Printer Friendly Forum Posts - Measuring voltage on tach wire

I am installing a remote starter on a 00 mazda 626 (4 cyl.) The literature says to use a red or green wire on the coil for a tach wire (for a 4 cyl.) I found those wires . My question is can I measure this first using a mutimeter before using? If so what should I get for a reading? I just want to be sure before hooking it up to the remote.

put the multimeter to AC and should be readin from 1-6voltage

-------------
Houston,TX
"The two most common elements in the universe are H+ and stupidity" (Ellison).

I was afraid someone would say that because it may mean i do not have the right wires. I dont see how they could be the wrong ones since they are the only red or green wires i see going to the coil. I tried AC and got
nothing. I only got 12 volts DC. I tried both an anaolog and Digital meter.

You also have to place the RED lead on 12 volts and the BLACK lead to the taregt wire. The voltage should read between 1 - 6 volts.

-------------
Jeff
Velocity Custom Home Theater
Mobile Audio/Video Specialist
Morden, Manitoba CANADA

Velocity Motors wrote:

You also have to place the RED lead on 12 volts and the BLACK lead to the taregt wire. The voltage should read between 1 - 6 volts.

ummm...no. Being AC voltage, why would you place the red lead on +12V??? Put the black lead on solid chassis ground, and probe with the red lead set to measure AC volts. THE CAR HAS TO BE RUNNING to see it. Its the wire that reads AC voltage when running, and goes to zero when not. Also, voltage will increase with RPM...

Perhaps your method applies to a specific vehicle Velocity? I've never ever heard of that..

i put the neg lead on the positive battery post and probe with the pos lead of the meter on AC. just grab the non common wire at the injectors if you can get to them.

Here is the method for testing a tach wire that has never ever failed me:

1.Strip back the target wire

2.Set your digital multimeter to AC voltage

3.Take the black lead of your meter and put it to the positive terminal of the battery.

4.Take the red lead of your meter and probe the target wire.

5.Start the engine. Your meter should show anywhere from 1 to 3 volts at idle speed.

6.If your meter leads will reach, take the meter inside the vehicle and rev the engine SLIGHTLY. If not, do it manually at the throttle body under the hood. The voltage should increase, however it should go no higher than 8 volts. If the voltage does not increase when the engine revs or if it goes above 8 volts, find a different wire or run the autostarter tachless.

This method may seem a little bit excessive or different than what most people do, but like I said, it has never failed me yet.

"Perhaps your method applies to a specific vehicle Velocity? I've never ever heard of that.."

You can see just from the replies in this thread that either method works..I put my black lead to ground as you do and red led to the wire for testing either tach or vss wires. For tach it should sit around 1 - 3 volts and rise as your rev the engine. For VSS is usually sits close to zero and will go up to about a 1 volt + with the car rolling.

Mike

Well I will give it a try and post back the results.I have done that polarity reversal (red to 12 volt) and black to target wire on DC to test for negative triggers but this one is new to me.

To test a tach wire the best thing i have found is my tach tester! Snap On makes a probe tester for tachs, abs, that type on things it detects electrical pulses in a wire. cost around $83.00

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 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).

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 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

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 . . .