How does the voltage regulator work in a PCM

[Matthew Jeschke]

This likely applies to newer trucks too with the voltage regulator in the PCM.

My sister called me. She has a 2018 Nissan Sentra. Apparently the battery wasn't charging. She took it to the shop. They said her charge voltage was over 15V. They wanted to change the alternator. This struck my sister as odd cause it's at least the second (maybe 3rd) alternator she's had now in a few years.

There must be a communication glitch... If the charge current is high, say 15+V versus ~14V, wouldn't the issue be the voltage regulator in the PCM? Or do these newer alternators still have circuitry in them to handle the voltage regulation?

I don't know the newer charge systems w/ voltage regulation in the PCM work. All my stuff is old.

My guess is the switching of the regulator could be in the PCM but the high current side would have to be in the alternator or elsewhere? Long story short, sounds like a bad PCM to me not alternator... But maybe I'm wrong, never worked on that stuff before.

Side note: I interviewed mechanics to find her a good one years ago. She moved now though and lives in Chicago. So if you know a good mechanic there let me know...

 

[Joseph Garcia]

Supposedly, the alternator has a built-in voltage regulator, but if she's on her 3rd alternator, I'd agree with you in that something else must be in play here. Unfortunately, I don't know what that would be in a Sentra.

 

[Matthew Jeschke]

Yeah I stay away from most newer stuff so I really am at a loss .. especially w/ Nissan.

So the voltage regulator is still in the alternator?. I sure thought they moved it to the PCM on the newer stuff... Sounds like that is good news. PCM would be a mess.

She said they told her it was outputting over 15 V so def voltage regulator if the case. Hopefully new alternator fixes it.

 

[tom3]

Quite often that high voltage is due to a weak or sulfated battery. Does the car sit a lot? Original battery?

 

[Matthew Jeschke]

It's her daily driver. I'm not sure otherwise... Kind of wish I was the one who diagnosed it. I assume the shop went through a full alternator check.

I've always had a low voltage when the battery is weak? How do you get a higher voltage without more cells? Sulfation? I'm confused, doesn't that lower the current...?

 

[strutaeng]

From me watching SMA/Pine Hollow Auto Diagnostic, etc., the ECU controls the duty cycle of the alternator via PWM. So in the day traveling on the highway, the commanded duty cycle of alternator is pretty low, maybe 30%.

Conversely, at idle with defroster, full fan heater, high beams, maybe radio at loud levels (with that aftermarket bass bada$$ system you installed) the PWM duty cycle is probably 100%.

I had an issue on our 2013 Chrysler a while ago where the battery wasn't charging. I did a quick DVOM test and concluded the alternator wasn't charging, so I got a replacement... AND, same issues!

It took me awhile to diagnose that, but I eventually got myself an older scope, and used a known-good signal from my 06 Suburban to establish baseline PWM data, then determined the ECU on Chrysler wasn't commanding the alternator to charge...I eventually had to get myself a remanufactured ECU and that solved the issue.

I took some photos of the signal from the suburban. You could see the waveform change as more load was added (me turning on more electronics). On the Chrysler, there was basically zero signal.

I guess you could have something going on where the alternator is charging 100%, all the time. I believe on the Chrysler the PWM signal is ground-controlled, so if that wire shorts to ground the alternator goes full charging mode, IIRC. Pull a wiring diagram on the Nissan to see how they do their charging system are controlled.

 

[exp500]

Whichever way it is regulated, intenal/external both are based on reference voltage. To me that is 1st step.Battery vs ref voltage.

 

[Matthew Jeschke]

From me watching SMA/Pine Hollow Auto Diagnostic, etc., the ECU controls the duty cycle of the alternator via PWM. So in the day traveling on the highway, the commanded duty cycle of alternator is pretty low, maybe 30%.

Conversely, at idle with defroster, full fan heater, high beams, maybe radio at loud levels (with that aftermarket bass bada$$ system you installed) the PWM duty cycle is probably 100%.

I had an issue on our 2013 Chrysler a while ago where the battery wasn't charging. I did a quick DVOM test and concluded the alternator wasn't charging, so I got a replacement... AND, same issues!

It took me awhile to diagnose that, but I eventually got myself an older scope, and used a known-good signal from my 06 Suburban to establish baseline PWM data, then determined the ECU on Chrysler wasn't commanding the alternator to charge...I eventually had to get myself a remanufactured ECU and that solved the issue.

I took some photos of the signal from the suburban. You could see the waveform change as more load was added (me turning on more electronics). On the Chrysler, there was basically zero signal.

I guess you could have something going on where the alternator is charging 100%, all the time. I believe on the Chrysler the PWM signal is ground-controlled, so if that wire shorts to ground the alternator goes full charging mode, IIRC. Pull a wiring diagram on the Nissan to see how they do their charging system are controlled.

A PWM. Simple enough. Wasn't sure if it was fancy MOSFET setup or what... A PWM is fairly simple.

I think my sister said hers was overcharging though at 15+ volts.

Whichever way it is regulated, intenal/external both are based on reference voltage. To me that is 1st step.Battery vs ref voltage.

Is there a spot where the reference voltage can be probed? I assume on older setups that would be on the voltage regulator inside the alternator? Newer would be inside the PCM?

 

[exp500]

Set your meter to freq.or break out the Oscope. Otherwise, an advanced scanner usually can give you percent of pwm and usually voltages on both sides of device if you can look in right place, maybe Alt output/inputs and PCM outputs. Like how to monitor VSS circuits.
If there is a voltage loss at PCM internally or at ground then PWM would go hi correct?

 

[Matthew Jeschke]

Voltage might go high if reference is low if the circuit is based off of power, not voltage. I'd love to diagnose it myself but car is over 1000 miles from me. I know on my P01 PCM there's several 12v reference lines. However, they are substantially below 12V so no clue why they call them 12V reference. I checked two (or three?) P01 PCM(s) all with same results.