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Old 02-19-2007, 08:42 PM   #1
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Alternator off question

OK, I have read some posts and want to try driving during the day without the alternator. At this time I want to do some testing to see if there are MPG gains by pulling the fuse or something simple along those lines. If I pull the fuse and then do some EOC will the battery be able to restart the car. Any ideas or advice is appreciated


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Old 02-20-2007, 05:40 AM   #2
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I've pondered this as well. Of course it would be best if you could totally disconnect it from the mechanical belt system for the most FE benefits (belt removal). But for convenience it would be neat to have a switch that you could use to disable the alternator electrically whenever you decide. As long as you could do it in such a way that doesn't damage the alternator. Gains could be had .

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Old 02-20-2007, 05:49 AM   #3
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Originally Posted by 95_corolla View Post
If I pull the fuse and then do some EOC will the battery be able to restart the car. Any ideas or advice is appreciated
Unfortunately, there's no simple way to answer this because there are too many variables:

- how healthy is the battery?
- what's the ambient temperature (battery performance declines significantly as temperature drops)
- what load are you putting on the battery?
- how long are you running this load?

Also, I see you have an automatic, so you face 2 additional challenges:

1) you have to use the starter (high amp draw) to reignite the engine;

2) you have no backup means of starting the car (clutch starting) should you draw the battery down so far that the starter won't work.

To be safe, you need to use a multimeter to experimentally figure out what you want to know, based on your specific circumstances.

Also, consider the point that from a financial perspective: you will almost certainly spend more money in battery replacement costs than you will get from fuel savings. Starting batteries are not meant to be charged/discharged any significant amount. Doing so shortens their lifespan.

Definitely caveat experimentor on this one!
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Old 02-20-2007, 06:54 AM   #4
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To get the full gain possible you'd have to mechanically disconnect the alternator from the belt that drives it. Since you need to use the alt at least sometimes, I think that full disconnection is not in the cards.

You can reduce the alt's load on the engine by reducing your electrical power draw. The alternator's mechanical load on the engine is directly related to how much current you are using while driving.

Some examples of high-level electrical power consumers are - - - cabin fans for heater and AC, extra lights, high-powered stereo, electric heaters for seats, mirrors, and glass. Ultimately all that energy comes from your gasoline.

Interestingly, high beam headlights don't use much more power than low beams - most of the difference is in the beam pattern. [You can check the wattage ratings for your bulbs; high/low bulbs have two wattage #s.] Some cars run four bulbs for high beams, most only run two. Of course any additional lights like fog lights or added running lights etc. will increase the electrical load on the alternator + thus the engine.

The electric radiator fan is also an electrical draw - but you don't want to risk overheating the engine by disconnecting the fan. However you probably can reduce it's use by avoiding situations that will make it come on, such as idling the motor.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 02-20-2007, 01:33 PM   #5
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Check out this thread, lots of good stuff in there.

I wouldn't recommend it unless your sure you have a good battery and another way to start it or charge it if it fails.
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Old 02-20-2007, 06:30 PM   #6
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I don't think I would do engine off coasting on an automatic trans car. They require a pump to keep the oil circulating unlike a manual trans. So if the engine is off and the car is going faster than around 35mph you are running the trans dry of oil. I don't think the little bit of gas saved would make up for the few hundred in parts it will cost to redo your trans. Unless you don't plan on keeping the car for long then it isn't a concern

let the engine idle with the trans in neutral going down a hill is probably the most I would do. You can refit the transmission probably like is done to cars being towed behind an RV but I have no idea what is involved in doing that. That way you would be safe coasting with the engine off.
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Old 02-21-2007, 04:21 AM   #7
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Thank for the reply everyone. Coyote X now you got me worried about EOC with my auto I am going to have to talk to a mechanic about that. Thanks for the warning though
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Old 02-21-2007, 04:38 AM   #8
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Does the corolla manual say if the car can be towed behind an rv? if so then no worries.

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