Voltage drops from 13 to 12 on center console voltage gauge when brake is pushed in. Where do I look first? 75 2.0

Phil

Probably all the chassis grounds.

Mine have done the same ... you're powering lights up and they draw volts ...

Being a barbarian, I ignore it.

Voltage regulator?

Normal. Cleaning grounds and other electrical connections is a GREAT way to 'spiff up' the electrical system. But because of the way that the voltage meter is wired into the system, it's normal for it to drop as higher wattage systems are enabled (wipers, headlights, blowers, etc).

does the voltage drop with other electrical loads also? if so load test the battery, even if it's weak (which would do what you're seeing), do a voltage drop on all grounds also.

I agree with McMark -- normal. At least, mine does the same thing. My needle jumps around when i turn on my turn signals too.

I think it's an advantage actually -- when the voltage drops you know your brake lights came on, which means the brake light switch on your pedal cluster is properly adjusted!

Some small drop is perfectly normal. I would expect the biggest drop when you hit the horn or high beam.

To diagnose better, you can check the voltage drop on the regulator itself. If none, clean the ground straps. If the drop is the same there, check the wires to the alternator.

A voltage meter is misleading a bit If you have an ampere-meter, you could monitor the charging state of your battery and realize if the alternator is keeping-up with the current load.

The voltmeter is wired through the brake light circuit, it's normal to see a drop in voltage.

I also see it as an advantage, I look at the gauge to make sure my brake lights are working.

Totally normal. Turn on the turn signal and have some real fun... up, down, up, down, up, down...

Corrosion on the ground points and within the wires them selves. Top that off with rust at each welded seam. The chassis only makes its ground path through tiny little spot welds. To avoid the drop, run a new wire directly to the battery.

Corrosion within the wires is an interesting possibility. I've seen this in home speaker wire. I haven't yet seen this in my 914. Not yet anyway.

Yep, exactly. Also noticed the same with the signals as well.
Nice to know i'm not alone. I'll still take a look at the v.r.and grounds but seems to be running great otherwise. And 32 mpg on first tank of gas since setting valves and timing. That's drivig to and from work 20 miles @ 35-45mph. Pretty good I guess, - anyway thanks for the input!

rear tail light bulb sockets and the associated ground in the trunk.
Both need to be cleaned up and cleaned out (sockets) every 40 years.

rich

I remember calling McMark when I noticed this on my own car. I think he told me to keep my eyes on the road!

Normal. I use a very small battery called an Oddesey and it doesn't cause a problem... It never drops below 12V with the headlights, fog lights, brake lights and, turn signals on at the same time. It's just normal draw.

Start exploratory surgery on your wire harness. Anything other than shiny copper is corrosion. Automotive insluation is cheap and pourus. Moisture gets in and the copper turns black. Wire actually has a life cycle. Our cars are way pass the life cycle of the electronics.

OK, don't actually cut into your harness. There is corrosion there, you just haven't found it yet...

Excessive resistance through the brake light switch? My car was doing this, brake lights were very dim and the switch was very hot to the touch. I pulled the switch off, opened it and cleaned the contacts.

most of the advise above is good but I recomend starting in teh fuse panel first
4th fuse from teh right is corroded
...they are all corroded and dirty. wipe the fuse ends on a green scotch bright pad. only wipe in ONE DIRECTION to prevent the fuse from coming apart. apply very light presure. should be clean in about 5 swipes
then clean the copper contacts.
that should fix many minor issues, if not proceed to the lamp sockets and chassis grounds
do the batt to chassis gnd first, then teh chassis to trans gnd strap

like Rich said, " once every 40 years"

I just find it funny that alot of the issues that come up are 40 year issues.
Dirt, corrosion etc.

First time I did the rear tailight sockets I was shocked at how much crap was in there...I even looked into buying new sockets to replace them.
that was after chasing all the circuits..fuses etc...
A few minutes with a dremel/wire brush and some dielectric grease...done.

I now just do it whenever I get a new car....all them porsches seem to lack a decent enough ground....and it creates loads of corrosion over the 40-50 years of never being cleaned.

First thing I do now...find, remove, and clean all the grounds.
There are what...10 maybe...I think there are less, but its pretty simple to do once you have the dremel setup...

Distributor grease....same deal.
I chased this for a long time...replaced all sorts of stuff on both the ignition and FI...turns out it was just dirty advance plates.

Sometimes the answers you get here may be short and sweet, but I agree with everything above...and it comes from all our collective wasted time chasing ghosts.

Rich

is there a way to take one of the wires off the back of the guage and wire it directly so that it is not on that circuit. I also was wondering that? My guage does the same thing.

My guage rests just under 14 since i upgraded the electrical system, and it drops to 12.5 w/ the brakes on.

What gauge?
If you unplug the volt gauge....life goes on and the car runs fine.
If you want it on another circuit, just sire it to another circuit.

Its a simple setup...I think post 73 all the cars had the wiring for the gauge in the center console...some cars (mine) had no console...

Should be a plug you can unplug right under the flat shifter panel cover...just pull it up and unplug the gauge..

Rich