Idle Speed Lowers When Fans Kick In

[elemgee]

I've noticed that when stopped at a light, when the fans come on (happening more frequently with the weather getting warmer), the idle speed will slow from its normal ~650 rpms, to maybe 600 or so, then back up to 650. It does this every couple of seconds as long as the fans are running. Other than that, the car runs fine, no CEL, no temperature problems.

Its more annoying than anything else but I'd like to fix it. Any idea what it could be? Thanks.

Larry

[1986nate]

It's the added electrical draw causing more draw on the alternator, causing more draw on the belt, slowing the engine down slightly.

[SVX92-97]

Time for a Husky Maniac high output 160A Alternator

[elemgee]

So then this is normal? As SVX 92-97 suggests, would a higher output alternator solve the problem?

[gwynethh]

Originally Posted by SVX92-97

Time for a Husky Maniac high output 160A Alternator

If they ever again sell them.

[svxcess]

Originally Posted by elemgee

So then this is normal? As SVX 92-97 suggests, would a higher output alternator solve the problem?

Not necessairly in our case. The HO alternators are designed to give half of their rated output at idle. The baseline for this is about 800 rpm.

Our cars have an extremely low idle. With my voltmeter installed, I can see a marked increase in voltage at idle by just increasing my RPMs with the accelerator by just 100 rpms.


When my fans kick on, my voltage drops very slightly due to the initial current draw, but rebounds and stays steady while the fans are running.

.

[elemgee]

I have a 400 watt (RMS) amplifier running my stereo so I'm guessing that may be largely causing the problem. I read that the theres an amp draw of around 16 amps per 100 watts, so theres an additional draw of 48 amps at idle.

[svxcess]

Depending on the vehicle application, alternator current (amperage) output at engine idle speeds of 600-700 RPM can be as low as 35% of the full rated output. With enough electrical loads engaged, it is easy to exceed the alternator current (amperage) output when the engine is idling at 600-700 RPM. This is a normal condition– the battery supplements for short periods of time.

The stock SVX alternator has a maximum rated output of 95A.

Read this to make things a little clearer.

http://www.subaru-svx.net/forum/atta...4&d=1270949812


And yes, a high-output alternator would help tremendously. A 160A unit would put out about 55% of rated output at idle. Would definitely need the 4 gauge wiring upgrade.

PM me for more about this issue.

.

[Chucksta]

My SVX toasted it's less than two year old battery by how much it flattens it with a high electrical load, and a low alternator output. High fan, heated seats, heated mirrors, headlights, rear window defroster.. Yup.. left to it's own devices with a 613 RPM idle.. You only have to do that a couple of times as you sno blow the driveway... It starts to stutter and within about thirty second, dies.

Put a multi meter across the battery terminals with all electrical accessories off, and the car running on low idle. You'll be surprised at how few things you can turn on before the voltage starts to slowly drop as the battery discharges.

As the fans come on, it is a load on the alternator, which now requires more engine effort to turn. The IAC compensating for the drop in revs, from the increased load, may manifest itself as fluctuations in RPM.


How bad is the discharge on the idle? Bad enough that batteries that are the right physical size, and of sufficient amperage, can be purchased, but would not be covered if they failed. The ones that were guaranteed, were rated for 800 deep discharge (80%) cycles.. and were also over $300.00