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Hello I have an 04 Trailblazer LT 4x4 4.2 that has stopped charging. I have checked the plug and wires for opens and they are fine. I had the alternator checked and it tested fine. I pulled the reference voltage off the plug and it is now reading 3v off the red wire at the plug. It should read 5 in order to get the alternator charging. I have no idea what is causing this. I have replaced the alternator with a brand new one because I thought it could have still been the alternator only working occasionally. I dont know what would cause the reference to be 3v instead of 5v but I'm pretty sure this is the issue. I have checked the battery and cables. Everything checks out okay except that reference voltage being low.
I would appreciate some much needed guidance. Thank you guys for your time.
 

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Right, and if you're looking at the schematic the red wire coming off the generator says "generator on signal". That should have a 5v reference and it has a 3v reference. IDK what would cause that considering I tested the wires for opens and there weren't any. Good continuity from the plug to the connector. So now what do I look for. I don't think it is the pcm itself because the car RUNS. Checked the #'s 10,22,23,and 28 fuses all fine, and the #47 fuse in the rear no problems. Frustrating to say the least. However, I do appreciate you taking the time and uploading the schematic for me. Thank you very much for your help.
 

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Just for kicks, unplug the electrical connector to the fan clutch and see if you now get 5 VDC instead of 3 VDC.
 

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Hey sorry for the delayed response. I will check that tomorrow (friday) I work alot of overtime and dont have any spare time during the week. I'll definitely try that and get back to you. Thanks for the help.
 

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Okay, I unplugged the fan and still 3 VDC. No idea what could be causing that.
 

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Just for kicks, unplug the electrical connector to the fan clutch and see if you now get 5 VDC instead of 3 VDC.
Okay, I unplugged the fan and still 3 VDC. No idea what could be causing that.
 

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Unfortunately, I don't have any hints or guidance. Hopefully, one of the other members will chime in and assist.

Good Luck!
 

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Discussion Starter #11
As I see it, you either have something loading down the 5V line or you have a bad PCM.
How would I check if something is loading down the line?
That is where my electrical skills end. I have no idea how to check that without a legit scanner.
 

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This recent thread has similarities with your issue. In particular have a look at the picture in post #12.

A degraded wire can check out OK with a meter under no load conditions and have a substantial voltage drop as current increases.

 

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pulled the reference voltage off the plug and it is now reading 3v off the red wire at the plug. It should read 5 in order to get the alternator charging

As far as I know this is a pulse width modulated signal. A varying duty cycle that can't be read properly with a multimeter. Need a scope for that.

You can however read the duty cycle reported by the PCM using an ELM327 obd adapter and a smartphone app. You can see in the second image the duty cycle is higher.... I created a load on the system by running the blower at the highest setting and running the headlights with the "flash to pass" on. This dropped the system voltage and the PCM responded by increasing the duty cycle.

Note: the "Volts (ad) is not a vehicle function. It is a function of the obd adapter and I have not yet calibrated it to the vehicle so it will not agree with other readings but will indicate a changing voltage anyway.

Screenshot_20200823-092600.png
Screenshot_20200823-092548.png
 
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