Tachometer Signal? Voltage?
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Race Director
Joined: Oct 2000
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Tachometer Signal? Voltage?
I have this Stupid Little White car, see sig, but it doesn't have a tach, and it really needs a tach.
I'm about to order a super cheap one online, price is about $30.
Anyways, for a '92, it's got some crazy electronics. It looks like the diagnostic side of the computer gets tach info from a green/red wire, and I've found that wire, so I might just splice it. On the other hand, there is a connector on top of the coil with only one wire coming out. There are two sockets on the connector, and I took it off, and there are two outs from the coil under that connector. I think the one that isn't used is for the optional tach, that my car didn't come with. The other looks like a ground, as per the POS Haynes manual.
Anyways, before I order this tach, I wanna make sure I'm getting a tach signal out of that output, what am I looking for? Pulsating DC? Am I gonna blow up my poor little multimeter?
Any ideas are appreciated.
I'm about to order a super cheap one online, price is about $30.
Anyways, for a '92, it's got some crazy electronics. It looks like the diagnostic side of the computer gets tach info from a green/red wire, and I've found that wire, so I might just splice it. On the other hand, there is a connector on top of the coil with only one wire coming out. There are two sockets on the connector, and I took it off, and there are two outs from the coil under that connector. I think the one that isn't used is for the optional tach, that my car didn't come with. The other looks like a ground, as per the POS Haynes manual.
Anyways, before I order this tach, I wanna make sure I'm getting a tach signal out of that output, what am I looking for? Pulsating DC? Am I gonna blow up my poor little multimeter?
Any ideas are appreciated.
Race Director
Joined: May 2001
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The tach output is a square sinewave pulse. If you don't have an oscilliscope you'll probably be very hard pressed to find it.
What type of ignition system does the car have? If it has a typical coil / distributor it shouldn't be any problem to hook up a tach. If it's got some kind of goofy coil per plug or coil pack system it might be more complicated.
What type of ignition system does the car have? If it has a typical coil / distributor it shouldn't be any problem to hook up a tach. If it's got some kind of goofy coil per plug or coil pack system it might be more complicated.
Drifting
Joined: Mar 1999
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From: Norcross GA
Blow up you poor lil mter,... no.
A tach is a frequency counter. The "tach" side of the GM coil is the "switched gnd" side. One side is Hot (+12vdc) the other side gets switched by the assoc electronics. (Be it points, HEI, Opti, or other.) This Tach side is very noisey. The tach's job is to filter it and count the # of pulses and come up w/ a RPM #.
You wanna measure it w/ your meter, use the 20VDC scale. Course you will be measuring voltage which wont tell you the RPM. If you have a meter w/ a freq counter in it,.. then you can come up w/ a #. It prolly won't match the RPM, because the signal needs filtering to get rid of all the switching noise.
(Prolly not the answer you were looking for, but enough info to make you danerous.)
For $30, give it a try.
A tach is a frequency counter. The "tach" side of the GM coil is the "switched gnd" side. One side is Hot (+12vdc) the other side gets switched by the assoc electronics. (Be it points, HEI, Opti, or other.) This Tach side is very noisey. The tach's job is to filter it and count the # of pulses and come up w/ a RPM #.
You wanna measure it w/ your meter, use the 20VDC scale. Course you will be measuring voltage which wont tell you the RPM. If you have a meter w/ a freq counter in it,.. then you can come up w/ a #. It prolly won't match the RPM, because the signal needs filtering to get rid of all the switching noise.
(Prolly not the answer you were looking for, but enough info to make you danerous.)
For $30, give it a try.
Thread Starter
Race Director
Joined: Oct 2000
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Thanks for the replies. So I should get a voltage jump if it is the tach signal wire, right? It's like checking if a speaker is getting power, the needle will move, but you can't get an accurate reading.
I'm not sure what kind of ignition it is, it's not a distributor and it's not Coil Packs. I'd say it's closest to opti. There is a coil under the four plug outputs, and the firing is completely controlled by the computer, I can't even adjust timing.
I'm not sure what kind of ignition it is, it's not a distributor and it's not Coil Packs. I'd say it's closest to opti. There is a coil under the four plug outputs, and the firing is completely controlled by the computer, I can't even adjust timing.
