Ok , so I have a 89 Gta 5.7 tpi . I have a new bat a 140 amp alt new bat cables and terminals. Iv cleaned every ground in and outta the car I can find. The problem I have is the car charges , it's reads as it should at the bat and back of alt , but with everything on in the car the dash will get as low as 8 volts and the interior dash lights are dim when this happens. But if I go test the bat and alt the voltage is 13.5 or higher . I replaced the instrument dimmer switch as well thinking that was it . I'm stumped and it's really pissing me off lol I just got the car so I'm sure it's been a on going issue . Any help will be greatly appreciated !!!!

 

You might take apart c100 look for corrosion.

 

What's c100 ?

 

A bulk head connection

 

You have a bad chassis ground.
Check the pigtail coming off the negative cable... it should screw to the fender or wheel well.

 

Ok I'll check the main connector , and the bat cables are new ground is good on fender new firewall ground cable as well, cleaned head light grounds the fuse box grounds the rear trunk grounds the ground on fender by relays next to master cylinder . I even added a ground strap from frame to a alt mounting bolt just in case

 

I did notice there's a few relays etc by the master cylinder that have the same tan and white wire that is on alt plug . The two relays run together and are soldered to the tan and white wire in the wire harnes running to the c100 . Not sure if they are all supposed to be tied together and soldered onto that wire ? They all same color tan and white

 

the relays bye the master cylinder are the est module, maf burn off relay, fan relay and fuel pump relay. the alt sense wire goes to the fan relay. here are some schematics that may help the colors on a bird should be the same.


http://www.austinthirdgen.org/index.php?pid=19

 

Ok so I fixed the shitty wiring by the fuel pump relay , Iv checked every wire I can lol its getting voltage on all the wires coming out of alt but from there it looses voltage the car still will run but the interior lights and if I have air on the blower will dim and slow down like its not getting power , the c100 is tight . Stumped

 

Are you sure the alternator isn't intermittently cutting out?
The small brown wire going into the alternator plug provides power from the ignition switch to "excite" the alternator and get it to charge.
Without power to this circuit, the alternator will just free spin.
I had a break in this wire, which wasn't easily found because the insulation wasn't broken.

 

Both wires have full power

 

Checked starter wires all are good .... Still stumped . The bat alt and starter all have 14.44 when car is in park and nothing on or running . 13.86 with lights , Ac and on break

 

the alternator voltages you measured are perfect-
if the block to body ground is good,must be something
under the dash-with the voltage drop you describe,
something will be getting very hot pretty fast-check
the ignition switch,bulkhead connector,behind fuse
panel,block to body ground for abnormal heating...
some warmth can be expected at ignition switch and
it's connections,but if burning hot there is trouble.
You can check the soundness of the block to body
ground by starting engine and with electrical loads
turned on,measure voltage between good clean
connection areas on block and body;if more than a
few tenths of a volt are measured the block to body
ground path is weak.

 

Grounds good no getting hot , with everything on the bat/alt voltage reads 12.8 fuses read 12.20 ? I found a few wires behind fuse box that were all tied together and one hot wire just chillin , I figured them out except for on the side of fuse box a fuse holder separate but that connects to the fuse block casing which has a red wire and a small red with white stripe wire on it I have no clue what it controls everything in car works even with it disconnected , but it was hooked up before to the mess

 

At this point I'm about ready to either drive it off a cliff or sell it ... I sold my 500rwh 2000 Ta for this car in hopes of making it faster and reliving my youth ... I feel hella bummed right now

 

Ok , here's where I'm at . The main connector connecting interior to engine harness had issues , if it all the way tight I'll get no interior power if it's alittle loose I can move (the engine side) cables and the power will come on , I took it apart and cleaned it all the Fi electric grease was hard and gross , wire brushed terminals clean , hook it back up same thing , there's not much to this thing am I missing something ?