Well I had just finished welding up my subframe differential mount when I took it for a test drive.
(88 SE)

Well everytime i would apply the brakes the radio would switch off then back on and the headlights would dim.

I tried it with the headlights off and hit the brakes and the radio did it again.

Anyone got an idea of WTF is going on?

I haven't tested the voltage while running but the battery shows 12.60 volts when off.

 

what does it show while on/running???

should be at least 13.6 volts and preferably 14.5 volts

 

I'm gonna check it tonight. The voltage gauage has been acting up lately and will occasionally not work along with my back lighting.

It's a good battery which I pulled out of my GTU when the engine blew. The car had only been sitting for 5 days while I located a welder to fix the subframe.

 

OK,
off it is showing 12.50-60 volts
running it is at 13.7-9 volts with or withour headlights

Now when I hit the brakes it drops to around 11 volts and then semi quickly goes back up to 12.8 then around 13.7-9

Maybe my alternator went??? any ideas? I have another alt from my GTU in case I need to switch.

 

anything under 12.6 volts and your alternator is not providing enough power to run the car.

Anything under 13.6 volts and the alt can't charge the battery.

 

Hmm. I'll swap the Alt tomorrow and see if it fixes it.

 

My father was saying it could be a bad ground for the tail lights. This sound probably since it only drops voltage when the brakes are applied?

 

There are four 27 watt bulbs + an additional 8 watts that light when you press the brake pedal.

That is almost 10 amps of current. The alt in the 88 FC is only 70 amps maximum when new. At 100k+ miles they probaly only put out around 50 amps.

So just by putting on the brakes you are using 1/5th of the power that the alt puts out. Well alone if the car is running the alt is already using another 15 amps.

Your father doesn't know what he is talking about in this case. A bad ground on the brake lights and the CPU would report a brake light failure.

 

Well.
The voltage only drops when the brakes are applied. With headlights off or on and roadio on or off.

 

OK I got another test done. This time it didn't screw up at all.

Upon cold start up it pulled 14.1 idle.
Running w/ headlights on it was at 13.8-9 volts.
I hit the brakes and it only droppped about .1-.2 volts.

I then drove to my friends house 3 minutes away and came back out after 15 minutes.
I ran the car and at idle it pulled 13.4
With headlights on it was around 13.2
I hit the brakes and the voltage slowly droppped to around 12.7.

It looks like the voltage drops after the engine is warm. Is it a bad altnernator maybe?Which doesn't keep up once warm?

I am going to swap alt's tonight to see if it remedies it but any help would be great.

 

sounds like the alt is on its way out. lower voltage when warm suggests either poor engine grounds or a bad alt.

 

I put on a new alt and it is pulling the same voltage.
It hasn't done the drop to 11v thing yet but it has gone to around 12.6 with brakes(held for a while) and headlights on.

It seems like the lights draw a bit of voltage since it drops a slight bit (like .01-2 every 2 minutes or so).

Hmm where are the main grounds on the engine?
I could only find one small wire going from a bolt on the bellhousing to the firewall.

 

In my mind.....the main ground is the one attached to the starters outboard attach bolt. Big **** wire that goes from there to the chassis near/under the left strut tower and then on to the battery negative post. And of course the one you mentioned attached to the bell housing and then to the chassis near the wipers.

Check the output wire on the alternator. Like inspect the lug that goes on the alternator. Take it off and lightly sand it and reinstall it. See if it has a loose swage on the lug. Well I mean, make sure that the lug and the wire are swaged good.

 

It'd be even better if when you see the low voltage, get out with the car in that state and put your meter on the stud on the alternator (not the lug or the wire, the stud), then put the meter on the lug or the wire. IF the voltage is big time different....fix the termination of the wire and lug at the alternator.