Tachometer does not go over 500 rpms. What can cause this?

[Tomzern]

Hi!

I have a problem with my tach. It only goes up to 500 rpms. It moves a little bit when I rev the engine, but no more than 550rpms at the most. When I stop the engine it goes back to 0 rpms, so there is a signal there.

What is most likely to be the problem here? I just got a new 1.6ohm resistor and the coil is stock (I think).

Regards,

Tomzern

[SteveJ]

Do you have a ballast resistor? Is it a 4 wire or 3 wire tach?

[Tomzern]

In my first post I wrote that I just installed a new ballast resistor. The car runs as normal. I don't know if the tach is 3 or 4-wire, but the car is a 1970-model and the tach has a small square connector if that tells you enough?

Thanks,

Tomzern

[Tomzern]

Anyone? =)

[Zed Head]

Why did you get a new resistor? What's the resistance of the coil you're using. You're not giving much information.

I've read that the 240Z tachs require a certain amount of current in the wire to the tach. You might have too much resistance on your primary circuit. Maybe you don't need that resistor.

[Tomzern]

Sorry about that :/

I bought a new coil resistor because I read somewhere that if the coil was a stock type the correct resistor value would be 1,6ohm. I measured the one I had to be 2,2ohm and the tach didn't work so I thought a new resistor with a lower value would fix it, but no.

So you suggest I should try to bypass the resistor? Can't that be harmful to the tach?

[Zed Head]

What is the resistance on the coil? The 1972 points coils had a resistance of 1.5 to 1.7. Looks like the resistor is 1.6 ohms, as you saw. So total resistance on the primary circuit would be 3.1 to 3.3 ohms. Maybe you have a low resistance coil and you need more resistance. Or a different coil.

Sorry, I might be confusing things. It's better to give too many facts rather than one at a time. If everything is right in the ignition system, it might just be bad tachometer. Has the tachometer ever worked correctly?

[beermanpete]

Using the incorrect ballast resistor won't hurt the tach but it can damage the coil and points if the resistance is too low or it is removed. From what you have said it sounds like the tach is bad. Check the wiring at the back of the tach. It needs 12 volts and ground from the primary circuit in addition to the signal from the coil.

[Tomzern]

Hi again

So I measured the voltage on the coil with the ignition turned on and it was only 3-4 volts. I then tried to bypass the resistor and got the voltage on the coil up to about 6-7 volts (where it should be). Tried to start it with the resistor bypassed, but the tach is still not working. Only shows about 500-600 rpms, and the needle moves just a tiny bit when I rev the engine.

Tried with a different coil and the resistor still bypassed. Measured the coil voltage to 8-9 volts, but tach still not working, so maybe it is the tach itself that is faulty.

I have just driven the car about 200 meters after I bought it, and the tach did not work then either, but I found out I had the wrong engine wiring harness and thought that could be the reason, but no.

[beermanpete]

It sounds like you have voltage drop in the wiring to the ballast and coil. There should be a solid 12 volts before the ballast and about 9 volts after the ballast (on the coil).