A/C not working when engine is under load?

Tiny
BOYD H
  • MEMBER
  • 1997 FORD AEROSTAR
  • 4.0L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 160,000 MILES
A/C works fine until you want to pass or go up a mountain. In that case the A/C switches to defrost.
Thursday, May 25th, 2023 AT 1:43 PM

1 Reply

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

The airflow direction is controlled by a mode door. The mode door functions from the engine vacuum. When you place a load on the engine, the vacuum in the intake drops. If there is a leak in one of the HVAC vacuum lines or the supply, what you described can happen.

I attached a vacuum schematic below showing you the hoses. There will be a white vacuum hose that supplies the engine vacuum to the vacuum tank under the dash. Check that first for leaks. Also, the other hoses are shown by color.

With the engine running, see if you can hear any hissing sounds from under the dash. Switch between defrost and vent to see if there are sounds.

The idea that they work until the engine is under a load, I suspect there is a leak between the engine intake and the supply hose I mentioned above. I would start with that area first. These are plastic hoses that get hard and brittle over time. It isn't uncommon for them to develop leaks or even crack and break.

Let me know what you find.

Joe

See pic below.
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Thursday, May 25th, 2023 AT 9:24 PM

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