Electrical Loss When Brakes Are Used

Hey trying to clear some issues with the new subie.

 

When I hit the brakes, the blower slows, dash dims and resume when the brakes are released.

 

Alternator?

 

1997 legacy outback limited 2.5h4dohc

chack all of your exteriot lights,

especially your rear brake and turn signal lights.

if they all work well,

remove one at a time until the problem stops.

you suspect one is shorting out whilst still applying power to the filament?

Checking it now.

It makes sense that it might be a shorting, but taking the bulb out won't affect the loss if there is a short.....

 

Electrical is always fun.... Good luck!

Electrical is always fun.... Good luck!

always.

 

and i'm no expert.

on the contrary, i know some basics of home wiring,

but very little about 12 volt automotive systems.

 

one thing i have heard / learned,

in lots od car circuits,

the negative wire is the one that is switched.

this makes for some complicated circuit issues.

and truthfully, ones i do not know much about.

 

but what ever the problem,

it is after the brake pedal switch.

 

 

the double filament bulbs on the rear can cause really weird thing to happen when they mess up.

pull the right bulb,

if the problem goes away ,

that is a big clue.

Drain still occurs with bulbs out...one or all...

 

the cable from the brake light switch at pedal to the lights must be shorting somewhere...another thing to note is the shift lock solenoid stopped working when the dimming started occuring...shorting solenoid?

I have this same issue and I believe it's related to my after market stereo as I've tried moving it and the shifter started clicking rapidly. Unfortunately for me I couldn't find the short...

K so I tested ground continuity on all the brake system wiring and there does not seem to be any shorts. (Solenoid for shift lock was unplugged slightly)

 

When I turn on certain high power items the power levels dim...rear defrost; bringing up windows. When I hit the brakes and then turn the rear defrost on you can here the further dim in the blower and see the dash dim out

 

New alt/bat i should think

I have the same issue. I was thinking about doing the big 3 upgrade to see if that helps.

It does sound like a bad charging system. What you can do is install some LEDs on all tail lights if you're on a budget. My car used to dim the headlights even with a new alternator. Problem was solved installing LED on the tail lights. They are even brighter, cheap and last forever.

I suspect charging system as well...i've been searching for a higher output generator.

 

Agreed; the electrical load seems to overpower, especially in winter when everything is blasting...cut electrical usage with leds might work.

 

Another note...If I stay on the brakes for longer than 3secs i can hear everything spool back up, and it returns to full even with brakes on...like the alternator turned on a switch to delvier the missing power.

 

Odd issue

This is normal. The ECU recognizes a voltage drop and accelerates the engine to a new idle RPM. The difference is minimal (a 50 or 70rpm increment) but it is enough to restore voltage. If you hear carefully you will notice the engine accelerating a bit. It is so little you can't even see the needle move in the rev meter, but you can hear.

So if the engine is brought up to compensate...means the alternator is under powering for full systems use at lower revs?

 

I thought any RPM above idle and the alternator puts out same power...doesnt the voltage regulator in the alternator control this?

My wagon is having a similar problem. I notice the headlights dim for a split second when I hit the brakes. I put a new a remaned OEM alternator on a few months back and my battery is about 2 years old. Now I have some ideas to go look at.

I've noticed this as well. I figured it was I was using the brakes which run on the master cylinder that in turn runs on a vacuum. So using the brakes increases the demand on the master cylinder which requires a greater load on the engine. But that's just my guess.

I suspect charging system as well...i've been searching for a higher output generator.

 

 

2006 outback H6 has a 110A alternator. only difference is the plug, youll need the 3 pin green plug from a 2000+ model for it. im in the process of doing this swap as well, when i get the wiring all worked out and the rest of the stuff i need for the redo of the big 3 i will post pics and the diagrams up.

So if the engine is brought up to compensate...means the alternator is under powering for full systems use at lower revs?

 

I thought any RPM above idle and the alternator puts out same power...doesnt the voltage regulator in the alternator control this?

 

Power = Voltage * Amps

 

The RPM of the alternator controls how much power you put out, so if your RPM remains constant but like you in your case your amps goes to high to power all the accessories, you're gonna get a voltage drop which is what you're seeing. The ECU will bump up the RPM to increase the total power but the voltage regulator will keep the voltage constant. This increases the total number of amps you can source, but won't change the 12v constant.

Perfect explanation..... I only knew that the service manual specifies different RPMs for different amount of systems turned on.

 

 

As for the vacuum if there is a leak in the vacuum system (when brake pedal depressed) the RPM can drop a bit too much and the dimming is actually caused by that instead of the lights consuming too much power.

I had this problem some time ago. I fixed it by finding that my positive connection at the battery had corroded almost in two. Fixed that and the power fail on brake use went away. Probably could get the same thing with the negative at the battery.

Mine was doing this not too long ago, when I hit the brakes my head unit would turn off and turn back on. Turned out to be a dead cell in my battery, would have perfectly fine voltage but couldn't handle a load. Changed out my battery and cleaned all my grounds while I was under the hood and had no more issues. You might have a short somewhere or excessive resistance too but I would double check the battery too, might be on its way out.

I forgot that I was getting the head unit reboot as well at that time.