Tachometer interference?

[65-440]

Just installed a new tach from charger specialties in my 68 coronet (standard dash), with the ignition off the needle slowly sweeps backwards and settles in around 6500rpm..key on it jumps to zero. Needle is erratic with engine running, bouncing around pretty good. Called them and one of the guys is saying it's the magnet from the dash speaker causing interference. Makes sense seeing as the needle is pretty much pointing in that direction with ignition off. Has anyone else experienced this and if so what is the fix? I've tried shielding it with some metal plate but no luck at all.

 

[Don Frelier]

Try lead

 

[Sinitro]

Just installed a new tach from charger specialties in my 68 coronet (standard dash), with the ignition off the needle slowly sweeps backwards and settles in around 6500rpm..key on it jumps to zero. Needle is erratic with engine running, bouncing around pretty good. Called them and one of the guys is saying it's the magnet from the dash speaker causing interference. Makes sense seeing as the needle is pretty much pointing in that direction with ignition off. Has anyone else experienced this and if so what is the fix? I've tried shielding it with some metal plate but no luck at all.

Hmm..
Very strange but makes sense.
1st I would uninstall the front speaker
and then check what happens??
A possible solution would be to install a 2nd magnet being
reversed on the front speaker. Note that this solution was
done in certain multi-media desktop speakers when their
magnets caused color distortion when near
a CRT monitor. But again U need to confirm
the situation then decide what to do.
Another solution would be to install a metal cannister
shield(grounded) over the rear side of the tach. Or install
a metal shield over the front speaker's magnet...

Just my $0.02...

 

[dadsbee]

Had an electronic tach in a customers airplane that would go full scale when you keyed the mic to talk on the radio. That puppy went straight in the dumpster!

You'd think if Charger Specialty knows there is such an issue they'd build shielding or required shunts into their unit.

 

[kiwigtx]

Try lead

You mean that guy from FABO?

 

[kiwigtx]

Hmm..
Very strange but makes sense.
1st I would uninstall the front speaker
and then check what happens??
A possible solution would be to install a 2nd magnet being
reversed on the front speaker. Note that this solution was
done in certain multi-media desktop speakers when their
magnets caused color distortion when near
a CRT monitor. But again U need to confirm
the situation then decide what to do.
Another solution would be to install a metal cannister
shield(grounded) over the rear side of the tach. Or install
a metal shield over the front speaker's magnet...

Just my $0.02...

I like the idea of a metallic shield around the body of the tachometer.

I also wonder if a screened cable between the coil (origin of tach input) and the Tach terminal might help.

A few years back I had to use a screened cable on a Hot Rod for the speedometer output (Hall effect) and the tach due to interference from the HT leads and coil. That worked a treat, and I was able to calibrate that speedometer easily after the interference was gone...remember to only ground one end of any screened cable.

 

[Speedbird]

I'm thinking this?
Sounds like exactly the same symptom. They don't tell you and must not even know about it.
Tach installation