tach problems

[82stangracer]

i installed a 3911 sport comp tach on my 82 and after about 6 months of working fine with no problems from either tach the sport comp starts reading only about half the rpm. then my factory tach started showing no reading. it comes and goes. sometimes they both work, sometimes its just one or the other, and sometimes neither works(the sport comp always reads something just not always correct). it really confused me since they are wired in two locations. (factory to dist. and s.c. to coil) i have looked for broken or loose wires and havent found any. its also embarrassing when people want to know why my $220 tach isnt working

[82stangracer]

yaw got any ideas on the promblem?

[SVT Rob]

You should try running a new wire from the tach directly to the coil. Most likely have a wiring issue. Check your grounds.

[82stangracer]

You should try running a new wire from the tach directly to the coil. Most likely have a wiring issue. Check your grounds.

would bad wiring from the coil to the tach mess with the factory tach also or do you think i have 2 problems

[82stangracer]

is there one thing that could cause 2 tachs, that are wired to 2 different locations, to malfunction or do you think i have 2 seperate issues.

[82stangracer]

any other thoughts?

[SRM351]

Well if I'm reading this right your tach signal is actually coming from the same place, it's just measured at either end of the wire between the coil and dizzy, correct? Assuming that's true and there's no symptoms of lost spark when one or the other tach is malfunctioning, I'd check out the circuitry that powers either gauge and see if maybe you have a loose connection somewhere. Like a previous poster said, bad grounds are often the cause of this sort of thing.