Tachometer Question

JohnnyC

JohnnyC

Member
I have a 77 Scout II with a 304/727/20/44 front and rear combo. The qusetion I have is why does my tach guage go crazy when my Scout gets driven for longer then 30 min?

I have replaced my cap, rotor, plugs, wires, and coil. It is hooked up correctly ( the black to the neg. Batt. Term, the red to the positive, and the green to the negative term. On the coil.

The tach works great when I first start the Scout up but when the Scout is at operating temp it starts going crazy. The longer I drive the crazy_r it gets. Any ideas would be greatly appreciated.

Thanks john
 
Newly installed aftermarket tach?

Do not know if this is the problem, but I think it would be better if you grounded (black wire) the tach to the firewall instead of running the black wire to the neg post of the battery.

The tach (36 yrs old) in my t/a only has two wires - 1 to coil; 1 to ground (besides the "dash" light for the tach).

Does your tach have a built-in light for night illumination? Is there a separate wire for the light? Besides red, green, black?
 
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Yup, it's brand spanking new sun pro.

Ok I'll try grounding it to the firewall.

Yeah my tach has 4 wires, red (pos), black (neg/ground), green (tach neg.), and white (for internal light). Ihave not hooked up the light on the tach yet. I know its not integral for the tach to work properly.

Thanks for your quick response robertc...
 
I have a '77 Scout II as well, and I ended up finding ground and key-on current on the fuse deck below the steering column. I was told it shouldn't be getting current until the key is on, but I'm also not sure if that would affect the tach's behavior.

Depending on your ignition module here - have you ensured it's in good condition? That's my only other idea - my own "gold box" Holley ignition would go into a failure mode after it got enough ambient engine heat, but it caused intense changes in engine performance - not something you mentioned.

Hope you can figure it out.
 
I'm with kyle on this. Find a keyed on power source for the red wire. While this May or May not cure the problem, there's no reason to have voltage going to the tach when the engine isn't running.
 
I've had tach wires ground before causing erratic behavior in the tach. It also caused a miss at the same time. Do you notice any miss in the engine, however slight it might be? All of the wiring diagrams I have seen for electronic guages call for switched power
 
An electronic tach really only counts voltage pulses so it can also error by counting noisy power sources like from an alternator voltage regulator.

I have had this on many occasions and currently my model a, Scout and boat all do it. Like yours they do it most after running for a while. I correlated that to the battery charging up after starting and the voltage regulator in the alternator regulating more to limit peak voltage.

Try a different positive and negative connection location like close to the same as what feeds the ignition.

Also load the alternator by turning on loads like head lamps and heater blower motor (hi) see if it stabilized

a bad engine ground can also do it.
 
I've fried three of the sun pro tachs on rigs with punky delcotron 10si alternators. When the internal voltage regulator goes intermittently haywire, the alternator can produce up to 80 volts! Typical voltage spikes for several seconds at a time will be in the 20>25vdc range.

Light bulbs can deal with that as long as the spikes are intermittent (usually), but those cheepo tachs cannot!

This is a very difficult diagnostic to make as you must have a voltmeter on the battery and alternator at the time the erratic alternator output occurs to catch the spiking. This is not som,ething that will show up in one of these quickee alternator test sequences in the parts house, especially when the unit must be run under load for a period of time before it begins to glitch.

There is a tremendous difference in quality of "reman" alternators out there, primarily in the quality of the voltage regulator module itself.

The symptom regarding this only occurring after a period of time or use is typical. Get that tach disconnected or it can literally go up in flames internally (the housing itself will fill with smoke!).
 
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