autometer tachometer
03-12-06, 06:57 AM
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autometer tachometer
i need to know where to put the wires to install a tach in my 85 rx7.
and if any body has good suggestions for locations to put the tach it would be helpful ( pics would be more helpful )
and if any body has good suggestions for locations to put the tach it would be helpful ( pics would be more helpful )
03-12-06, 02:01 PM
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All tachometers should come with at least four wires: Red, Black, White, Green. Red is key on power. Black is common chassis ground. White is for dash illuminating power. Green is for the tachometer signal. It shouldn't be that hard to figure out where to place the first three. The wire for the tachometer should be yellow/green and leads up to the ignition coil. Connect the green wire to that.
Why would you want to install an aftermarket tachometer? Did your old one break?
Why would you want to install an aftermarket tachometer? Did your old one break?
03-12-06, 02:20 PM
#6
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Originally Posted by trochoid
The stock tach works quite well
The stock tach sucks major bunghole, even if it is one of the better OEM tachs from the era.
Here's what I posted in another forum:
Originally Posted by from ohiorotaries.com
And boy howdy the stock tach is inaccurate! For one thing, it's very heavily damped. At idle the needle is rock-steady, when the drivetrain is bouncing around on/off throttle the needle barely moves, and the needle "seems" to not change as quickly as engine speed does when shifting and such.
I never would have BELIEVED how much, though, until I put this unit in! The Sport Comps are race tachs and use a special movement that responds very rapidly and is completely unaffected by outside forces. Cheap units (Sun, Autogage maybe?, and stock type tachs) will let the needle move if you hold the tach in your hand and shake it around. Good tachs do not let the needle move, period, unless the mechanism says so! (A nice side benefit is that you can mount the tach off-vertical, mine is set so the 5000-8500 band is at the top)
But remember that rapid respinse? I started the engine after making all of the connections and the needle was shaking. A lot. "Crap," I thought, "the tach is hosed". (Remember: screaming deal...) As it turns out, what I was seeing was the engine's actual speed fluctuations at idle! The needle is an almost 100rpm wide blur, reflecting the engine's actual speeding up/slowing down. Then I took it for a drive... after a shift the needle moves *instantly* to the new speed. Period. The drivetrain "bouncing" after a shift is actually reflected as a 300-400rpm fluctuation. Wow! More proof that a torque strap is a really good idea, and that I should get off my lazy butt and make one.
And furthermore, the stock tach on this car is about 10% off! 3000rpm on the good tach is about 3250 on the stocker. 7000rpm on the stock tach is about 6400-6500 on the good unit! So all that time I was shifting at 7700 at Norwalk, I was really shifting at about 7000. (But that's where it was fastest...) And all that time I was short-shifting the car at Rallycross because i couldn't trust the stock tach, I was *really* hurting myself. This upcoming season is gonna be a lot more fun
I never would have BELIEVED how much, though, until I put this unit in! The Sport Comps are race tachs and use a special movement that responds very rapidly and is completely unaffected by outside forces. Cheap units (Sun, Autogage maybe?, and stock type tachs) will let the needle move if you hold the tach in your hand and shake it around. Good tachs do not let the needle move, period, unless the mechanism says so! (A nice side benefit is that you can mount the tach off-vertical, mine is set so the 5000-8500 band is at the top)
But remember that rapid respinse? I started the engine after making all of the connections and the needle was shaking. A lot. "Crap," I thought, "the tach is hosed". (Remember: screaming deal...) As it turns out, what I was seeing was the engine's actual speed fluctuations at idle! The needle is an almost 100rpm wide blur, reflecting the engine's actual speeding up/slowing down. Then I took it for a drive... after a shift the needle moves *instantly* to the new speed. Period. The drivetrain "bouncing" after a shift is actually reflected as a 300-400rpm fluctuation. Wow! More proof that a torque strap is a really good idea, and that I should get off my lazy butt and make one.
And furthermore, the stock tach on this car is about 10% off! 3000rpm on the good tach is about 3250 on the stocker. 7000rpm on the stock tach is about 6400-6500 on the good unit! So all that time I was shifting at 7700 at Norwalk, I was really shifting at about 7000. (But that's where it was fastest...) And all that time I was short-shifting the car at Rallycross because i couldn't trust the stock tach, I was *really* hurting myself. This upcoming season is gonna be a lot more fun
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03-12-06, 02:51 PM
#8
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I personally just poked a wire through the firewall to run to the coil instead of trying to find the stock wire in the mess of a dash. Then again I only had a half hour or so to install the tach.
The coil negative should have spade connections on it, the coil positive should have a threaded connection. Only a test light or voltmeter can know for sure.
The coil negative should have spade connections on it, the coil positive should have a threaded connection. Only a test light or voltmeter can know for sure.
03-12-06, 03:12 PM
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Peejay, I'm glad you posted that about the tachometer. I may start looking into getting an aftermarket one to replace it. Altho, I'm not sure if it is off as well. There isn't a way to figure that out is there? Minus having a new one next to it to compare.
03-12-06, 03:45 PM
#10
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You could try checking the calibration. Granny's Speed Shop has a page somewhere on their site detailing how to calibrate using a cheap battery charger. Cheap chargers still have a 60Hz voltage ripple that can make a reading on a tach. 60Hz is 3600 cycles per minute, divided by 2 pulses per revolution = 1800rpm.
Actually making the *adjustment* requires removing the instrument panel and diddling with a trim pot. Because of the "cheap" design of the stock tach, you have to ake the adjustment with the unit at the same angle as installed in the car.
Actually making the *adjustment* requires removing the instrument panel and diddling with a trim pot. Because of the "cheap" design of the stock tach, you have to ake the adjustment with the unit at the same angle as installed in the car.
03-12-06, 06:28 PM
#16
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No, the signal wire goes to negative.
Ignition coils recieve a constant power and ground side is switched. When the coil negative is grounded, electricity flows through the coil and builds up a field. When you remove the ground, the field collapses and induces a much higher voltage current in the secondary side, which finds its own ground through the spark plug.
The positive side should see pretty much a constant 12V or so, the negative side switches back and forth from 12V to ground (12V because it becomes the end of the circuit when the points are open/ignitor goes "open") so that is the only place you *could* get a signal from.
Ignition coils recieve a constant power and ground side is switched. When the coil negative is grounded, electricity flows through the coil and builds up a field. When you remove the ground, the field collapses and induces a much higher voltage current in the secondary side, which finds its own ground through the spark plug.
The positive side should see pretty much a constant 12V or so, the negative side switches back and forth from 12V to ground (12V because it becomes the end of the circuit when the points are open/ignitor goes "open") so that is the only place you *could* get a signal from.
03-12-06, 07:15 PM
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*L* Actually just hearing how peejay described it nearly threw my head into a spin. 
Plus sometimes I prefer tapping into stock wiring just because I find it simpler. More along the lines of a simpler approach. But, if I were to do a tach replacement and found that finding the wire and tapping into it was a big pain, then I'd just go with peejay's advice.
Plus sometimes I prefer tapping into stock wiring just because I find it simpler. More along the lines of a simpler approach. But, if I were to do a tach replacement and found that finding the wire and tapping into it was a big pain, then I'd just go with peejay's advice.
03-12-06, 07:36 PM
#20
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That's what I ran into... I had no time to work and I was looking under the dash at wrist thick bundles of wiring... Screw it, I'll just run a new wire. 
