voltage increase when brake pressed

Hi my name is Jason.

 I have a 1991 Mazda familia interplay, when the brake is pressed the stereo lights dim a bit along with the dash lights at night when the head lights are on. I have checked the voltages in different places and at the battery to. When the car is idleing it is 13.8V and when the brake is pressed it is 14.2V. The battery is only two weeks old.  I have replaced the earth wire on the battery. When the plug at the foot brake is unpluged the problem goes away, maybe their is something wrong with the brake light circuit or something. Any help with this would be great

Thanks Jase

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jrkerley

Have you had your alternator inspected, for proper current output?

If the alternator passes the AVR test. Then either the bulbs are drawing down too much current, or there is a fault in the circuit.

If the lights dim only slightly, on your model year vehicle a little dimming is normal, and I would agree with Rage237 in that regard.

You could change the rear bulbs to a lower wattage, to see if there is a difference. If there is a difference, then either one or more of the bulbs are about to expire.

Regards

EVIL Teken . . .

go to autozone or a similar place.  theyll test the charging circuit for free.  you could look for corrosion on your own or just upgrade the factory wiring under the hood for the batter grounds and alternator wire to something like 4gauge with some ring terminals.  i know a lot of GM vehicles use 10gauge wire from the alternator to the battery.  but then the batter has 4 gauge to the starter.  so think about it, if you have anything more that 10 gauge running to you amps... is it worth it?  you only get power from the alternator (which is running the whole car) through 10gauge wire... but the stereo has super duper 0/1 guage... does that make much sense?

thanks for that I have taken the brake bulbs out and when i did the problem got better although the voltage did change a bit but not alot so maybe its time to get new bulbs and see what happens. I haven't had the alternator tested yet that was next on the list. I thought about changing the wire from the battery to the alternator at some stage if all else fails I might have to do this

thanks for your help

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jrkerley

Are the earthing points for the rear lamps corroded?

does your car have cruise control, and - does it work properly?  voltage INCREASING when you step on the brakes is the reverse of what it should be....  if you have cruise but it doesn't work right, and/or you said when you unplug the wire at the brake pedal the problem is gone (i assume you mean the voltage goes back up to 14.2 at idle with it unplugged) then you've got either a short in the brake pedal switch, or (for one reason or another) a high current drain at the cruise control.   try doing this - if you have a multimeter with a 10A range on it, pull the 12v wire from the brake pedal switch, hook it to your meter, then the wire from your meter to the brake pedal switch.  it should be pulling 0amps with the pedal up and probably about 4-6 with the pedal down.  if it's pulling current with the pedal up, check the switch itself (move it around with your hand, see if readings change) for shorts.... you might try removing the switch from its mount and see if holding it away from anything metal, if it still pulls current...... replace the thing if it's drawing current in the up position and you can't find any obvious damage or flaws with it.   btw - new fully charged battery sitting static with key off and no load should = 13.8 volts (2.3 volts per cell on standard lead acid batteries) and around 14.2 when charging (can be higher or lower depending on temperature - but that's the base level to make your comparisons on - just so ya know)

well I have replaced the bulbs but it didn't change anything. the next thing to do would be to check the earthing points for the rear lamps, which i just have to find? The car does not have cruise control. the car is really strange I just checked the voltage when idleing and it is about 13.8 volts and when the headlights are on the voltage goes up to 14.2 volts????????  I haven't checked the brake switch yet to see what the current drain is but I think that the problem must be between the battery and the alternator.

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jrkerley