Alternator will not charge at idle.

[rlane547]

Hey guys, I'm trying to help out a co-worker with his truck. He has a 98 12v and told me today that his alternator will not charge at idle. He said that he replaced the alternator but it didn't change anything. I know that the voltage regulator is built in to the PCM. I read somewhere that one of the sensors it takes it readings from is the crank position sensor but I don't know what the others are. Could anyone share how the system works and any ideas what might be going on? Any help would be greatly appreciated.

[rancherman]

if you find it's internal pcm, I'd just convert it to an 'inline' type regulator. I've read it costs about 25 bucks... and bypasses the pcm altogether.

how quick 'off idle' will the gauge sweep upwards, showing a charge? And when the rpms drop, does the gauge slowly drop, or does it just cannonball to the low reading?

[AH64ID]

Is he certain it's not a bad battery, or other load?

 

Has he verified voltage with a volt meter?

[MnTom]

The way the charging system works on the 12v trucks is the PCM looks at the crank sensor to determine if the engine is running. If it sees a signal it then looks at the battery temperature sensor located under the left battery. Then by the magic of computers it determines how much power to put into the battery. Something you can try is to unplug the small connector on the side of the alternator. Hook two wires up to the connector in the side of the alternator. Ground one and put the other on the positive terminal (it doesn't matter which one goes where). That will put the alternator into full charge mode. If it charges like that then check the wiring that you unplugged earlier. Chances are there is nothing wrong with it, but you never know. The next step if it charges with the jumper wires then a regulator is needed. If it does not charge then most likely the alternator is faulty.

[rlane547]

I can't seem to get a hold of him right now. I'll let you guys know when he calls. I'm pretty sure he said the tach was working, no lights were on in the dash. Not sure about the batteries, I'll ask him. I remember him saying that he attempted at some point in the past to install an external regulator so that leads me to believe this may not be a new problem. He also said it didn't work out for some reason.  If I trace it to the PCM I may give him the instructions for the voltage regulator mod on here. I'll post when I get more info.

[rancherman]

Thinking out loud here:

would a bad battery temp sensor shut down/reduce the output of the alternator?.. but should be at all engine speeds, not just idle?

Unless it reduces it so much that @ idle it's actually basically zero output??

[AH64ID]

Battery temp sensors help dictate voltage, not amperage.

[rancherman]

Battery temp sensors help dictate voltage, not amperage.

true.

isn't the gauge that  this  guy is  watching  a  VOLTMETER???

[AH64ID]

true.

isn't the gauge that  this  guy is  watching  a  VOLTMETER???

 

Yes, but they don't take the battery out of a "normal" range.

 

Only way for a battery to drop too low is to have too low of amperage going to it.

[rancherman]

is  the  battery temp sensor  for   both   'too hot'  and  'too cold'    conditions?   I would've  sworn I read  somewhere that  if the batt is  REALLY  cold,   the  pcm will  modulate  the  full output  of  alternator   until   batt is   close to  normal operating temps.

[AH64ID]

I don't recall that tidbit, but that shouldn't be effected by rpms. 

 

I have had my truck in some very cold weather and the battery always comes up to voltage as quick as the alternator can get it there. Set voltage is a hair above 15V in COLD weather. 

 

I don't think the PCM has control over anything but the voltage, and the alternator supplies the amps it needs to get to the proper voltage.

 

The only time I have seen voltage effected by rpms was on low batteries, and once I spent a couple minutes at cruise rpms the idle voltage came back up. If the idle voltage is dropping after driving for a while there is an issue.

 

The crossover cable should also be checked, as the ECM monitors voltage on one battery but the alternator charges thru the other one.

[rancherman]

I need a  refresher again on   Volts-Amps.

 

I  thought of  volts  as  the  speed of  current,  which is  tied  in  with  how fast the  armature is  turning in the  alternator.     Faster the  alternator,  the more volts...   this is  where the  voltage regulator comes in.

 

Then   when  the  battery is  being  discharged,  or  is   already  low,  the  amps  come up  to  refill the  batts.      I've  associated   AMPS  with   'force or pressure'  of  the  current. 

 

What   'changes'  in these  alternators  to  vary the amount of  amps?    The amount  of  'field'  being supplied to the windings?

[AH64ID]

RPMS also effect amps, it's why output increases with rpms even with a constant voltage.

 

Voltage isn't directly tied to rpms, unlike Hz on a AC generator. There does come a point where rpms are too low to maintain the desired voltage, but that's well below idle rpms for normal idle loads (no gird heater, big stereo, winch, etc).

 

The EVR adjusts the field to increase/decrease output based on the voltage/temp of the batteries. This increases the amperage being put out, which will (as long as output is higher than draw) increase the voltage. To maintain a constant voltage the amperage must vary.

 

Or is that not what you were talking about?

[rancherman]

let me ruminate  this over...  thanks for now!

[MnTom]

let me ruminate  this over...  thanks for now!

Uh oh...... The BIG words are coming out!!!

[Cowboy]

Volts is like pressure (psi), Amps is like Current (gpm).  When the pressure in the batteries are low, the PCM tells the alt by means of increasing the pressure in the windings  (just like water, more pressure = more current).  The windings then become stronger magnets, and when they pass the stator, induce a larger voltage.  The faster the rotor spins, the larger the voltage that it induces into the stator.  

 

Just like any wire, when current passes through the windings, there is a voltage (pressure) drop.  That voltage drop is linear to the Current going through the wire/winding.  So, the voltage at the battery may be at 14.5 volts, but if there is a lot of current load, you may need 20 volts peak in the alternator to overcome the voltage drop over the windings and  wires to, in the end, still be at the batteries 14.5 volts.  

 

Where does the voltage go?  It gets turned into heat (hence alternators get hot).  If you know the amount of current going through the wire, and the voltage drop over the wire, you can calculate it (volts x amps = watts = 3.41 BTU/ hr). 

[MnTom]

Dang, Cowboy, I like that explanation! That is exactly what I was taught including the BTU statement! Anyhow, I have a difficult time expressing that stuff in writing. I am glad you are more 'articulate' with the keyboard than I am.

[AH64ID]

Dang, Cowboy, I like that explanation! That is exactly what I was taught including the BTU statement! Anyhow, I have a difficult time expressing that stuff in writing. I am glad you are more 'articulate' with the keyboard than I am.

 

Agree... excellent statement that put to words what I couldn't come close to.

[rancherman]

Gotcha too!

 

I  was  'off'   on my  parallelisms.        I think  my  Junior High  shop teacher   used  a  river  as  his   example..      but  that was 40 years ago!     I screwed the pooch  trying to remember   something  taught in a  room full of  juvenile  delinquents.