Low Alternator Output and Fuel Mileage

[TallPaul]

Can low alternator output affect fuel mileage? I was getting around 15 mpg on the Mountaineer I recently purchased. One day it would not start. I charged the battery, then started it. The alternator was putting out 12 volts, 12.6 when revving the engine.

After getting a new alternator, which charges close to 14 volts, the fuel mileage is running around 16+. So I assume the alternator was not charging at full rate for a while (had truck about 2000 miles) and that it affected fuel mileage by either 1) weaker spark for poorer combustion, 2) hindered function of electronic fuel injection, or 3) both.

What say ye?

 

[Quest]

low alternative output as in what? an overloaded alternator (then yeah, no doubt that it will impose additional load to the engine, thus lowering the fuel mileage as a consequence). If it's just a regulator issue, then no, it will not have an impact on your lowering of fuel mileage.

if you alternator, even with a partially discharged battery as a major load, shouldn't go lower than 12.8VDC (or lower than 13V nominal, when you revv it). If it goes lower than that, your alternator is shot.

Q.

 

[tinmanSC]

I bet wrcsixeight could answer this question. He's pretty pro when it comes to vehicle electrical stuffs. Maybe he'll chime in.

 

[George7941]

If your battery was strong enough to crank the engine, then it is strong enough to power the ignition system and the fuel injection system

Any misfiring or wrong fuel mixture would trigger the MIL light.

So, to answer your question, unlikely. You need to look elsewhere for the cause of your improved fuel mileage.

 

[GeneralEclectic]

You could be on the right track. Since the alternator wasn't completely dead there are a couple of possibilities, such as a shorted turn in the stator or a blown (i.e. melted-down into bidirectional conduction) diode that could have caused the alternator to draw huge amounts of power from the engine. If that had been the case though, it would have gotten VERY hot in the process of dissipating all that power. Since average engine output to drive the vehicle down the road is only on the order of, say, 10-15hp normally, a failed alternator that's dissipating an extra 1 or 2KW (1hp = 0.746KW) would DEFINITELY cause a decrease in mileage on the order of what you experienced.

 

[wrcsixeight]

I am hardly an expert in auto electric systems. I do have some knowledge on alternator charging, cause I live off 12 volts.

I can relate one experience that might be applicable to the OP's question.

One time I was deep in Baja with a recently installed reman'd alternator. I ran 3 batteries way too low, and while they had enough juice to start my engine, shortly afterward my ammeter and voltmeter indicated no charging was occurring. The alternator had quit under the stress of 3 super hungry batteries asking for everything the cold alternator could produce..

Later on, after I got all three batteries fully charged, I was driving around without a charging system, the voltage while running was 11.5 volts. I noticed the engine was kinda sluggish, and at one point when climbing a hill then engine surged and felt normal again. I noticed my ammeter was indicating charging and my volts was back up at 14+.

The higher voltage definitely made my engine run way better, even with the alternator sucking up 2 horsepower to produce 50+ amps.

 

[Warstud]

It does seem logical that if you had low alternator output on a vehicle with EI that the fuel wasn't being burned efficiently. But you say you were getting 15 mpg.... now it's 16mpg...could be alot of factors with that small of a difference.

 

[JHZR2]

Slightly as lower voltage equals higher current, which means higher losses.

But you're talking watts in a tens of kilowatts situation, so it is so tiny ghat it is NBD.

You have something else wrong...

 

[Quest]

Originally Posted By: JHZR2
Slightly as lower voltage equals higher current, which means higher losses.

But you're talking watts in a tens of kilowatts situation, so it is so tiny ghat it is NBD.

You have something else wrong...


+1. I'm with you on that one.

Q.

 

[flacoman]

Methinks the slightly lowered voltage de-calibrated the sensors slightly, and lowered the fuel pressure a wee bit , thus leaning it out ever so slightly . Hotter spark and injectors firing cleanly would affect mileage.

 

[Colt45ws]

The sensors are run off a 5V feed from the computer which also takes care of the DC-DC Buck converter for that. Its setup to maintain that output down to 10ish volts.
If anything, its probably the ignition system getting weaker. Since they pretty much use a straight transformer to get the HV output, there is no way to dynamically correct the voltage output.

 

[mechtech2]

Your gas mileage should have gone down, not up.
More load.

 

[TallPaul]

Originally Posted By: mechtech2
Your gas mileage should have gone down, not up.
More load.
Good point. When running the generator on my motorhome the engine speed takes a big dip when the air con compressor comes on. But on the Mountaineer maybe it was still generating full voltage, but a bad electronic component or voltage regulator was keeping it all from getting to the battery.