RPM occasional drop with AC on - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

Dboe · 08-14-2014, 10:11 PM

Trying to figure out why the RPMs jump between 500 to 1000 with the AC on.

I know there is a TSB, but the fix says to re-flash the computer.
I have an OpenFlash and already flashed mine to a different setup. I figure that would have taken care of it.

Thanks for any help. I would have searched the forum, but "rpm" and "ac" are too short of terms to search for.

nivek111070 · 08-14-2014, 10:26 PM

my ecu was replaced, and my rpm's still drop and it's starting to surge again while in 1st gear at low speeds going through the parking garage

Sigh-on-Rice · 08-14-2014, 10:50 PM

Google search does the job well.

https://www.google.com/search?site=&....0.w-IFRhx-IxE

l0aded · 08-15-2014, 03:37 AM

normal. It is just the AC power kicking on and off.

strat61caster · 08-15-2014, 11:49 AM

Here's why:

The air conditioning needs power to operate, it gets that power by utilizing the engine under your hood. However it doesn't need power all the time so some of the time it's disconnected from the engine, sometimes it's connected and spinning if you ask for it (hit the A/C button, defrost) via a clutch similar to the one in a car's manual transmission. In a low powered car you can feel the loss of power due to the A/C, i.e. an old 4-banger pickup or econo-box (civic/camry), however in modern cars the A/C pump will disengage under heavy acceleration so you get your powah.

In past vehicles and many more mass market consumer oriented vehicles the added load of the A/C is compensated by raising the idle rpm in a way that most people don't even notice a difference when it pops on. Toyota has decided on many vehicles that lowering the idle rpm to the lowest possible level results in better fuel economy and to suffer the consequences of having it too low to effectively be stable when the A/C is kicked on and we get this lovely idle fluctuation when we're sitting at stoplights on warm summer days.

No harm, just a bit annoying and sometimes alarming when it dips too low, several aftermarket tunes/tuners raise the idle rpm to prevent this at the cost of decreased fuel economy.

Dboe · 08-15-2014, 06:03 PM

Great explanation. Thanks

08-14-2014, 10:11 PM	#1
Dboe Member Join Date: Dec 2012 Drives: 2013 Red FRS Location: Santa Clara Posts: 96 Thanks: 0 Thanked 12 Times in 11 Posts Mentioned: 10 Post(s) Tagged: 0 Thread(s)	RPM occasional drop with AC on Trying to figure out why the RPMs jump between 500 to 1000 with the AC on. I know there is a TSB, but the fix says to re-flash the computer. I have an OpenFlash and already flashed mine to a different setup. I figure that would have taken care of it. Thanks for any help. I would have searched the forum, but "rpm" and "ac" are too short of terms to search for.

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08-14-2014, 10:26 PM	#2
nivek111070 Senior Member Join Date: May 2014 Drives: 2014 GB Scoobie BRZ Location: Fort Worth, TX Posts: 115 Thanks: 121 Thanked 37 Times in 29 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	my ecu was replaced, and my rpm's still drop and it's starting to surge again while in 1st gear at low speeds going through the parking garage

08-15-2014, 11:49 AM	#5
strat61caster - Join Date: Nov 2012 Drives: '13 FRS - STX Location: SF Bay Area Posts: 10,383 Thanks: 13,790 Thanked 9,502 Times in 5,013 Posts Mentioned: 94 Post(s) Tagged: 3 Thread(s)	Here's why: The air conditioning needs power to operate, it gets that power by utilizing the engine under your hood. However it doesn't need power all the time so some of the time it's disconnected from the engine, sometimes it's connected and spinning if you ask for it (hit the A/C button, defrost) via a clutch similar to the one in a car's manual transmission. In a low powered car you can feel the loss of power due to the A/C, i.e. an old 4-banger pickup or econo-box (civic/camry), however in modern cars the A/C pump will disengage under heavy acceleration so you get your powah. In past vehicles and many more mass market consumer oriented vehicles the added load of the A/C is compensated by raising the idle rpm in a way that most people don't even notice a difference when it pops on. Toyota has decided on many vehicles that lowering the idle rpm to the lowest possible level results in better fuel economy and to suffer the consequences of having it too low to effectively be stable when the A/C is kicked on and we get this lovely idle fluctuation when we're sitting at stoplights on warm summer days. No harm, just a bit annoying and sometimes alarming when it dips too low, several aftermarket tunes/tuners raise the idle rpm to prevent this at the cost of decreased fuel economy.

08-15-2014, 06:03 PM	#6
Dboe Member Join Date: Dec 2012 Drives: 2013 Red FRS Location: Santa Clara Posts: 96 Thanks: 0 Thanked 12 Times in 11 Posts Mentioned: 10 Post(s) Tagged: 0 Thread(s)	Great explanation. Thanks

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