Alternator disable switch -- how much MPG improvement?
 

I know a few of you guys have installed a alternator disable switch. Just trying to get an idea of how much that will improve MPG (actual number or %). A "guesstimate" is fine since I'm still months away from completing all my mods.

My daily round trip commute with my 99 Saturn SL2 automatic is 64 miles (56 highway, 8 city) with some construction and traffic.

Forgot to mention that I did read the second half of DOAX's Paseo thread which includes the alternator switch.

Darin (metrompg) tested this a while back. He saw a 10% improvement at steady state cruising.

http://ecomodder.com/forum/showthrea...n-70-a-98.html

Thanks Daox

technically, Darin had removed the belt from the alternator so that it wasn't even turning. There would be less gains from just turning off the field. And it was a metro.

I did it on my 94 Toyota PickUp. I was not able to measure any change, I don't have a computer and tank to tank fillups with variations in temp/load didn't see any changes.
I do know I can feel a change in acceleration up a hill or a quick left turn in traffic with it off. Maybe I'm just using the switch in improve performance not improving gas mileage.

Don't get discouraged. I think if I had a scanguage or similiar and did A-B-A testing I would see a difference. Removing belt gets 10% so I would expect field switch at 5% or so.

I have a field switch, so when I get my MPGuino, I will be able to tell you a % gain. I won't be able to say MPG improvement, because I won't have it calibrated by then.

Sometimes, however, when I have the alternator off, the engine feels funny. Like the throttle is closing partway. It goes away when I turn the alternator back on. I suspect that it is the ignition getting less powerful with the low battery volts.

This alternator disable switch is one of the more extreme mods/tactics that I may end up not doing ... in the same boat as power steering pump delete and turning off the engine (during P&G). Maybe after exhausting the aero mods, I might reconsider.

Thanks for the input, though.

Were you also recharging the battery from an external power source?

busypaws: saw you answered my question in another thread:

If you're not using an external power source to re-charge the battery, that explains why you're not seeing any fuel economy benefit.

Some vehicles use smart alternator control to eke out a few percent savings, but mostly by maintaining a minimum voltage under accel/cruise, and boosting alternator output under engine over-run and braking conditions.