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RPM needle goes down when AC is turned on?

 
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Old 10-10-2012, 06:22 PM
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RPM needle goes down when AC is turned on?

I tried researching couldn't find anything. So I notice my RPM needle (at idle) goes down rather then going up when I turn on my AC. This is only when it's warming up. I never really paid attention but shouldn't the needle go up when the AC is turned on? When it's warm the needle doesn't move at all when I turn on AC. I just recently deleted cruise control and capped off everything tho I don't think that would make a difference. Thanks.
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Old 10-10-2012, 06:28 PM
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Re: RPM needle goes down when AC is turned on?

mine does the same thing whenever I do anything electrical when the engine is cold i.e. turning on headlights. also happens when I turn the steering wheels slightly. as far as i know, it's normal, but i'm interested to see if anyone has some insight on the subject.
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Old 10-10-2012, 06:32 PM
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Re: RPM needle goes down when AC is turned on?

Yep I have the same issue if it is an issue. But mines worse, my idle is pretty rough so if I turn the corner with everything on it shuts off :/
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Old 10-10-2012, 06:35 PM
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Re: RPM needle goes down when AC is turned on?

A little drop is okay. not alot though.
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Old 10-10-2012, 06:43 PM
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Re: RPM needle goes down when AC is turned on?

Yeah it's a small drop not big. Happy it's normal thought I had another problem on my hands lol. And I'd get that checked out outdorsman the car shuts off or the AC?
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Old 10-10-2012, 07:28 PM
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Re: RPM needle goes down when AC is turned on?

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Originally Posted by 01PreludeBlue View Post
Yeah it's a small drop not big. Happy it's normal thought I had another problem on my hands lol. And I'd get that checked out outdorsman the car shuts off or the AC?
The car itself just stalls while I'm driving and I shift to neutral. People have told me its my main relay/ignition switch/IACV
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Old 10-10-2012, 07:58 PM
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Re: RPM needle goes down when AC is turned on?

What year is your Prelude? I know the 97-98 preludes had a problem with the igntion switch I think 99 too but I can't remember when they fixed it.
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Old 10-10-2012, 08:14 PM
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Re: RPM needle goes down when AC is turned on?

Typically happens when additional load is added. A slight drop is ok but if you're idling rough and/or stalling, you need to address that issue.
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Old 10-10-2012, 08:17 PM
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Re: RPM needle goes down when AC is turned on?

Yeah it was a small drop nothing past 0.5k which is my normal idle point didn't turn off my car or anything.
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Old 10-10-2012, 08:27 PM
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Re: RPM needle goes down when AC is turned on?

98 Prelude SH, I'ma get the ignition switch replaced, its only like 30 bucks if it hasn't been changed already.
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Old 10-10-2012, 08:28 PM
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Re: RPM needle goes down when AC is turned on?

take it to honda, they'll replace it for free
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Old 10-10-2012, 09:15 PM
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Re: RPM needle goes down when AC is turned on?

Yup Honda does it for free.
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Old 10-10-2012, 09:36 PM
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Re: RPM needle goes down when AC is turned on?

It's normal, don't sweat it.
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Old 10-10-2012, 11:57 PM
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Re: RPM needle goes down when AC is turned on?

My Rpms jump when I turn on the ac.
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Old 10-11-2012, 12:36 AM
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Re: RPM needle goes down when AC is turned on?

Your A/C and power steering are driven by belts ran by the engine. it is normal for the RPM(engine speed) to change- Drop, and or raise when you turn on the A/C compressor, or turn the wheel (which runs the pump for power steering), these 2 things are drawing power from the engine in order to run. when not used they draw less power, but when they are used they draw more power, causing your engine speed to drop or raise slightly.

The drop means that it is drawing power from the engine, which takes away from the engines speed when engine is at a cooler or cold temp (My opinionated answer)

The raise is when the engine speeds up to try to replace the power being drawn by the A/C compressor, or power steering, which happens more often when the engine is warm, or at operating temp (middle of your temp gauge), but also happens when the engine is cold. (also my opinionated answer.)

hope this helps

Last edited by ProjectPrelude95; 10-11-2012 at 10:16 PM.
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Old 10-11-2012, 12:40 AM
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Re: RPM needle goes down when AC is turned on?

Just take the A/C out. Problem solved
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Old 10-11-2012, 05:35 AM
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Re: RPM needle goes down when AC is turned on?

same here. idle drops when a/c is on :[ almost hits 0 rpm
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Old 10-11-2012, 08:58 AM
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Re: RPM needle goes down when AC is turned on?

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Originally Posted by ProjectPrelude95 View Post
Your A/C and cruise control are driven by belts ran by the engine. it is normal for the RPM(engine speed) to change- Drop, and or raise when you turn on the A/C compressor, or turn the wheel (which runs the pump for cruise control), these 2 things are drawing power from the engine in order to run. when not used they draw less power, but when they are used they draw more power, causing your engine speed to drop or raise slightly.

The drop means that it is drawing power from the engine, which takes away from the engines speed when engine is at a cooler or cold temp (My opinionated answer)

The raise is when the engine speeds up to try to replace the power being drawn by the A/C compressor, or cruise control, which happens more often when the engine is warm, or at operating temp (middle of your temp gauge), but also happens when the engine is cold. (also my opinionated answer.)

hope this helps
for someone who has common sense.. no offense to anybody here. but your a/c is run by a belt so why wouldnt your rpm differ a little bit. the only thing i didnt know about was the steering wheel
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Old 10-11-2012, 10:33 AM
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Re: RPM needle goes down when AC is turned on?

All of my cars pretty much do the same thing. THis is normal and no need to worry.
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Old 10-11-2012, 07:10 PM
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Re: RPM needle goes down when AC is turned on?

the honda service manual states for certain items diagnosing your rpm is a bit higher without a load.
example:
1000 rpm when cold. 700 rpm when warm. 675 rpm headlights on. etc
every vehicle i've ever owned the rpm drops when turning on the a/c while motor is cool.
like dude said don't sweat it..his avatar scares my kids lol.
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Old 10-11-2012, 09:13 PM
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Re: RPM needle goes down when AC is turned on?

Thanks guys info has made me smarter :) And lol! taking out AC in this weather is a stupid idea.
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Old 10-11-2012, 10:11 PM
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Re: RPM needle goes down when AC is turned on?

Pretty sure Crusie control is not driven off of a belt......
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Old 10-11-2012, 10:15 PM
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Re: RPM needle goes down when AC is turned on?

lmao, i didnt mean cruise control, i meant power steering ill edit that
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