car in question is a 85gs if that matters.

My tach will bounce around like crazy when my headlights/parking lights are on. Also if I turn my hazzards/turn signals on the tach will get even crazier and bounce between 500-2,2000 rpm.

What would should I do to fix this, I'm pretty sure there is a short or some sort of electrical problem, but I have never really done any electrical work on a car and am very newb when it comes down to that so I need all the help I can get in english since I'm a electrical newb.

Also on a semi unrelated note, when I push on the brakes the cars idle will get drop (don't have any hard numbers cause of the tach problem) Could I have a vacume problem/leak and how could I diagnoice where and what to fix?

Thanks -aaron

 

Tach bouncing at idle, with a large strain on the charging system, is actually quite common. Check your grounds, and the smaller wire that attaches to the positive battery post. That small wire is the only place where everything EXCEPT the starter gets power.
A slight drop in idle speed when the brakes are pressed can be from the drag of the alt (brake lights) or a vacuum leak.

 

i will take a look at the ground on my batterie, what does it look like I don't think I have ever noticed it there before ?

what would I need to use to check for a vacume leak ?

 

Mine acted like that (somewhat) when I lost a fusible link. But it wouldn't crank afterwards either so that's probably not it. Clean all connections and try again...

 

replaced all fuesable links within the last six months, checked them recently and they all look fine.

how do I clean the connections and which connections to clean ? lol if that makes sence...

 

Battery and starter, and where the ground wire attaches to the strut tower...

 

my 79 was doing that when i got it, it turned out to be the points, which any 80+ car doesnt have, but it still coule be a loose coil wire, or ignitor connection

 

I had the same problem as you. I had to rewire the lines to my fuel pump to get it to rum. It shorted at the fuse panel, so I wired in a breaker. I still havent figured out how to fix my tach yet though. Its getting its signal from the ignition, and it gets ground. I figured it was internal and gave up.

 

I am also have the same problem and I had clean my battery wires. I also replace my sparkplugs and sparkplugs. First of all I lose my oil pressure and so I replace it with a mechanical one. Now my tach is going crazy just like notveryhappyjack said and this is on a 84 GS. I gave up after a while but occassionaly it makes a clicking sound and my tach stay down reading zero. When I give it gas and drive around it gets fixed and sometimes it start to act up again. Weird huh?

 

You know, probably 90% of these issues could be tracked down to the crappy battery cables installed when new. They tend to corrode internally, so you don't always see the problem before it causes issues. I've added a number of extra ground wires, and have added mostly all new wiring to all other power sources bypassing the battery cables. I still use the original cable to power the starter, but all else has been rerouted. I just thought I'd throw that in to confuse matters even farther.