Hey all i'm helping a friend out on tohis one. he's got an 83 z28, with a motor swap, 5spd trans ect ect. anyways, when his headlights are on, the tach completely maxes out, if the tach kept going, id say it'd be at the 12k range. however, when the headlights are off, it works like it normally would. engine isnt revving that high of course, and both his turn signal indicators in the dash stay lit up green, wether you turning or not. not a huge issue, just kind of annoying. any input would be great.

 

As there is no connection between the two systems , ground problem ( at IP ) sounds most likely

Do the lights flash correctly when operated ?

 

yeah signals work correctly, the lights on the dash both stays on

 

if it was a ground wouldnt it be constant? makes no sense that when the headlights are off it works properly being on differnt circuits

 

for the indicators in the dash staying on, check that all the park lights work. i don't recall if a park light problem in back will cause the indicators in the dash to stay on, i don't believe they will. but i know if the fronts don't work right the dash indicators will stay on when ever the park lights are turned on.
when this is a problem, it sets up a feed back so the indicators stay on so you know something is wrong.
with both indicators staying on means there is a problem of some kind with the parking lights on both sides of the car.
check at the front park/turn lights if the bulbs are burned out or for a bad ground.
its pretty common for the sockets to develop problems.
if it is a problem with the sockets, you find replacements at most parts houses.

on the tach, i agree with vetteoz.
a bad connection at the dash connectors or with the printed circuit on the back of the cluster can do strange things.

 

alright thanks guys, ill let him know. i know the rear parking lights dont do that, least on my 86 iroc. but i'll check his out and see what happens from here. not a huge issue like mentioned.

 

No because problem only arises when the headlights on, so something to do with the headlight circuit is causing problems with tacho
Connection to ground is the only thing they have in common
Ground problems are always fun to troubleshoot
if power can't find it's way to correct ground it can backfeed to places that shouldn't get power