# A/C goes to defrost when accelerating

**URL:** <https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337>\
**Category:** Maintenance/Repairs\
**Tags:** airconditioning\
**Created:** [July 28, 2008, 2:46pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337 "2008-07-28T14:46:13Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![mrhildey](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@mrhildey](https://community.cartalk.com/u/mrhildey)\
**Post date:** [July 28, 2008, 2:46pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/1 "2008-07-28T14:46:13Z")

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On my 1999 Dodge Ram, when accelerating with the A/C on, the air stops coming out the vent and comes out the defrost. When I lift off the gas, it comes out the vents again. This happens when both A/C or non-A/C vent is on. Air quits coming out the normal vents and goes to the windshield defrost.

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**Author:** ![ken\_green](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/ken_green/32/816_2.png) [@ken\_green](https://community.cartalk.com/u/ken_green)\
**Post date:** [July 28, 2008, 2:59pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/2 "2008-07-28T14:59:05Z")

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Sounds like a vacuum leak within the mode switch.

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**Author:** ![SteveF](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/stevef/32/2_2.png) [@SteveF](https://community.cartalk.com/u/SteveF)\
**Post date:** [July 28, 2008, 2:59pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/3 "2008-07-28T14:59:26Z")

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This is a familiar problem with Rams. The setting is controlled by engine vacuum. Acceleration increases the vacuum and the the airflow setting changes by itself, temporarily.

Dodge has a TSB and a fix for this situation. It consists of installing a valve in the vacuum line somewhere. You’ll have to consult your dealer. Or you can consider this a minor nuisance and just live with it.

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**Author:** ![tardis](https://avatars.discourse-cdn.com/v4/letter/t/f0a364/32.png) [@tardis](https://community.cartalk.com/u/tardis)\
**Post date:** [July 28, 2008, 3:00pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/4 "2008-07-28T15:00:17Z")

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There is a one-way check valve on the vacuum line from your engine to the climate controls. Either that valve has failed, or you have a vacuum leak somewhere in the climate control system. The air doors in your truck are vacuum controlled. Defrost is the default position if all vacuum fails.

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**Author:** ![Ranck](https://avatars.discourse-cdn.com/v4/letter/r/aca169/32.png) [@Ranck](https://community.cartalk.com/u/Ranck)\
**Post date:** [July 28, 2008, 4:33pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/5 "2008-07-28T16:33:07Z")

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As SteveF says, this is a common problem on Dodge Rams of that vintage. I have the same problem with a '99 Ram Van. In researching this on the Internet I found there is an improved check valve/accumulator that costs less than $10. I still haven’t gotten one, but that’s because I don’t use the van very much and it’s not a big deal.

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**Author:** ![Joseph\_E\_Meehan](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/joseph_e_meehan/32/21_2.png) [@Joseph\_E\_Meehan](https://community.cartalk.com/u/Joseph_E_Meehan)\
**Post date:** [July 28, 2008, 8:00pm UTC](https://community.cartalk.com/t/a-c-goes-to-defrost-when-accelerating/17337/6 "2008-07-28T20:00:18Z")

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I remember that one from back in the 1960’s You have a vacuum problem, likely a leak