Front defroster problem

[Fred Ritchie]

'04 SR-5 front defroster works fine when on defrost but when set to floor/def. windshield fogs immediately and severely, have to switch to full def. to see enough to drive. This is first time I've seen this but haven't had truck through cold weather before. Not really cold today (mid fifties with light rain, very humid). When I select def/floor split I still feel air from upper vents in dash (stronger on pass. side than on driver), floor air is fine but windshield begins to fog from sides until fully obscured.
Seems like some air direction mechanism isn't working correctly to close dash vents and direct air to def. vents. Temp. setting nor fan setting prevent fogging.
Any advice where to start?

 

[4by4th]

Maybe blend doors aren't working. I had similar issues, heater blowing through dash vents. Dealer fixed it after local mechanic worked on the AC and denied anything was wrong afterward after numerous trips back.

 

[Fred Ritchie]

Thanks for response. Can you recall if this was electrical (solenoid?) or physical problem with door/flapper directly?

 

[commuter64]

Fred,

I'm going through a similar issue with my 04 SR5 V6. Assume you DO NOT have a Limited model, just SR5 as the controls vary... so, there are two servo motors on the right hand side of the heater box, behind the glove box assy. They are both black. The upper servo unit operates all of the "Mode" settings. The lower servo operates by changing the temperature control knob from max hot to max cold. If you can remove the glove box assy, you can see both operate while you move the settings for Mode and Heat. I suspect that the upper servo may not be moving the doors properly.

My issue is no, or limited heat on the right side of cabin. All servo's appear to be moving as they should. Heater core flush is next for me.

 

[07V8]

Make sure your AC is on. Yes, even in the winter.

 

[4by4th]

Thanks for response. Can you recall if this was electrical (solenoid?) or physical problem with door/flapper directly?

Found the invoice. They replaced a servo motor. Parts and 3.5 hours labor $623. I recall they cautioned me that if a blend door was broken it would be expensive to fix. Didn't ask how expensive. Couldn't stand the hot air blowing from the vents, really dried our eyes out.

 

[4by4th]

Make sure your AC is on. Yes, even in the winter.

Why?

 

[Fred Ritchie]

Thanks all.
No its not a Limited, just SR-5
Since turning on full defrost works like a charm but the split doesn't block airflow thru dash vents it sounds like a door to me, especially since I can feel greater drop in flow on driver side vent than pass side which seems hardly reduced.
Will see if I can check servos. Does one servo operate all doors and should there be any airflow thru dash vents when on split setting?


Oh yeah, I agree, why must AC be on?

 

[07V8]

Why?

Because the windows fog without it on in cold weather.

 

[commuter64]

Thanks all.
No its not a Limited, just SR-5
Since turning on full defrost works like a charm but the split doesn't block airflow thru dash vents it sounds like a door to me, especially since I can feel greater drop in flow on driver side vent than pass side which seems hardly reduced.
Will see if I can check servos. Does one servo operate all doors and should there be any airflow thru dash vents when on split setting?


Oh yeah, I agree, why must AC be on?

Fred,

The upper black servo motor is the one that is activated by the Mode control. As you select each mode, the servo makes adjustments accordingly to which position you selected on the Mode button. If you can see the servo, and run the truck, select each mode position and see what the servo is doing. That may provide clues as to what is or isn't happening.

On the SR5, there is only one air mix control servo motor that controls the heat temperature range from cool to hot. It is the lower one. This unit moves a white pivot arm that has a movement of about 90 degrees while adjusting heat from cold to hot and vice versa. Again, you can see it move while adjusting temp from max cold to max hot. This also has a built in sensor function that can be measured with an Ohm meter.

There should be no airflow through dash vents with split setting. Not sure, but seems to me the upper servo unit is either not responding to the input signals to move the correct door(s), or it it an internal door issue. Perhaps the cheapest fix it to close the vent doors on the dash, this may reduce some of your flow issue.

Tom

 

[Fred Ritchie]

Thanks again, I have tried closing vents to increase airflow with marginal improvement but still too much fogging to drive. If its the doors, are they located inside the blower unit itself or within the duct(s)?

Sorry, still don't really get why the AC must be on when I'm calling for heat.

 

[07V8]

Thanks again, I have tried closing vents to increase airflow with marginal improvement but still too much fogging to drive. If its the doors, are they located inside the blower unit itself or within the duct(s)?

Sorry, still don't really get why the AC must be on when I'm calling for heat.

Have you tried it yet? I leave my AC button illuminated all year - even in -20F weather. The windows never fog. You'll also notice that if you use the Auto functionality - its on in all temperatures as well.

 

[commuter64]

Thanks again, I have tried closing vents to increase airflow with marginal improvement but still too much fogging to drive. If its the doors, are they located inside the blower unit itself or within the duct(s)?

Sorry, still don't really get why the AC must be on when I'm calling for heat.

Doors are inside the box behind the HVAC control panel. The blower fan assy is behind the glove box. On top of the blower box is a door that opens and closes if you activate the air recirculation button.

 

[Fred Ritchie]

OK, I dropped glove box, don't really see servos, nothing black, but looking to the left lower corner behind the white blower housing(?) I can see white plastic lever which moves according to input from dashboard controls. But when I set to split def/floor there is virtually no reduction in air coming from pass side vent and very little defrost air. Remember, full defrost works like magic, fastest I've ver encountered. Looks like a deflector door? flapper? isn't working.
Correct me if I'm wrong but it looks like removing entire unit and replacing is only remedy. I am assuming deflector is located inside unit housing? Essentially dash has to be dismantled and unit removed, no?

 

[4by4th]

I don't know if this helps or not, but when I was talking to the service manager before having mine fixed, I asked if it would be any cheaper to wait until the cracked dash is being replaced and he said no because they wouldn't have to remove that to get to the blend doors or servo motor.

 

[Fred Ritchie]

I'm beginning to wonder if problem might involve non-use of defroster, I haven't had to use it since buying truck a year ago +\-. Could it be that a door was sticking and is loosening with use? Can still feel some airflow thru vents but drove 70mi +\- this morning, mid 50's & light rain, with passenger and no fogging even without using defrost. I'll have to wait for the right levels of humidity to see if something has improved, around here I'm sure that won't be long. Prices for repairs are horrifying from what I'm seeing.

 

[COn4mer]

Thanks again, I have tried closing vents to increase airflow with marginal improvement but still too much fogging to drive. If its the doors, are they located inside the blower unit itself or within the duct(s)?

Sorry, still don't really get why the AC must be on when I'm calling for heat.

07V8 is correct. Don't just associate the AC system with cold air. The AC will suck moisture from the air and clear up a fogged window faster. There is a reason why there is a puddle of water under your car when you leave it parked for a while after having your AC running.

 

[Fred Ritchie]

I think I understand purpose of evaporator/dryer in AC systems its just strange that no other vehicle I own or have ever owned required AC to be on for heat/def to work. Granted, I now live in area of serious humidity, even in the colder months, not like the dry winter air up north but since moving here no other vehicle I have owned, from '69 Chevy C-10 w/no AC to BMW 325 w/climate control, has presented this problem. Just shift lever to floor/def split and enjoy warm tootsies as well as still be able to see the road. Drove vehicle the other day with two people in it on chilly rainy day, like the one where the problem showed up with just me aboard, and no fogging occurred. I'll certainly try AC ON remedy if fog shows up again but for now I just right it off to what I find as the somewhat erratic design and behavior of these vehicles.
If you've actually read all this, I do have a question. What is actual purpose of AUTO setting on control panel?

 

[07V8]

I think I understand purpose of evaporator/dryer in AC systems its just strange that no other vehicle I own or have ever owned required AC to be on for heat/def to work. Granted, I now live in area of serious humidity, even in the colder months, not like the dry winter air up north but since moving here no other vehicle I have owned, from '69 Chevy C-10 w/no AC to BMW 325 w/climate control, has presented this problem. Just shift lever to floor/def split and enjoy warm tootsies as well as still be able to see the road. Drove vehicle the other day with two people in it on chilly rainy day, like the one where the problem showed up with just me aboard, and no fogging occurred. I'll certainly try AC ON remedy if fog shows up again but for now I just right it off to what I find as the somewhat erratic design and behavior of these vehicles.
If you've actually read all this, I do have a question. What is actual purpose of AUTO setting on control panel?

Set a temperature and forget about it - just like your house thermostat. It will automatically switch from full defrost to the bottom vents in the winter based on ambient temperature and humidity.

I never take mine out of auto. My windows never fog and the AC light is on 365 days a year.

 

[ExtraCaucasion]

I have been scouring the forums with a similar issue I have with my recently purchased 07 Sport. When I switch from the front vents to defrost, the front vent air slows but there is no air movement above the dash. Same if I switch to floorboard/defrost, no air above the dash but floorboard and front vents still blow. From what I can gather, it is either my upper servo or possibly a stuck vent? Im leaning towards the servo but any input would be appreciated.

Thanks in advance!