PWM Control of GM Alternators

[MPC001]

Have searched the forum and this subject hasn't really come up in detail as far as I can see apart from a few mentions.

Anyone played with alternator controls much on Commodores/G8/Chevy SS or other GM vehicles?

Applicable to Gen III & Gen IV PCM/ECM's & probably Gen V as well, GM uses a PWM control to manage the alternator via the "L" (lamp) terminal, and the alternator says hi back on the "F" (field) terminal.

ACDelco TechConnect article covers the detail best of how Duty Cycle of a 5V 128Hz signal to "L" is used to control alternator output voltage. (etc).

GM ECM Controlled PWM Alternators 1.jpg

GM ECM Controlled PWM Alternators 2.jpg

Also a good how-to thread on LS1Tech by LSswap which gets into using a micro to sense & control a PWM alternator:

https://ls1tech.com/forums/conversions- ... -away.html

What got me down this path was an LS Gen IV conversion with VZ Holden alternator run by a Corvette C6A6 E38 which regardless of duty cycle pushed to the "L" terminal via ECM alternator setting changes, the charge voltage sat solid at 13.8-14.0 Volts (not a bad thing - but no ~15V boost like a VE/VF Commodore runs by BCM control to it's trunk located battery.)

[rjdrew1986]

Thank you for sharing.

[AngelMarc]

What got me down this path was an LS Gen IV conversion with VZ Holden alternator run by a Corvette C6A6 E38 which regardless of duty cycle pushed to the "L" terminal via ECM alternator setting changes, the charge voltage sat solid at 13.8-14.0 Volts (not a bad thing - but no ~15V boost like a VE/VF Commodore runs by BCM control to it's trunk located battery.)

It says 100% is for diagnostic purposes. It also says it (alternator) stays at 5 volts (pull up resistor) when nothing is connected. 100% duty cycle probably sets it to old school constant voltage mode (more or less). I'd say you either have the wrong pin wired up, or your ECU is switching between open circuit and 5 volts, effectively doing nothing. Needs to send ground and open circuit, or push high and low signal. Probably that. A lot of variety in the gen 4 stuff; wouldn't be surprised if this is more of that variety. All just a guess though. Should be easy enough to test. Unplug it and probe each side while it's running. I'm curious what 0% does.

[MPC001]

What got me down this path was an LS Gen IV conversion with VZ Holden alternator run by a Corvette C6A6 E38 which regardless of duty cycle pushed to the "L" terminal via ECM alternator setting changes, the charge voltage sat solid at 13.8-14.0 Volts (not a bad thing - but no ~15V boost like a VE/VF Commodore runs by BCM control to it's trunk located battery.)

It says 100% is for diagnostic purposes. It also says it (alternator) stays at 5 volts (pull up resistor) when nothing is connected. 100% duty cycle probably sets it to old school constant voltage mode (more or less). I'd say you either have the wrong pin wired up, or your ECU is switching between open circuit and 5 volts, effectively doing nothing. Needs to send ground and open circuit, or push high and low signal. Probably that. A lot of variety in the gen 4 stuff; wouldn't be surprised if this is more of that variety. All just a guess though. Should be easy enough to test. Unplug it and probe each side while it's running. I'm curious what 0% does.

Have checked the L wire with a CRO & it's got the correct PWM signal & Duty Cycle being fed to it from the correct L pin. Also used a PWM generator at 5V 128Hz from 0-100 DC on the same wire (based on a factory harness) and no change.

So either the connector at the alternator is at fault (very hard to get to), or the reg in the alternator is ignoring the PWM - which it must've been doing for the last 17 years .

But thanks anyway .

[AngelMarc]

K
And it's not some generic replacement alternator?

[MPC001]

K
And it's not some generic replacement alternator?

Correct . Authentic GM part.

[rjdrew1986]

Opening different calibrations in HPT, it appears the desired Commanded Duty Cycle PWM % is a tunable parameter on some PCMs while not on others. Does anyone know if there is a way to adjust the output voltage on the PCM OS's where this commanded duty cycle % is not accessible & adjustable?

Commanded Duty Cycle Generator Output Voltage:
10% 11.00 V
20% 11.56 V
30% 12.12 V
40% 12.68 V
50% 13.25 V
60% 13.81 V
70% 14.37 V
80% 14.94 V
90% 15.50 V

[MPC001]

Opening different calibrations in HPT, it appears the desired Commanded Duty Cycle PWM % is a tunable parameter on some PCMs while not on others. Does anyone know if there is a way to adjust the output voltage on the PCM OS's where this commanded duty cycle % is not accessible & adjustable?

Commanded Duty Cycle Generator Output Voltage:
10% 11.00 V
20% 11.56 V
30% 12.12 V
40% 12.68 V
50% 13.25 V
60% 13.81 V
70% 14.37 V
80% 14.94 V
90% 15.50 V

That's interesting. Haven't seen that table in HPT. Which OS?

The duty cycle can be set if CAN control is set to enabled & the CAN messaging from the BCM is absent. , plus there is a timer to how long this DC lasts for before going to 100%. (C6 OS). (HPT).

[rjdrew1986]

My apology, I should have been clearer, it's not a table in the OS that supported it, I recall that it was a single parameter, I think it was in the System tab. I'll look for a PCM & OS that supports it in HPT and post it here (I just don't recall) but I recall it was a newer PCM, but it wasn't a P04 (which is the PCM that I'm focused on).

[MPC001]

My apology, I should have been clearer, it's not a table in the OS that supported it, I recall that it was a single parameter, I think it was in the System tab. I'll look for a PCM & OS that supports it in HPT and post it here (I just don't recall) but I recall it was a newer PCM, but it wasn't a P04 (which is the PCM that I'm focused on).

Ok cool. Understand now.

Probably the same as what I mentioned. Under System/Alternator - With "voltage control" set to fitted, "ECM Override" set to enabled, "Serial Request Init" DC can be set to run for "default time" up to 13million seconds, default 120 sec.

FYI FWIW - Reflecting back to Lucas alternators on earlier GM Holdens, they maxed at 13.8V or so, maybe with the hot climate this was the norm, whereas Bosch seemed to be 14.5V (my experience on Deutz/Bosch in colder climes). Maybe Holden VZ Mitsubishi alternators followed the Lucas specs.