yup tach problems....

[speedy]

I have been trying to figure out my tach. I have searched and searched the forums but nothing to help me out. here is what I do know. my tach is getting power and ground. the blk and puple wire has continuity from tach to cdi unit. I do not know how to see if its sending a signal to the tach? I have a digital multi and when I place it on 200 millivolts nothing. the is power or signal coming from coil both the brown/wht wire and the white wire. how can I check the blk and puple wire. when I try and follow it....it goes into the engine harness and then to cdi unit. what else can I do??? help!!!its driving me nuts!
thanks

[speedy]

ok....did some more research.....I need to know how to use the ac part of my multi meter to test the blk/puple wire for a signal????

[Early_S_Man]

It would help if you told which car/engine you are working on ... 3-pin or 6-pin CDI unit???

One of the following threads has documentation that may help:

6-pin SC/Turbo CDI unit repair documentation

Perma-tune CD or Bosch CD?

[speedy]

thanks warren. you seem to be the god of cdi stuff. every search I do, I find you in on it! but what I really need to know is if my blk purple wire (trigger signal) is working. I have an 81 sc 3.0 with 6 pin cdi. how do I test for the signal. I have a digital multi meter but loss instructions.......thanks

[Zeke]

Set your meter to 0-20v DC. Ground the black lead and stick the red lead in the black/purple wire. Start the car. As you rev it, you will see the voltage rise....or not.

[Early_S_Man]

speedy,

Zeke's instructions are good, except that the multimeter needs to be set to AC Volts.

Not knowing what model of digital multimeter you have ... it would be hard to say what kind of reading you will get. A true RMS-responding meter with 10 MHz bandwidth would give you around 1.0 Volts AC at the TD terminal of the tach [but the tach has to be powered and connected to the Black/Violet wire] ... but, an average-responding, inexpensive Voltmeter circuit would probably read less, say, in the 100 - 300 milli-Volt range, and may not respond at all ... because of low bandwidth in the audio range! A Fluke 88 multimeter, or maybe Actron 7676 [the one in the Bentley SC manual] would be preferable to a plain-Jane $20 DMM.

A CMOS logic probe [or TTL/CMOS probe set to CMOS] equipped with a pulses indicator could also be used to check for a pulse train at the TD terminal.

An alternative test would be to disconnect the Black/Violet wire from the tach and connect it to a 1000 Ohm pullup resistor connected to +14 Volts. Again, with the red test lead to the TD wire and Black lead to ground ... the reading should be in the 2 - 5 Volt range. And, a CMOS logic probe at the TD wire should show pulses with the engine running and logic probe power leads connected to +14 Volts on the Red and ground on the Black power clips.

An oscilloscope should show an approximate 10 Volt peak-to-peak pulse train varying from about +3.5-4.0 Volts to +14 Volts at the TD wire in either test mode above. The duty-cycle of the pulse train is very low, and the pulse width is only a few microseconds wide ...

[Early_S_Man]

BTW ...

If the engine runs OK, but the CDI isn't putting out a TD signal, there is only one component that can be the problem ... D8, a 1N4148 silicon switching diode, very readily available -- Radio Shack, or any other electronic supply store!!!

[speedy]

thank you thank u thank you!!!! you guys were a great help... I now know that I have a singnak at the the tack but still didn't work. I pulled apart the tach and found some promlems.

does any know how much north hollywood charges to rebuild a tach?
thanks again guys!
sean