Tach jumps when brights are turned on?

[rikubot]

Hi guys, quick question: my tach jumps to about 800-900 when i turn on the brights in my car. The car is off, lights are on, then I hit the brights. Is this because of the 12v feed going into the tach, or do I have it on the wrong fuse? Thanks bros

[John_Kunkel]

Where do you have the tach 12V wire attached?

[rikubot]

I have it wired into the second little tab. I'm going to double check tho cuz my memory is questionable

[rikubot]

Here's the link I followed:

http://www.1970chargerregistry.com/mboard/index.php?PHPSESSID=f4a86fb79939cd9e382548a7458146b2&topic=40.0

[rikubot]

Bump

[rikubot]

Bump

[Bronzedodge]

Well... what about when the car is running - does it act up then also?  Something is amiss, My guess would be a bad dash ground perhaps.

[rikubot]

I'm not sure, but I will check as soon as I get off work and report back. I should also note that it looks like my tach might be a couple hundred rpm slower than what the motor sounds like it's turning

[rikubot]

Sorry for the late response, work has been a crap shoot. I hit the brights while she was idling, and the tach stayed consistent. I do have another issue: my trash seems to read a couple hundred RPMs lower than what it sounds. Any way I can check this or fix this?

[2002TransAm]

Sounds like a bad ground to the dash body or engine if you make a jumper wire and hook
It to the ground at the battery and go to each location and then turn the brights on see where the tach stops jumping. I would expect it to be the body ground by the headlights and its backfeeding through the tach dash ground circuit. As for testing the tach rpm setting a nice timeing light will show rpm or a photo tach from the parts store will tell you rpm.