passes through the evap core. The blend door only allows or prevents a pathway through the heater core. What made me realize that this might be significant is the realization that there is some difference in air temp out of the defroster. If I havebit set to full Defrost and full heat the air coming out is around ambiebt temp, maybe a little warmer. If I then change the temp setting from full heat to full cold the air exiting the defrost vents doesnindeed get colder. Further, I can hear a change in airflow as the blend door closes and prevents access to the heater core.

So, it seems the most reasonable explanation is the evap coil is getting cooled when the heater only is supposed to be on. To confirm this I need a way of positively preventing the ac compressor from turning on. Is that possible? Is there a plug I can take out to ensure that the ac remains off? I’m thinking if I get the compressor to not turn on I’ll likely have heat back in the defroster.

Then it’s just a matter of finding out what is causing the AC to turn on at inappropriate times.

quote:

Originally posted by SgtGold:

quote:

Originally posted by thumperfbc:

quote:

Originally posted by Shaql:
You flushed the core, did you flush the whole system. Are you sure you don't have air in the system?

Yes, it’s flushed. It’s an entire new engine. All new coolant. The only thing that wasn’t flushed with the new engine was the heater core and I did that myself. No overheating issues with the engine and I get good consistent hot air front the Panel selection so the air seems to be out of the system.

If you can get hot AC then that means the heater core is responding to the controls. If you are getting no hot air from the non AC settings my guess is something is disconnected. It all worked before the engine swap, correct?

Hot air works from the “Panel” setting. See my post above...

Pull the fuse or relay to the AC. That will keep it from coming on.

quote:

Originally posted by SgtGold:
Pull the fuse or relay to the AC. That will keep it from coming on.

Easy enough. I will do that next.

Pulled the “A/C Compressor Diode” and the “A/C Clutch Relay” from the underhood fuse block, which seems to have succeeded in killing the A/C for now.

Further, as I hypothesized my hot air has returned to all locations. So it seems certain that something is triggering the A/C to come on when it isn’t supposed to, the question is what? My first thought is the selector switch but I don’t know what I don’t know. It seems to be a relatively cheap part that looks easy to replace, so I can start there.

Is there anything else in play that I’m not aware of?

It could be as simple as the relay you pulled being bad. You will need to know which poles are closed to allow the current to pass through, but you can check it with a multi meter.

I thought that the defrost uses the AC to dry the air. I think my Bronco was that way.

quote:

Originally posted by SgtGold:
It could be as simple as the relay you pulled being bad. You will need to know which poles are closed to allow the current to pass through, but you can check it with a multi meter.

Swapped out the relay with another identical one (from the windshield cleaner pump) and the problem remained with the heater and the washer fluid pump continues to work. So I think we can eliminate the relay as a potential cause.

I guess next is the climate control switch?

The A/C will run when in defrost.

If the control switch were faulty, I'd imagine you'd hear a vacuum leak at the switch. I'm thinking you have a door that is not closing fully when in any of the right side positions. Could be a faulty vacuum motor, an obstruction or not enough vacuum due to a compromised vacuum line from the engine to the climate control switch.

While the engine is running and the fan on low, stick your head under the passenger side foot well and listen for a vacuum leak, if none heard there, check under the hood. Check all vacuum lines.

quote:

Originally posted by Shifferbrains:
The A/C will run when in defrost.

If the control switch were faulty, I'd imagine you'd hear a vacuum leak at the switch. I'm thinking you have a door that is not closing fully when in any of the right side positions. Could be a faulty vacuum motor, an obstruction or not enough vacuum due to a compromised vacuum line from the engine to the climate control switch.

While the engine is running and the fan on low, stick your head under the passenger side foot well and listen for a vacuum leak, if none heard there, check under the hood. Check all vacuum lines.

Are you fairly familiar with the switch? Is there one vac line leading into the switch and then multiple lines out from the switch going to the various door motors? Could I, in theory, attach a vac gauge to the line leading to the switch from the e give bay to confirm good lines downstream (or upstream, whatever term is correct)?

The only door that makes sense (since it’s a temperature problem, not a flow problem) would be the blend door over the heated core, right? It appears to be working though, as I can adjust the temp successfully in other modes, for instance the Panel mode. The blend door is an electric motor, not a vac motor, I believe.

You would need to put a gauge on the outlet side of the switch and test each circut. Vacuume coming into the switch only verifies that the switch has no leak from it's sourse.