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Measuring voltage on tach wire


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jayts95 
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Posted: December 08, 2003 at 7:13 PM / IP Logged  
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.
brunetmj 
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Posted: December 08, 2003 at 8:18 PM / IP Logged  
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.
HamiltonAudio 
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Posted: December 08, 2003 at 8:51 PM / IP Logged  

I didn't know it would read on both the red and black sides of a battery (cool!).  worst case scenario....grab the tach wire you suspect, but don't run the wire nicely or solder it....connect it temporarily and program the unit...some units will tell you if you have the right wire, some won't.  If not, try to start the car...if the r/s holds the starter in and doesn't let go after firing, your tach wire is wrong (or either too low/high for the unit to see).

b

Teken 
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Posted: December 08, 2003 at 9:04 PM / IP Logged  
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 . . .
JMaxx93 
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Posted: December 09, 2003 at 2:05 AM / IP Logged  
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.
Velocity Motors 
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Joined: March 08, 2002
Location: Manitoba, Canada
Posted: December 09, 2003 at 9:43 AM / IP Logged  

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 Measuring voltage on tach wire - Page 2 -- posted image. 

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
brunetmj 
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Posted: December 09, 2003 at 10:11 AM / IP Logged  
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.
Teken 
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Posted: December 09, 2003 at 5:23 PM / IP Logged  
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 . . .
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