Need a diagnosis for an electrical issue please

[reznunt]

Setup: Dual batteries, both less than 6 months old, one standard and one AGM, connected with a Blue Sea 7610 ACR and switch. ACR works perfectly, charges when it should, disconnects batteries when it should. All 2 gauge wiring and multiple grounds to frame/block/body. Haven't had any dead batteries, truck hasn't died, just recent peculiar activity with the charging voltage.

Background: Cold starts, alternator puts out 14.2-14.3V until engine is warm and the output slowly drops for the next 10 minutes until leveling out at 13.5 to 13.8V. This is the average charging voltage (for my truck) for the duration of any trip with or without any heavy loads. Others on this forum state just over 14V is what's normal so I have a rebuilt Denso ready to drop in.

Issue: When I apply the brakes while in motion the voltage reads 13.5V-13.8V but as soon as I come to a complete stop with the brakes still applied (such as at a stop light) the voltage drops to 13V. As soon as I release the brake pedal to get going the voltage returns to 13.5V-13.8V. If that isn't weird enough,
when I apply the brakes and come to a complete stop (say to park in a parking space) the voltage drops to 13V, but when I put it in Park and keep the brake pedal depressed the voltage jumps back to the normal 13.5V-13.8V. I don't feel any rough idle during the voltage drops. No dimming of the dash lights, etc. Only the voltmeter tells me something is wrong.

Summary: Trans in Drive, brakes applied at complete stop, voltage drop... brakes released, voltage returns to normal. Trans in Park, brakes applied or not, voltage remains normal.

What's the deal? Almost certain once I swap in the new alternator all will be well.

 

[bjarnep]

sound quite normal - with trans in drive and with brakes applied, the RPM drops and therefore the output of the alternator drops. In park you will have higher idle RPM and therefore normal charge.
The alternator need an minimum of speed to produce enough voltage to charge

 

[Sandroad]

I agree with @bjarnep the voltages sound normal. The voltage is higher right after you start because the alternator is recharging the battery from the starting. That happens fairly quickly and then the voltage drops to a float charge rate just like you indicated. And, as noted above, the alternator output will normally be pretty low at the low idle speed when stopped in D. That, along with the lack of any problems as you state in the setup, indicate you don't need a new alternator. I think all is well.

 

[PADDO]

FSM spec for standard, no load voltage measured at 2000rpm is 13.2-14.8 volts. Me thinks all is well too.

 

[reznunt]

Thanks for all the replies. Forgot to mention that the drop in voltage in Drive with brakes applied only occurs intermittently. This is why it concerned me. Sometimes it drops to 13V, sometimes it stays constant.

Regarding the float charge, there was another thread here where everyone was saying 14.2-14.3V is normal and our 100's are supposed to always charge at. If my 13.5V-13.8V is normal I'll just sell the new alternator. I'm almost at 100k now.

 

[abuck99]

It's possible the brake booster is cycling to build pressure during full stop, that motor draws some amps, at the same time your foot is off the gas, in drive, rpms are down so then alternator isn't putting out much juice. Just a thought.

 

[reznunt]

It's possible the brake booster is cycling to build pressure during full stop, that motor draws some amps, at the same time your foot is off the gas, in drive, rpms are down so then alternator isn't putting out much juice. Just a thought.

I thought that too but I listened and the booster does not come on and off concurrently with the voltage drop. Soemtimes yhe booster does not come on but the voltage still drops. The voltage drop and pedal press/release seem to be correlated.

Since I have the alternator here and don't plan on selling it, I'll just drop it in when I have time and see what happens. I'll report back with any findings.

Thanks for all your help!