Factory Tach Inaccurate / Reads Too High

[68Runner]

I recently purchased a 1968 Road Runner with the stock 383 and 4 speed trans. I've noticed that the factory dash tach is reading about 1K rpms too high at cruise. The car has the orange electronic box on the firewall so I'm assuming that it's some sort of electronic ignition. I don't recall my original Road Runner having this type ignition so I'm guessing this may have something to do with the tach input?? Any ideas?

 

[Russ69Runner]

Tach wire should be connected to the coil. You still have a distributor it has a cog in it. Electronic ignition don't know if the 68 still had the point system. If I am wrong on the wire to the coil. Sure some one will correct me. It may need to calibrated. the Tach. Good luck my friend.

 

[ACME A12]

The orange box indicates an electronic ignition. The factory tach was not designed to work with electronic ignition. It can be modified however. See the link below:

http://www.tachman.com/mopar.htm

 

[Russ69Runner]

So do you think the replacement tach that classic sells would be able to work with the Electronic ignition. Thinking of putting one in my gauge panel.

 

[68Runner]

Thank you for the info ACME. I’ll check it out.

 

[HD539]

So do you think the replacement tach that classic sells would be able to work with the Electronic ignition. Thinking of putting one in my gauge panel.

I got my tach from classic and it worked fine for several months (I have electronic ignition). Recently it has been drifting when the ignition is off but as soon as I turn the key on it goes back to 0 and functions normally.

 

[Sixtynine_Bird]

I got my tach from classic and it worked fine for several months (I have electronic ignition). Recently it has been drifting when the ignition is off but as soon as I turn the key on it goes back to 0 and functions normally.

Good to hear as I have bought the same, but not tested yet. Did you do some special wiring, or just plugged it like the instructions says, with the blue wire directly to the fuse box and the grey one to the coil in the engine compartment?

 

[RonLiv]

So do you think the replacement tach that classic sells would be able to work with the Electronic ignition. Thinking of putting one in my gauge panel.

Pretty sure the Tac in the Dash on my car isn't factory, but is a reproduction and looks factory. When I spoke with the previous owner, he told me he put it in. I don't know how accurate it is, as I really don't pay much attention to it. But it works. The car has been converted to Electronic ignition, but the Tac itself is connected to the coil as indicated already. I will say for some reason any time I use the turn signals, the Tac jumps back and forth keeping perfect time to the signal. A little annoying, but again I really don't pay much attention to it and since the signals don't go off automatically once the turn is complete, its like a little warning device on the dash. The car has the 727 torque flight, column shift and by ear seems to shift on key.

 

[HD539]

Good to hear as I have bought the same, but not tested yet. Did you do some special wiring, or just plugged it like the instructions says, with the blue wire directly to the fuse box and the grey one to the coil in the engine compartment?

I wired it per the instructions.

 

[Russ69Runner]

Their it is someone pulled the power off the light circuit. What ever they could find hot. Seen a lot of wiring when some one adds a radio and other things. The tach should have its own source.

 

[69RRBeepBeep]

I got my tach from classic and it worked fine for several months (I have electronic ignition). Recently it has been drifting when the ignition is off but as soon as I turn the key on it goes back to 0 and functions normally.

Where is classic, what is the web site address?

 

[HD539]

Classic industries.com

 

[Al Fiandaca]

Must say, I was having the same problem with my RR. Just got her engine reinstalled and the tach was working fine as it had before tear down. Detailed the engine compartment a little and all the sudden my tach is reading about 2K high. After examination, I realized my tach wire was nice and tucked along side of the #8 plug wire and I couldn't help but wonder if it was inducing a pulse in the tach wire. Sure enough, moved the tach wire away, factory tach reads fine again. Just something to try

 

[HD539]

Good to know, thx, I’ll try it.

 

[Russ69Runner]

Any wire that moves current could effect other thing's. The wire carries a field around it as voltage passes through. This is how you measure Amp's in AC power wires. The meter has a clamp to go around the wire to get the reading. So Dc dose the same thing. The plug wire is a high voltage wire so it would have a lot more current and produce a bigger field. Remember having to put a resistor on the radio to stop the sound of the plug wires from disrupting the radio with a tic tic. Or the buzz as you speed up. Good luck glad you found the problem. Something so simple but will drive you crazy trying to find it.

 

[halifaxhops]

Must say, I was having the same problem with my RR. Just got her engine reinstalled and the tach was working fine as it had before tear down. Detailed the engine compartment a little and all the sudden my tach is reading about 2K high. After examination, I realized my tach wire was nice and tucked along side of the #8 plug wire and I couldn't help but wonder if it was inducing a pulse in the tach wire. Sure enough, moved the tach wire away, factory tach reads fine again. Just something to try

AC signals will transfer into a DC wire if touching. Good find.