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

 

[Don Frelier]

In the US it's called shielded cable.

 

[65-440]

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

Yes! Ok so all you did was add another ground? That screw into the metal socket is supposed to be the ground to the gauge as it is....your gauge rests at zero now with ignition off?

 

[RemCharger]

What kind of ign components? I wonder if charger specialties starts with an autometer tach.

 

[Speedbird]

Yes! Ok so all you did was add another ground? That screw into the metal socket is supposed to be the ground to the gauge as it is....your gauge rests at zero now with ignition off?

Exactly. After installing needle moved to 2 o'clock with key off. Turn key on needled moved to 7 o'clock and stayed till key off.
Then with key off gauge returned to 2 o'clock. Very disappointed till realized that the gauge MUST have a ground some way.
And it's an all plastic housing.
If not via the screw to light socket, is there some other tach metal touching the metal housing like an OE would to provide a system ground?
I don't think so.
Should have thought of that during bench assembly process.
I've spent untold hours working in a avionics compartments installing shielded wires and can't imagine that's a problem with the gauge.

 

[65-440]

Exactly. After installing needle moved to 2 o'clock with key off. Turn key on needled moved to 7 o'clock and stayed till key off.
Then with key off gauge returned to 2 o'clock. Very disappointed till realized that the gauge MUST have a ground some way.
And it's an all plastic housing.
If not via the screw to light socket, is there some other tach metal touching the metal housing like an OE would to provide a system ground?
I don't think so.
Should have thought of that during bench assembly process.
I've spent untold hours working in a avionics compartments installing shielded wires and can't imagine that's a problem with the gauge.

Man, I'll do this Monday and see what happens. I don't see how a ground wire will stop the needle from being pulled by the speaker magnetic but definitely worth a shot! Sounds like we had the same exact issue and that added wire worked for you....I'll keep you posted!

 

[65-440]

What kind of ign components? I wonder if charger specialties starts with an autometer tach.

I'm running pertronix igniter 2, the main issue is the needle pointing at 6500 with the key off!

 

[RemCharger]

I'm running pertronix igniter 2, the main issue is the needle pointing at 6500 with the key off!

I differ in opinion, your main issue is the needle is erratic while running. Which would point to an ignition compatability problem.
Many tachs point oddly when shut off, and zero out when turned on.
Need more info about the production origins of the tach.

 

[Speedbird]

Man, I'll do this Monday and see what happens. I don't see how a ground wire will stop the needle from being pulled by the speaker magnetic but definitely worth a shot! Sounds like we had the same exact issue and that added wire worked for you....I'll keep you posted!

I doubt it's a magnet problem. Note how close mine is to the speaker. And I'm using an aftermarket dual speaker with a FM converted radio.

Radios for antique and vintage cars

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garytayman.com

If you are working under the dash be sure and do not losen the screw any more than absolutely necessary to slide a forked spade terminal under it.
Small is better.
I was nervous about the screw coming all the way out which should be a nightmare.

 

[RemCharger]

I doubt it's a magnet problem. Note how close mine is to the speaker. And I'm using an aftermarket dual speaker with a FM converted radio.

http://[URL]https://garytayman.com/

[/URL]
If you are working under the dash be sure and do not losen the screw any more than absolutely necessary to slide a forked spade terminal under it.
Small is better.
I was nervous about the screw coming all the way out which should be a nightmare.

Very interesting. Was that the factory bulb holder? Did the light work before the addition of the ground?

 

[Sinitro]

In the US it's called shielded cable.

View attachment 1831972

Good point!!
We use the Belden foil alot for sensitive audio circuits. It has the traditional copper braided shield and all wrapped in a foil shield,
provides excellent shielding for interference from close by components.

Just my $0.02...

 

[65-440]

I differ in opinion, your main issue is the needle is erratic while running. Which would point to an ignition compatability problem.
Many tachs point oddly when shut off, and zero out when turned on.
Need more info about the production origins of the tach.

You're not wrong, it's bouncy when mounted in the dash...when I took it out and hooked it up under the hood with jumper cables it's was far more steady. Still jumped but much less. They sent me a replacement, will be to me on Monday and I'll mess with it some more.

 

[65-440]

I doubt it's a magnet problem. Note how close mine is to the speaker. And I'm using an aftermarket dual speaker with a FM converted radio.

http://[URL]https://garytayman.com/

[/URL]
If you are working under the dash be sure and do not losen the screw any more than absolutely necessary to slide a forked spade terminal under it.
Small is better.
I was nervous about the screw coming all the way out which should be a nightmare.

Same here, dual voice coil 4x10 with a modified factory radio

 

[Speedbird]

Very interesting. Was that the factory bulb holder? Did the light work before the addition of the ground?

Yes. And not working I think. Seemed no light there until I gave it a ground. All lights on the car are LED by the way.