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JohnNeiferd · 03-18-2010, 11:13 PM

I just installed an alternator kill switch on my 1993 Geo Metro 1.0L 5-speed today. I used a 50amp toggle switch to break the connection between the alternator output line and the 60amp fuse it connects to.

I plan on testing this for a few tanks to see if it does improve fuel economy, but I have noticed something strange already. I have an MPGuino installed and it seems like my instant MPG drops very slightly when I turn the alternator off. Not dramatically enough to be sure though. However, at night with my lights on and heater blowing, the battery was under a lot of load and was putting out far below 12V (my cigarette lighter voltmeter said 8.6V, but its not vary accurate, I'd estimate around 10V - 10.5V).

I kept my foot in a constant position as best as possible as to not change the throttle position. With the alternator off and the battery under high load, MPGuino should only about 45 MPG, when I switched the alternator on, it jumped up to 56 MPG. I flipped it on and off several times with the same results. From 45 to 56 MPG is a 24% increase with the alternator on, this doesn't seem plausible at all. Especially considering my foot was keeping the throttle at a constant position.

After looking at the MPGuino, switching the alternator on and off caused my gallons per hour to jump from around 0.77gph to 0.98gph with the alternator on.

Is the fuel injector signal getting messed up from the low voltage causing the MPGuino to give an incorrect reading? I was hoping to see an increase, similar to when MetroMPG removed his alternator belt, but this has me puzzled.

03-18-2010, 11:13 PM	#1 (permalink)
JohnNeiferd EcoModding Lurker Join Date: Jul 2008 Location: Ohio Posts: 28 Johnny's Metro - '93 Geo Metro 4-Door Hatchback 90 day: 47.5 mpg (US) Jetta GLS TDI - '01 Volkswagon Jetta GLS 90 day: 45.83 mpg (US) Thanks: 0 Thanked 0 Times in 0 Posts	Alternator Kill Switch, Worse MPG? I just installed an alternator kill switch on my 1993 Geo Metro 1.0L 5-speed today. I used a 50amp toggle switch to break the connection between the alternator output line and the 60amp fuse it connects to. I plan on testing this for a few tanks to see if it does improve fuel economy, but I have noticed something strange already. I have an MPGuino installed and it seems like my instant MPG drops very slightly when I turn the alternator off. Not dramatically enough to be sure though. However, at night with my lights on and heater blowing, the battery was under a lot of load and was putting out far below 12V (my cigarette lighter voltmeter said 8.6V, but its not vary accurate, I'd estimate around 10V - 10.5V). I kept my foot in a constant position as best as possible as to not change the throttle position. With the alternator off and the battery under high load, MPGuino should only about 45 MPG, when I switched the alternator on, it jumped up to 56 MPG. I flipped it on and off several times with the same results. From 45 to 56 MPG is a 24% increase with the alternator on, this doesn't seem plausible at all. Especially considering my foot was keeping the throttle at a constant position. After looking at the MPGuino, switching the alternator on and off caused my gallons per hour to jump from around 0.77gph to 0.98gph with the alternator on. Is the fuel injector signal getting messed up from the low voltage causing the MPGuino to give an incorrect reading? I was hoping to see an increase, similar to when MetroMPG removed his alternator belt, but this has me puzzled.