2005 Saturn Ion 3, not charging while on drive

When vehicle is on park or neutral, alternator charges just fine. Soon as vehicle is set on drive, the alternator stops charging. I replaced serpentine belt in case it’s slipping, I put a new alternator and replaced battery. Still, no charging battery when set on drive. When it’s shifted to the drive position the voltage drops on the generator turn on signal red wire going from the ecu to the alternator, from 5v to 0v then the voltage on my cigarette light tester starts showing the battery voltage dropping way lower than 12v the. Charge indicator light on dashboard comes on too at this point.
When it’s on the park position my cigarette light tester shows a good 14.5 volts, showing the alternator is charging and at the same time the generator turn on signal shows 5 volts.

Do you have any codes? Though I'm not sure why it would in your instance, the PCM might be detecting a fault and grounding the wire when it goes into drive.

Just out of curiousity, does alternator output drop when car is put into reverse?

I plugged in the scanner to see any codes but there are no codes.

Yes, alternator charge drops or alternator stops charging when car is put into reverse as well.

When I put it into reverse or drive it stops charging. The 5v on the generator turn on wire goes down to 0v but if I rev up the car from its 700 rpm up to 1200 rpm it starts charging, 14.5v . Then I let go of the gas pedal and rpm drops down to 700 rpm and the new alternator stops charging.

Does the alternator ALWAYS start charging with rpm at 1200 or more and does it keep charging while driving above this treshhold?

Does your scanner access data related to the charging system ? The same thing happened with the previous alternator? 

Does this alternator only have one wire? The 5V L-terminal wire?

I'd suggest disconnecting the alternator and probe the single wire. Monitor the voltage in Park and Drive. Does the voltage continue to drop out? If it does, I'd suspect a wiring or PCM problem. If it doesn't, the alternator is grounding the L-terminal in response to an internal fault.

Recheck the voltage drop across the positive and alternator case, then get another alternator. 

Hello, when the engine is cold the rpm is about 1000 then rpm drops down when it warms up between 600 or 700 rpm. and it used to charge just fine at that rpm. Before all this issues, if I had to stop for any reason e.g. red light to wait for the green the alternator charged just fine and the car was on drive, of course. Then one day we had some light rain, the first red light I had to stop on my way to work the charge indicator light came on my dashboard. I revved the car up to 1200 rpm and started charging but soon as the rpm settle down to about 600 the charge indicator light came back on. Later that day I plugged the scanner to see if there were any other issues but there were no check engine codes. Unfortunately, my scanner does not show the charging system data.

What size engine does the vehicle have any how many pins are on the alternator?

There might be a current clamp over the one of the cables that come off the battery. If you disconnect it, it should make the alternator go into "dumb" mode, and output a steady charge voltage regardless of PCM commands. It might help you in the meantime til you get things figured out.

The engine size is 2.2L. The alternator has only 2 pins F and L. I have an schematic of the charging system I want to upload but for some reason it wont let me upload it for you.

Thank you, I will do that and will keep you posted...

Is it this one?


 
 

Okay, right. The PCM supplies a 5Vdc PWM square wave to that "L" terminal. How long that 5V stays "on" as compared to "off" (duty cycle) controls how high the voltage of the alternator goes. The PCM determines what that duty cycle should be based on a bunch of factors including what that current clamp i mentioned to you earlier reads and what the "F" terminal is reporting back to it (F provides alternator feedback).

The following video (and the one that comes after it) provides comprehensive info on how the system works and how to diagnose.



I'm not sure what you're using to measure that 5V on the "L" terminal, but, as the video alludes to, a scope would give you the clearest picture of what's going on there. Using a regular volt meter probably isn't going to cut it. The same goes true for reading the return signal on the "F," The video does show how you how to use a multimeter's frequency setting to do a cursory test on the pin terminals if you don't have a scope, however.

Being able to read the PID data for the charging system would be a big help to you, as this system is rather complex and depends on inputs from a number of sources. Not having access to that info isn't much different than trying to diagnose an engine misfire without having use an OBD scanner. You're kind of flying blind.

In addition to watching the video, you may want to have your battery checked (conductance and/or drain test), clean its cable connections and one on the alternator. Checking the condition of the plug/wiring on the current clamp might help, too,

I wanted to update you all on my car not charging issue. After testing here and there and looking under my car I found a ground screw on the engine that was loose and it had mud and corrosion. I remover the nut and used a wire brush to clean the threads then applied some dialectic grease and tightened the nut. The car has been charging fine for these past two weeks and we've even had some rain. So no problems at all. I hope this can help anyone else who might come into this problem.
Thank you everyone of you who helped me with your car advice.

Thanks for the update! Did you notice a voltage drop on the alternator case before you found and cleaned that ground?