97 expedition 4wd eddie bauer edition. 5.4 2v v8 250xxx miles

Long story short, voltage gauge drops to 8/9v when i hit the brakes or use turn signals. When this happens i lose power to my interior lights, brake lights, radio windshield wiper and cluster dims. Alternator seems to be good, ran the truck without the battery and it did not stall. Battery tested fine as well. Problem was intermittent up untill last week. Cant remove the alternator at the moment to bench test it as its my daily, any advice?

 

Not a good idea to run an alternator without the battery connected. Can wipe out the alt.
Your daily driver has a poor connection, and it could be in the power side or the ground side.
Check and clean any and all grounds you can see coming from the battery negative terminal to the sheet metal, also a good check of the fuse panel(s) both under the hood and inside the left side dash . Corrosion in either place will give the results you are seeing..
If there were a possible ,not completely, shorted circuit it could produce similar symptoms, but hopefully it would blow a fuse first..
One way to attack the problem would be to connect a voltmeter to the stoplight feed wire at the rear of the vehicle and see what voltage is present there when stepping on the brake. It should read 12 volts , if less, problem is in power side of circuit. You have to make sure the voltmeter has a good connection to ground and you can place the positive probe on the bulb case and the negative probe on a clean part of the chassis . If you get any reading the problem is in the ground side. You can do it !!

 

Im not a wiz in electronic wiring but i checked the brake light wires for continuity and the multimeter showed low resistance and then 1257 (i assume is voltage?) its been very foggy and moist recently. Maybe my problems are coming from the GEM with moisture trapped inside?