Measuring voltage on tach wire

To test for tach set your multimeter to AC, black probe to ground, red to wire being tested. Readout should be between 1 and 6 it will go up in volts as the rpm of the car increases.

Well I used that method (red to 12 volt and black to target wire) an sure enough I got 4 to 8 volts AC while revving the engine. The literature says use the green or red wire on the coil and now it tests properly. So that is sufficient evidence for me to give it a try with the remote I am installing. I am waiting for a transponder bypass kit so it may not be a week or so before I will know for sure. Thanks everyone for your help as it is greatly appreciated.

I think alot of people learnt some important information in this thread, with regards to finding a proper tach signal. But the basics should be remembered, the souce voltage is AC, and hence that is why both methods work. If the target wire was simply DC, and you were to have probed the wire with a negative source, it would have shorted to ground and killed the circuit. Regards EVIL Teken . . .

I just read this thread and i agree w/ the last post.  since its obviously an Alternating current then it will go to 1-6 Volts Positive and negative. thats why both will work.  when it alternates to 1-6 volts negative then it will pickup a reading w/ the other lead on positive and visa-versa.

brunetmj wrote: Well I used that method (red to 12 volt and black to target wire) an sure enough I got 4 to 8 volts AC while revving the engine. The literature says use the green or red wire on the coil and now it tests properly. So that is sufficient evidence for me to give it a try with the remote I am installing. I am waiting for a transponder bypass kit so it may not be a week or so before I will know for sure. Thanks everyone for your help as it is greatly appreciated.

...................... " and the TRUTH shall set you FREEEEEE " Jim Carrey - Liar Liar  

Many ways to skin a cat........... just not all of them are used regularly.

Jeff

Velocity Custom Home Theater

Mobile Audio/Video Specialist

Morden, Manitoba CANADA

I agree that this thread was most useful. Perhaps to make it even more helpful I wanted to include a discovery I made on this topic after my post. It also may explain why there is confusion on the topic. swinstal mentioned the use of the snap -on meter for finding the tach wire. If I was doing a lot of installs that is what I would use and disregard the use of a multimeter altogether. Why? My electronic experts tell me that a tach wire is not really AC current. It is a form of pulsed DC current who's waveform can only be measured with a scope.These other methods may indeed work in some cases, however it may not be the most useful way when doing this in volume. A dedicated meter would probaly be the best bet.

Brunetmj, You are correct in that it is not a true AC wave form. It is actually a half rectified DC output, which is pulsed in succession. Because the wave form is sent so quickly, it appears on a DMM to be a AC wave form, when in fact it is not. When viewed by a scope, the DC wave form will appear to be a somewhat jagged. This sort of DC pulse mimmicks the AC wave form, which the DMM picks up. It is technically called a *Sawtooth* wave form, and should not be mistsken for a Triangular wave form. The reason it can be considered a AC wave form is that it has both equal positive and negative portions. Hope that answers some of the Q's some members may have. Regards EVIL Teken . . .