Alternator Kill Switch, Worse MPG?

[JohnNeiferd]

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.

[Christ]

You'll probably have to do something to get your electrical system powered back up while the alternator's not charging.

13.6-14.4VDC would be a good idea.

[nayeliesuncle]

I'd say such a low voltage could cause a problem. Maybe you need to run two batteries.

[JohnNeiferd]

I work at AutoZone. I've tested my battery before, and it is a bad battery. One of the previous owners put in a large MaxxLife battery with 875 CCA, so it can still crank the engine over easily. I plan on using as large of a deep cycle marine battery as possible when my battery finally dies.

I did purchase a 5 watt solar panel from Rural King for $21 that I plan on hooking up to help keep the battery topped on when I'm not driving it. I'm on the highway a lot and plan on cycling the alternator on and off for 10 minutes each. During town driving, I'll probably only have it off when I'm accelerating and on when I'm cruising, slowing down, or idling.

[Christ]

Get a voltmeter, and don't drain the battery past 11.0-10.8VDC.

[RobertSmalls]

Originally Posted by JohnNeiferd

I'm on the highway a lot and plan on cycling the alternator on and off for 10 minutes each. During town driving, I'll probably only have it off when I'm accelerating and on when I'm cruising, slowing down, or idling.

You would see the biggest gains from grid charging, but the intelligent alternator control you describe is better than stock.

You could have the brake lights drive a relay that turns the alternator on, which is pretty much what I have in my Insight.

I'm curious: How much does your gph at idle rise when you increase the load on the engine?

[busypaws]

I disagree with the way you did the alternator kill switch. The way I did mine was not to disconnect the output of the alternator but to disconnect the field wires.
I think of an alternator as working this way:
When the voltage on the battey drops below some number (say 12.4 volts) the voltage regulator turns on the field wires and the alternator pulls power from the engine and makes current to put the battery back above 12.4 volts. When the battery gets back above 12.4 volts the field wires go back off and the alternator just spins freely not pulling the energy through the belt and making power.
In your system I think: You disconnect the output. The voltage drops in the battery. Your voltage regulator calls for the alternator to put out more power. It turns on the alternator by turning on the field wires. Your alternator pulls power from the engine and makes power but the power doesn't make it out. So I think your alternator is running full time.
Think about it this way. Disconnect the belt from the alternator. With the field wires off the alternator just spins with bearing friction. With the field wires on you are turning a magnent inside a coil which makes it harder to turn. It is just the reverse of a small fan. Stoping a fan with your hand with power off is easier then stopping a fan with the power on. In the first case you are just overcomming the bearing friction. In the second you are (with your hand) overcomming the power of the motor.

In my setup I turn off the field wires. This forces the alternator to just free wheel. Not pull any power from the engine and use it to make current. Even though the voltage regulator is turning on the field wires I have them turned off with a switch. Now when going down hill (not EOC) I turn on the field wires and now the alternator takes power from the engine and makes current and starts to recharge the battery.

Hopefully I've explained it --- This is the way I think most modern alternators work.

[silverinsight2]

Originally Posted by RobertSmalls

You could have the brake lights drive a relay that turns the alternator on, which is pretty much what I have in my Insight.

details please

[Daox]

It sounds like you're using a starting battery. You need a deep cycle if you are gonna do any worthwhile time with the alternator off. If you don't, you'll end up with a dead starting battery very quickly.

Cycling the alternator on the highway will likely not give you much if any mpg improvement. You're simply avoiding the load hit now to take it a few minutes later.

I agree with busypaws about how to do the alternator disconnect. The field control wire is much easier to use. However, I disagree with the idea that its full on all the time now. If there is nowhere for the electricity to go (since its physically disconnected from the battery), the alternator can't put a load on the engine.

One more thing, the mpguino depends on an accurate calibration of injector pulse timing. Altering your system voltage will cause the injectors to open slower or faster, so it is going to throw off the mpguino readings somewhat. Its no problem for the engine to compensate for this, but the mpguino can't. I can't say how much this will effect things, but it will to some extent.

[RobertSmalls]

Originally Posted by RobertSmalls

You could have the brake lights drive a relay that turns the alternator on, which is pretty much what I have in my Insight.

I drive around with the IMA disabled with a clutch switch mod, and when the brakelights come on, it energizes a relay that reenables the IMA. Since, in the winter, I don't have enough high-voltage juice for assist, it's more like a really fast alternator than regenerative brakes.

http://ecomodder.com/forum/showthrea...tml#post150133

Btw, I have an Optima yellow top in there, so I'm cooking up a DC-DC converter disable.