Car stutters at idle when power locks are operated - Subaru Outback - Subaru Outback Forums
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post #1 of 8 (permalink) Old 02-09-2015, 05:09 PM Thread Starter
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Question Car stutters at idle when power locks are operated

The Outback is my wife's car but 100% of the time when I drive up to the garage and unlock the doors with the power door switch on the driver's side, the RPMs drop a little and the car shakes a tiny bit. Trying to figure out what could be causing this - anyone else experience this?

When I just sit there at idle in a warmed up car and hit the lock/unlock switch I can feel the idle change. Lights/radio/AC don't seem to affect it - all of which I would think draw more than the locks.

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post #2 of 8 (permalink) Old 02-10-2015, 11:10 AM
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I think the locks momentarily draw enough current to make the idle change. The same thing happens with the power windows when they hit the end of their travel. Perhaps electronic items like the radio, lights, AC, etc don't have a spike in power demand.

To answer your question, I do experience this. I think it's normal.

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post #3 of 8 (permalink) Old 02-10-2015, 12:22 PM
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Quote:
Originally Posted by rockthebeef View Post
I think the locks momentarily draw enough current to make the idle change. The same thing happens with the power windows when they hit the end of their travel. Perhaps electronic items like the radio, lights, AC, etc don't have a spike in power demand.

To answer your question, I do experience this. I think it's normal.
Mine does this with the windows and locks.
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post #4 of 8 (permalink) Old 02-10-2015, 12:31 PM
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[quote=rockthebeef;2520970 To answer your question, I do experience this. I think it's normal.[/quote]

Same here. The Outback does seem to be more sensitive to additional loads while idling than other vehicles I've driven. I'm assuming all of this has to do with the engine idling at the bare minimum RPMs. Any additional load on the alternator will force the engine to rev. This happens with any electrical system in my car like locks, seat heaters, HVAC, and headlights.

The increase in idle RPMs will even force the vehicle to lurch forward just a little bit if I am idling at a red light (at a red light for instance) and holding the brakes lightly. This forces me to press a harder on the pedal for a moment.

I experience the same thing on my backup generator @ home when I plug in anything over 200 Watts.

Last edited by soobienewbie; 02-10-2015 at 12:32 PM. Reason: fixed quote selection
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post #5 of 8 (permalink) Old 02-10-2015, 01:29 PM
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Well, here's a surprise. I was curious about the instantaneous peak current draw of all those door lock motors firing in unison, and it was a lot more than I would have thought. Per an article in Motor Magazine:

Fig. 11 shows the current draw when all five power door locks are commanded. Again, the nearly vertical rise (or in this case, fall) indicates a fast, high-powered motor moving from a standstill. This capture was made with a high-current probe set to 1mV/amp. The initial surge was 75.2 amps, or an average of about 15 amps per lock motor, although it tapered off quickly from there. This waveform is from a known-good vehicle.

There is also a strong inductive kick in voltage as the motors quit, and the fields collapse. It's the kind of jolt that could blow sensitive electronics if you removed the battery from a running car.

So yes, the sudden huge current uptick is like suddenly applying the brakes to the moving rotor in your alternator. You will certainly feel it.

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post #6 of 8 (permalink) Old 02-23-2015, 06:10 PM Thread Starter
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Thanks for the replies everyone - just wanted to make sure I didn't need to adjust my idle or check for something affecting it.
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post #7 of 8 (permalink) Old 02-23-2015, 07:04 PM
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My 2010 does this. my 2001 did it and my toyota does it.

As pointed out the power draw shocks the system and the alternator adds load to the engine and you get a minor stumble. Never an issue unless the stumble turns into a stall where you need to turn the key and start the car back up.
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post #8 of 8 (permalink) Old 02-23-2015, 07:48 PM
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Unfortunately the last car I owned with an adjustable idle screw on an idle air control pot went to it's grave over 10 years ago. Everything is computer controlled and non-adjustable.

75 amps likely represents more than the total capacity of your alternator at idle. It's a huge retarding load on the engine.

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