Heater only blows on defrost

2003 FORD F-150
230,000 MILES • V8 • 4WD • AUTOMATIC
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F150FXRGUY
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Electronic climate control unit. Heater is currently only blowing on defrost. A/C works good on vents, floor and defrost. Heater will not blow on floor. I replaced the heater unit once and it worked ok for about 6 months, but then defaulted to the same defroster setting only. Is this a vacuum leak or did the ECU fail again? I now have 2 suspect failed heater units... are these repairable? If it's a vacuum leak, where should I start looking?
Jun 20, 2010 at 11:58 PM
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KASEKENNY
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Yes, this could be a vacuum leak. I would suggest going to the panel/floor actuator and pull the line off and put a vacuum gauge on it and then turn the system on. If there is no vacuum on the gauge then you most likely have a vacuum leak.

Here is a guide that will help explain this as well:

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

I attached the process for your review. Let us know what you find. Thanks
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Apr 19, 2021 at 3:01 PM
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SKEPTICHAN
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Maybe this will help with some further assistance because I cannot find this leak. I feel stupid because I didn’t realize that the vacuum actually controlled that vent. So, still stuck on what is actually leaking.
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Apr 7, 2022 at 10:56 AM
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KASEKENNY
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Thanks for the video.

The best way to check vacuum operated systems like this is with a handheld vacuum pump.

Basically, you want to work your way through the system and disconnect different lines through the system and apply vacuum to it and either plug the other end so that it holds vacuum. Or leave it hooked up and see if you are able to move the actuator.

If you cannot hold a vacuum or move the door, then you have a leak. At this point you need to shrink the connections even more until you find the portion that has the leak.

Some people blow through the line with one end plugged but that is not as definitive. Basically, if you have resistance when blowing then there are no leaks.
Apr 7, 2022 at 1:08 PM
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