Flatrater Diagnostics is excited to bring you another excellent case study by Miles Wada
Thank you Miles!
Alternator not Charging
How hard is it to do a diagnosis on an alternator that isn't charging? Very difficult if you don't know what you are doing. This involves a 1997 Chevrolet Cavalier with a 2.2L engine that came in with an alternator that wasn't charging. This vehicle came in with a rebuilt alternator that another shop installed for the same symptoms. Be warned all of the following information is boring until you encounter this particular problem and really need this information..
I must say that the key to this diagnosis was to follow the trouble tree. My thinking on a trouble tree is it is the last thing you look at when trying to do a diagnosis. I treat trouble trees like the assembly instructions that came with my computer. I look at the instructions only after I have assembled all the components and when the computer doesn't work. I think this mentality has something to do with me being a mechanic.... I could put anything together without reading the instructions.
Here is a little information on how I think this alternator works. There are two control wires that go to the alternator. The "L" terminal (red) and the "F" terminal (gray). Both of these wires come from the PCM. The "L" terminal switches (on/off) the alternator and also controls the alternator light through the PCM. The "F" terminal is a duty cycle signal that commands the regulator for charging purposes. Very simple right? All we need to do is find the proper signals at these terminals and move on with the diagnosis.
Both of the above images were captured with the KOEO. With both images the upper screen is the "L" terminal and the lower screen is the "F" terminal. The left image is showing the bad system and the right image is showing what a good system should look like. The far left of both images (with voltage low) is when the key was off.
Same setup as the above images, left showing bad and right showing good. With these images I am using the scanner to command the "L"terminal On (inactive) and Off (active) while the engine is idling. When I commanded the alternator on with the bad system (left) nothing changes, both terminal voltages stayed at about 3.5 volts but when I commanded it off both voltages dropped low and then went high when I commanded it on again. With the right image, since the alternator was already on (charging) nothing changed when I commanded it on but like the bad system both terminal went low when I commanded it off. I know it is difficult to see the pulsing voltage(solid black bar) with the good system. There is no diagnosis value to look at this signal close up with this particular failure but if you need to, this is how it should look like.
I also ran some additional voltage checks with the alternator connector disconnected from the alternator with the bad system. All of the above images were captured with the connector on the alternator. With the connector off the "L" terminal measured 5 volts and the "F" terminal measured 0 volts with the engine off or running. I don't know if there is a code for a malfunctioning charging system but there were no codes stored. I also have to mention that the alternator light does not come on with the KOEO or while the engine is running but if you turn the key on, off and back on you will see the alternator light flash momentarily.
I believe that the momentary downward spike is when the alternator light flashed.
I don't believe that we need to see any scanner images but I will provide them for the scanner users.With all of the images below I believe that the data parameter for "Generator L Terminal" is reversed.
Bad system commanding alternator Off (first high), On, Off.
Bad system, no commands, KOEO, start-up, idle.
Good system KOEO, startup, idle.
Good system, commanding alternator Off, On Off.
Ok, with all this information what is wrong and more importantly why? Just a note, both wires and connections leading from the alternator to the PCM are good. The battery terminal wire at the alternator is also good. Also, if I power the alternator up with a positive test lamp on either the "L" or "F" terminal the alternator would start charging. All of you have an advantage over me when doing this diagnosis. I didn't know what a good system should look like. Even with all this information I still don't understand how this system works. You will be surprised how many different configurations GM has with these charging systems.
Stay Tuned For Part 2
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