Loosing tach signal

[dartguy340]

I've been removing a bunch of wiring left over from an alarm system and remote
door lock setup someone had installed in a car I bought, and noticed when I was checking
bulbs and outside lights last night that the tach went away when I turned on the parking or
the head lights! But then, when I turned the brightness wheel up on the dash display lights, it
came back on. Also, the tach doesn't even come close to displaying the actual RPM's. It's
off by about 1500(on the high side). I am thinking that it is loosing it's ground for some reason
or another. I looked for a ground to the instrument cluster,(it might be buried behind the
speedo where I can't see it), but haven't found one yet. Any ideas guys? Thanks,

 

[dartguy340]

I think I found the problem. I was experimenting with the WH-1
car that I have and I noticed that when I turn the dash lights
down(dimmer)to almost off, the tach bar lights go away. So I figure
that the rheostat that controls the brighness of the cluster bulbs must be bad.