Strange symptoms
 

Have 1985 Supra with auto that has been running great. Two days ago the dashboard brake light came on and has yet to go out. Next once the car is warmed up it surges from a high idle to a low, if you put the car in gear it kicks back to a high idle and runs but also about the time the brake light came on the low voltage light at the dashboard gauge now stays on and it using the battery for juice to run. The first thing I replaced was the oxygen sensor no help. The alternator could be frozen up but why would the brake light come on. The motor still sound great when cold, HELP??

intake manifold leak?

does the engine bay go pshhhhhhhhhhhhhhhhhhhhhhhhhhhhhh in there? while idling?

brake light: you haven't got a blown bulb? are you talking about the brake warning light that comes on the dash or literally a brake light is coming on?

have you checked the water in the battery?

get an auto sparky to quickly check out ur alternator to make sure it's charging

when it come my brake light on the dash board. first thing i check is my brake fluid... if its low then yeah it does that on us the mk2 owner... fill it up to the position... n double check the brake light again. if that doesnt help n the light still show then double check on your alternater.... if those doesnt work then check on your head light n ect... where all the light bulbs at.... if still didnt work then go to the shop... for electricity testing... i mean dignos.... hope this work for you. it work for me tho... hehe. have a nice one... n let me know how you get to work alright. thanx.

not alternator
 

Alternator ok, after disconnecting battery to charge, guage and dash brake light functioned perfect. drove car for 10 miles and light started coming back on along with a red light on the dash voltage guage. I'll try disconecting the battery again to see if will reset itself again. Unfortunatley still searching???

Pulled negative off battery after charging all night and vehicle dies immediately,

try contactor at alternator since pulling and checking alternator corrected the problem for a short time.

just make sure while you are at it all grounds are tightened well :)

i pretty sure its your aternater then... n another thing is maybe your batterry too... cause thats what it use to be happen on my mk2 way back than. hope i right... hehe

oh one more thing... double check on that one wiring on the alternater that were tight up with the washer or bolt i didnt remember... but not the one that you plug it in to the alternater... this is what it happen on my alternater tho... that wire n the clip that clip on the bolt it got broke... n it didnt connect tight to the alternater... so it happen on me to... n it make me buy a new batterry... cause i have my battery for couple years... n i toght its the batterry but after i got new one i put it on n it die in the next few min... n i untight the alternater n i found out thats the wiring that cause me the trouble... n i recut the wired n rewired the wired n it work.. n now i have two batterry now... hehe... hope this will help you little bit...

Fixed
 

Turned out to be the alternator, the first time it was tested it passed. Took it out again and to a differant store were it failed. New alternator fixed the problem. The brake light is tide into voltage for all instrumentation and thus the reason for it to light up on voltage warning light. New O2 sensor makes a noticable differance, will update the increased mileage.
Thanks for the help!

you welcome