Charging system- excessive voltage variation

Marlin Technician Oregon Posted   Latest  
Resolved
Electrical
2005 GMC Sierra 2500 HD SLT 6.0L (U LQ4 CNG) 4-spd (4L80-E) — 1GTHK29UX5E169130
Headlights And Instruments Lighting Flicker Very Noticeably
Voltage Fluctuates 1.1V

While it is typical for these trucks to have more voltage fluctuation than most vehicles, it is bad enough on this one to be quite annoying. With the headlamps on and the HVAC blower on high with the engine idling, alternator output voltage varies 1.1V. The PCM is cycling the field circuit on and off at 5.7 Hz,, which is causing the voltage to cycle 13.9 to 15 volts at 5.7Hz. Scan data doesn't refresh this fast, but it does reach 0% and 100%, so indications are that it is simply going between full and no field control. This is confirmed by monitoring the circuit with a scope; it simply switches between two fixed pulse widths of about 10% and 90%. I already corrected the battery cable connections.

I found PIT3170E, which relates to this issue, but I am not sure that it applies to this vehicle. It includes this statement, "This modification will not work if the vehicle is utilizing the new "Regulated Voltage Control" system.", but without definition of what specifically that is. Not only that, but the bulletin doesn't make sense anyway.

What baffles me is why the PCM doesn't vary the pulse width instead of just cycling between full and no output. PCM software is current level.

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Rod Diagnostician
California
Rod
 

What part of the bulletin does not make sense?

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Marlin Technician
Oregon
Marlin
 

Most of it. For instance, step 4 says that a new connector is not needed if a red plug is present, but step 11 says to use a new connector. Following the procedure, you would end with only one wire. It seems as though the idea is to add a jumper wire from the output terminal to pin D, but I don't know the purpose of a new connector. I also still don't know if it applies, due to the statement…

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Jeffrey Mobile Technician
Illinois
Jeffrey
 

Have you tried adding another battery. I have come across battery issues like these before on other vehicles. The capacitance of the battery is too low and PCM is going a little crazy trying to control voltage

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Marlin Technician
Oregon
Marlin
 

Yes, no noticeable change.

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Mark Owner
Pennsylvania
Mark
 

RVC alternator control uses 2 pin regulator(not really a regulator, its a field control unit). I had a customer that fixed his flicker with a new OEM GM not ann ac delco alternator. Mark

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Marlin Technician
Oregon
Marlin
 

Is it an RVC system? GM used PCM control before 2005, so what is the differentiator? I am not aware of any difference from … with the basic system.

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Michael Owner
Alabama
Michael
 

Does it have a Generator Battery Control Module or Current Sensor on/around the negative battery cable near the battery terminal? If not, it isn't RVC… I believe that only the half-ton (not the … trucks were RVC-equipped that year, but I have been wrong before.

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Marlin Technician
Oregon
Marlin
 

No and no. Per the link that George provided below, and SI for a 1500, it is not an RVC system. I just finished testing a … with RVC, so that was a … coincidence.

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Marlin Technician
Oregon
Marlin
 

Any comment on the bulletin? When it refers to “a red cavity plug”, is it just the wire insertion point plug, or a plug in place of the terminal? It seems as though what is intended is to simply add a circuit, from the output terminal to pin D in the connector.

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Michael Owner
Alabama
Michael
 

Marlin, I think you're reading/interpreting the PI correctly. The referenced “red cavity plug” seals the hole in the plastic connector body in lieu of the wire and terminal. Adding the circuit (I believe) lets the regulator sense output terminal voltage. Again, I've been wrong before, but that's my interpretation FWIW…

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Marlin Technician
Oregon
Marlin
 

I might just try it. I spent 15+ minutes trying to make sense out the mess in that bulletin.

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Michael Owner
Alabama
Michael
 

It'd be easy enough to start with a temporary alligator-clipped jumper to see if making the connection smooths out the output voltage fluctuation.

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George Owner/Technician
Alabama
George
 

Do you have a alternator make &.part number ? If it is a RVC 2 pin alternator It should energize with regulator unpluged….Try and see if voltage stablizes unpluged js-alternators​.​com/gm-rvc-2pin-al…

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Marlin Technician
Oregon
Marlin
 

Delphi made, AD230 per cataloging. Per the link that you provided, and SI for a 1500, it is not an RVC system. I just finished testing a … with RVC, so that was a … coincidence. Thank you for the link.

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George Owner/Technician
Alabama
George
 

A ad230 alternator is a 4 terminal regulator (l,ign,sta,S) Light,,,,ignition…stator …Sense….) never seen one with a (D) terminal…Where is this info coming from ? Bulletin number …

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Marlin Technician
Oregon
Marlin
 

The PI referenced above, which refers to them as in a wiring schematic; A, B, C, D. D = Sta.

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George Owner/Technician
Alabama
George
 

Seems to me that PI has nothing to do with your problem….Just a fix if a bad plug is found….This is not a Computer controlled alternator ….What you are seeing on the (FR) terminal is a output from the regulator.

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Marlin Technician
Oregon
Marlin
 

Wow, you are correct. It has been so long since I worked on this type that I just pulled up the F-terminal Signal PID and thought that it is command from the module. Duh!

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Marlin Technician
Oregon
Marlin Resolution
 

Thanks to all who helped. I was off track regarding control in this system; the PCM has no control over charging voltage, it just receives the signal of what the internal regulator is doing. Adding a jumper wire from the output terminal to space D in the connector completely stabilized the voltage, eliminating the light flicker and the engine idle speed bounce (which I had not mentioned). I was…

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George Owner/Technician
Alabama
George
 

Glad it works….. I have rebuilt 100's of these alternators and have never seen one with the (P) phase hooked to B+. Doing this modification to me would seem to defeat some functions of the regulator….. One of the functions would be to not have the alternator energized during cranking. Not that it makes any difference if battery & wiring are up to specs…. Generally a (P) phase is a single…

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Marlin Technician
Oregon
Marlin
 

I don't know how the regulator is designed on this; according to the schematic, pin D is connected to terminal “S” of the regulator, which must be “stator”. Why did this problematic system get designed as it did? Flickering lights is extremely common, but is usually just tolerated. Yes, the PI is from GM, but I do wonder why it was never made a Service Bulletin.

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