Voltage Regulation (Alternators)

[Jubbsta]

Hey Everyone,

Please forgive me if any of my terminology doesn't make sense or my questions are redundant - I'm an extreme novice here. I work as an auto repair technician, and a coworker of mine has been having an issue with his vehicle and I've gone down a rabbit hole, problem is any question I google results in basic responses of how alternators work - I'm already familiar with the basic workings and anatomy of an alternator. The issue is: he replaced his alternator with a new one due to the output dropping to ~11V with several loads turned on and the vehicle at idle - repair manual for his vehicle specifies no lower than 13V. Per the schematics, the Voltage Regulator is integral to the alternator. We've checked voltage drops across both ground and power circuits and both are within specification. The diagram lists terminals - C to the PCM, B to B+, and L for the dash indicator. No where in the circuit does it show S for signal, and under the PCM input/output list it DOES have Battery Voltage listed under inputs (though I can't find any voltage sensor in any other diagram, assuming it's integral to PCM?). When de-pinning & isolating the C terminal from the alternator harness there is no change - we did this with the hopes that it would throw a "model" type value resulting in an increase of output voltage. The main question I have is there a way to find out what exactly the PCM is outputting under high electrical load, and then to spoof it? - I don't mean to add load to the sensing circuit, but rather change the output of the PCM to the IC. The thought of the PCM not going into a "fail safe" when disconnecting the C terminal is leaving a lot of questions unanswered for me.

I hope this make sense, all basics have been checked to the best of our knowledge.

Thanks!

 

[rjenkinsgb]

There are at least three different alternator "C" terminal systems that I am aware of.

With the oldest one, it was power out from the star point of the stator for such as an electric choke.


Ones connected to the PCM include:

A simply full output / limited output control, using high / low voltage (around 8 - 9V and near 0V).

PWM control using a fairly low frequency signal. With that, the duty cycle controls the regulator output voltage setting, as long as it's a valid PWM signal, never continuously off or on.
A disconnection or fixed level switches the alternator back to internal regulation at normal battery full voltage, ~14.5V

There are likely others as well.

In general, the C pin is used to reduce alternator output (and engine load) during acceleration, and possibly increase it during engine braking, with PWM controlled types. PWM is also used with AGM batteries to improve charge control on start/stop systems.


The alternator itself should usually default to conventional operation with C disconnected; no PWM or an internal pullup setting it in full output mode.

It's always possible the replacement is faulty!

What's the exact vehicle make, model and year?

 

[Jubbsta]

There are at least three different alternator "C" terminal systems that I am aware of.

With the oldest one, it was power out from the star point of the stator for such as an electric choke.


Ones connected to the PCM include:

A simply full output / limited output control, using high / low voltage (around 8 - 9V and near 0V).

PWM control using a fairly low frequency signal. With that, the duty cycle controls the regulator output voltage setting, as long as it's a valid PWM signal, never continuously off or on.
A disconnection or fixed level switches the alternator back to internal regulation at normal battery full voltage, ~14.5V

There are likely others as well.

In general, the C pin is used to reduce alternator output (and engine load) during acceleration, and possibly increase it during engine braking, with PWM controlled types. PWM is also used with AGM batteries to improve charge control on start/stop systems.


The alternator itself should usually default to conventional operation with C disconnected; no PWM or an internal pullup setting it in full output mode.

It's always possible the replacement is faulty!

What's the exact vehicle make, model and year?

Thank you for the extensive reply, there’s a lot of info they dont give us as auto repair techs! The vehicle is a 1996 acura slx