Tach won't work now.

[ZCCOR#109]

Ok, today I removed the Capacitive Discharge system I install 37 years ago. (1972 240Z)

Re-established the stock wiring on the coil etc.

Car started ok! yea.

BUT

The tach doesn't work well. It works to about 3,000rpm, but jumps around above that. Also, when throttle is closed the tach drops to zero immediately and bounces up and down before settling down.

The CD system had a wire coil behind the tach that replaced the stock wire, that of course was removed.

When removing the CD system I reinstalled the white wire loop back on the tach.

So what do you think is the problem?

Edited March 25, 2010 by ZCCOR#109

[alhbln]

Based on your description i guess that the incoming pulse signal is too weak. The polarity of the small white wire loop could be an issue, try to install it the other way round in the white plastic mount. The loop has two turns, one won't be enough with the stock ignition.

[ZCCOR#109]

Based on your description i guess that the incoming pulse signal is too weak. The polarity of the small white wire loop could be an issue, try to install it the other way round in the white plastic mount. The loop has two turns, one won't be enough with the stock ignition.

Thanks,

The thing is, the wire loop is established within a plastic block that has only one orientation. There is a hole in the plastic block for the stud, and the plastic block has a small protrusion that matches a small hole in the sheet metal "U". Thus there is only one way to attach the loop and one loop is all that is made by the factory.

[alhbln]

Have you changed the calibration of the tach after installing the CDI unit? The calibration trimmer (accessible through a hole between the two indicator bulbs on the back) changes the sensivity of the tach, you might want to recalibrate it for the current setup. Use a plastic trimmer screwdriver. While the engine is idling, turm the trimmer pot slowly around (clockwise) until the tach needle is stable around 800 RPM. Then check 4000 RPM, ideally with a digital RPM meter as calibration reference.

[ZCCOR#109]

Have you changed the calibration of the tach after installing the CDI unit? The calibration trimmer (accessible through a hole between the two indicator bulbs on the back) changes the sensivity of the tach, you might want to recalibrate it for the current setup. Use a plastic trimmer screwdriver. While the engine is idling, turm the trimmer pot slowly around (clockwise) until the tach needle is stable around 800 RPM. Then check 4000 RPM, ideally with a digital RPM meter as calibration reference.

Thanks,

However, the tach calibration was never changed for the CD system. Shouldn''t require a change/adjustment when going back to the original wire loop should it?

Could the ballast resister be bad. The resister is certainly in line with the input to the tach. What do you all think?

[alhbln]

If the ballast resistor is broken (normally they rarely break), then your engine would start but switch off when turning the key back to 'ignition', or run quite rough.

If you are sure that the stock wiring has been fully restored, the trying to recalibrate the tach would be a good next step.

Added: Just another test, if you short cut the resistor by connecting a temporary wire to the two resistor terminals, does the tacho work then? Shortcutting the resistor would increase the primary current and deliver a stronger signal to the tach.

Edited March 28, 2010 by alhbln
Added a second test idea

[ZCCOR#109]

Solved!!

I removed the nut, and repositioned the wire loop and plastic retainer paying particular attention to the fit of the sheet metal "U" with respect to the metal fingers on the tac that the "U" matches.

The parts must have not matted up well. Tac works fine now. WHEW.

"Life is all about the details"