Battery idiot light on - whats the problem - help!

[98v70dad]

The charging system idiot light went on yesterday. My battery is only a few weeks old and should be good. The truck still starts up fine but if its not charging that wont last. Battery connections are tight and not corroded. Whats the next most likely thing in the charging system that could be bad? I'm thinking its the rectifier but really not sure.

A few days ago I was on a trip and noticed I was a quart low so I tried adding oil without a funnel and spilled at least a cup. My problem may related to this - don't know.

 

[Uncle Gump]

I would put a voltmeter across the battery and see if it is charging...

 

[98v70dad]

Thanks. I can do that but have no idea what the reading means. Is the voltmeter in AC or DC mode?

 

[ericbphoto]

Use the DC volts range. Check the voltage with the engine running. It should be around 13.5 volts after the engine has been running for about 5 minutes. Will be a little higher immediately after startup.

 

[Uncle Gump]

Sorry about that... eric got you straightened out though.

You check it right after start up... it needs to be minimum 14 volts.

 

[98v70dad]

Sorry about that... eric got you straightened out though.

You check it right after start up... it needs to be minimum 14 volts.

Thanks. I figured it out. It reads 11.5 volts. This truck usually reads 14.4 v - so something is out. The truck only has about 78,000 miles on it so I'm thinking its something electrical that's failed instead of mechanical. Its 23 years old. Is the voltage regulator part of the alternator, a separate item or part of the rectifier?

I've got to go winterize the RV and will be back in a few hours. Thanks for the help. I'm trying to round up my parts so I can fix it tomorrow. Any help on what might have gone bad is appreciated.

 

[RonD]

+1 ^^^^

The Battery Light means alternator voltage is LESS than battery voltage which is a bad thing in that you are powering the vehicle using your new battery which will cause it to drain and it would no longer start the engine

The battery in any vehicle is ONLY used to start the engine, thats its only purpose, after start up the alternator powers all electrics
Key OFF, a new battery will show 12.8-13.0 volts DC
3 year old battery 12.5volt
5/6 year old battery 12.3volts, and time to shop for battery sale

12.2volts or less is a failing battery

Yes, after startup voltage AT the battery should be 14-14.8volts DC, that "fast recharge voltage"
After a few minutes it will drop to 13.5-13.8volts, and stay there as long as engine is running regardless of lights or heater fan speed

If you see under 13.0volts AT battery with engine running then alternator is not working, Battery Light confirms that

Why did you get a new battery?

Alternator wiring test;
Use alternator case for ground
Key OFF
B+ wire, larger wire on back of alternator, leave it connected for test, should show Battery voltage, so test battery first

Unplug the Voltage regulator 3 wire connector, yes its inside alternator
Test Yellow wire should show Battery voltage, if not ALT Fuse is blown in engine fuse box

Test light green wire, should be 0 volts
Turn key on, should now be Battery voltage, this is the Battery Light wire

If wires test OK then replace alternator, you can just replace voltage regulator but.................its been my experience that it will only last a few months before another issue pops up like a loose bearing

 

[98v70dad]

Use the DC volts range. Check the voltage with the engine running. It should be around 13.5 volts after the engine has been running for about 5 minutes. Will be a little higher immediately after startup.

Hey thanks. That's what I figured since its a 12 volt system but electrical isn't really my thing. This truck usually reads around 14.4 right after start up with the engine running. 11.5 V this morning - something's not working.

 

[98v70dad]

+1 ^^^^

The Battery Light means alternator voltage is LESS than battery voltage which is a bad thing in that you are powering the vehicle using your new battery which will cause it to drain and it would no longer start the engine

The battery in any vehicle is ONLY used to start the engine, thats its only purpose, after start up the alternator powers all electrics
Key OFF, a new battery will show 12.8-13.0 volts DC
3 year old battery 12.5volt
5/6 year old battery 12.3volts, and time to shop for battery sale

12.2volts or less is a failing battery

Yes, after startup voltage AT the battery should be 14-14.8volts DC, that "fast recharge voltage"
After a few minutes it will drop to 13.5-13.8volts, and stay there as long as engine is running regardless of lights or heater fan speed

If you see under 13.0volts AT battery with engine running then alternator is not working, Battery Light confirms that

Why did you get a new battery?

Alternator wiring test;
Use alternator case for ground
Key OFF
B+ wire, larger wire on back of alternator, leave it connected for test, should show Battery voltage, so test battery first

Unplug the Voltage regulator 3 wire connector, yes its inside alternator
Test Yellow wire should show Battery voltage, if not ALT Fuse is blown in engine fuse box

Test light green wire, should be 0 volts
Turn key on, should now be Battery voltage, this is the Battery Light wire

If wires test OK then replace alternator, you can just replace voltage regulator but.................its been my experience that it will only last a few months before another issue pops up like a loose bearing

First, I'm in Atlanta. I lived many years near the Canadian border with -25 F degree nights so I'm pretty familiar with battery death by cold weather. Batteries die differently here. The heat kills them here and they usually last about 4 years regardless of the the "warranty time" A 72 month battery lasts 4 years and a 36 month battery usually does too. That said, a dead battery is a dead battery. I replaced it 3 weeks ago and the battery light first showed up yesterday.

I bought the new battery because it wouldn't hold a charge and it was well past 5 years old. I recharged it twice and it held the charge for a day, then dead. This is an extra truck so I don't drive it everyday.

Thanks for the help and comments. I will troubleshoot the alternator tomorrow. Does a new alternator usually come with a new rectifier? Would like to buy a motorcraft alternator from Rockauto (if needed) but not sure what I'll get since every vehicle can be different.

 

[ericbphoto]

I would expect a new regulator installed in the new alternator for that truck.

 

[Uncle Gump]

Your replacement alternator will have the rectifier and voltage regulator in it.

 

[98v70dad]

Thanks folks! 78,000 miles for an alternator. A new low end record for me. Usually they last around 150,00 - 200,000 miles for me regardless of age.

Can anyone recommend a good value alternator? I don't want to have to screw around with it by having to replace the pulley to save a few bucks. RockAuto has a Motorcraft reman for around $130 and an all new Bosch for about the same price. I'm now leaning towards the Bosch - any other thoughts?

 

[Josh B]

This may not help your situation, but for what it's worth.
Earlier you mentioned cleaning the battery cable connections. I've found in times past the grounding cable to the engine can also cause major issues.
If it sits a lot etc, might not be a bad idea to check it. On your Ranger it probly goes to the starter underneath. Take a small wire brush under there with you and scrub both sides of the cable end and the bolt it connects to.
You might be surprised at the times I've found that simple fix to have been causing a major problem (it was a major cause why I got my first Ranger, for free!)

 

[98v70dad]

If it turns out I need the new alternator will someone suggest a new or re-manufactured one that I can get that isn't junk.

 

[Uncle Gump]

I kinda feel they can all be a crap shoot... maybe a Motorcraft reman might be the best bet.

Last time I replaced one... I had to do it 3 times before I got a good one.