# 99 Expedition. A/C comes on when Heat is turned on? **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [April 26, 2008, 4:33am UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285 "2008-04-26T04:33:08Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Robert-Gift](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/robert-gift/32/356_2.png) [@Robert-Gift](https://community.cartalk.com/u/Robert-Gift)\ **Post date:** [April 26, 2008, 4:33am UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/1 "2008-04-26T04:33:08Z")
No Owner’s Manual. 10:00 O’Clock = A/C 11:00 = Panel (dash vents) 12:00 = OFF 1:00 = Panel & Floor (dash and floor vents) 2:00 = Floor At 1:00 and 2:00 did A/C compressor come on?! Sounded like A/C compressor engaged and RPMs dropped from the load. At 11:O0 (Panel) RPMs does not drop and no sound as though A/C came on. Any ideas? Thank you ---
**Author:** ![admitted\_amateur](https://avatars.discourse-cdn.com/v4/letter/a/94ad74/32.png) [@admitted\_amateur](https://community.cartalk.com/u/admitted_amateur)\ **Post date:** [April 26, 2008, 4:39am UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/2 "2008-04-26T04:39:25Z")
I have no idea why, but in many vehicles there is a nice little A/C button so that A/C on-off is independent of the selector for the vents. (Only exception is defrost which usually activates the compressor). But in others A/C is one of the positions on the same dial as the vent selection (as in yours). And…it seems that some of those positions also activate the A/C. It makes no sense to me, but I have this [innovation?] in my car as well. The compressor runs when the dial is at Max A/C, A/C, Panel/Floor or Defrost. Panel only or floor only don’t activate the A/C. So I think your compressor is kicking on as you suspect and I think it is by [bizarre] design. ---
**Author:** ![Robert-Gift](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/robert-gift/32/356_2.png) [@Robert-Gift](https://community.cartalk.com/u/Robert-Gift)\ **Post date:** [April 26, 2008, 11:28am UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/3 "2008-04-26T11:28:11Z")
Thanks. What dopes to have the compressor running and lowering gas mileage when heating. In Colorado’s low humidity, especially low in Winter, we don’t need the air dehumidied by the air conditioner. Yes, for window defrost it makes sense. But for heating it should be optional. If I disconnect the compressor wire, will that cause a Check Engine Light or other problem? Thank you. ---
**Author:** ![Craig58](https://avatars.discourse-cdn.com/v4/letter/c/3da27b/32.png) [@Craig58](https://community.cartalk.com/u/Craig58)\ **Post date:** [April 26, 2008, 1:08pm UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/4 "2008-04-26T13:08:22Z")
You really need to find a manual for your car. Having the AC run in all modes isn’t unusual. For example, my AC runs in all modes except “economy” to dehumidify the air and keep the windows from fogging. You need to figure out if your car has the equivalent of “economy” mode. ---
**Author:** ![Scudder](https://avatars.discourse-cdn.com/v4/letter/s/b9bd4f/32.png) [@Scudder](https://community.cartalk.com/u/Scudder)\ **Post date:** [April 26, 2008, 1:36pm UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/5 "2008-04-26T13:36:21Z")
This might help - [http://www.analogstereo.com/pdf/om/expedition\_owners\_manual\_1999.pdf](http://www.analogstereo.com/pdf/om/expedition_owners_manual_1999.pdf) ---
**Author:** ![Beadsandbeads](https://avatars.discourse-cdn.com/v4/letter/b/c57346/32.png) [@Beadsandbeads](https://community.cartalk.com/u/Beadsandbeads)\ **Post date:** [April 27, 2008, 4:29am UTC](https://community.cartalk.com/t/99-expedition-a-c-comes-on-when-heat-is-turned-on/13285/6 "2008-04-27T04:29:16Z")
A good place for FOMOCo owner manuals is [https://www.fleet.ford.com/maintenance/owners\_manuals/default.asp](https://www.fleet.ford.com/maintenance/owners_manuals/default.asp) I would not mess with the compressor controls. In cold weather the compressor will not run anyway. The pressure switch will prevent it from running when the evaporator is cold.