tach wire or voltage sense?

better with TACH .

Any good installer and most manufacturers will tell you tach is recommended. That said, i can honestly say i have installed hundreds of RS using voltage and rarely do i have an issue. As a matter of fact, i think i may have more issues using tach just due to things like bad connections, wires touching hot parts, mechanics disconnecting tach wires to when they aren't smart enough to know why it is there.

I guess the best answer is, it depends!!!

I've never seen a car come back because a tach wire was connected.  I've seen more then I care to talk about coming back because they were set to voltage.  I live/installed in Ohio so we have weather from -10 - 110 to deal with.

Of course when working underhood there are things to consider - if you arn't going to solder the wires you might as well go with voltage.  If you arn't going to loom the wires and secure them so they don't touch hot or moving parts again you are better with voltage.  Butt connectors and scotchlocks should NEVER be used under the hood (or anywhere else in the car.... :)  )

If you look at the way the two systems work it is easy to see which one is "better". 

If you are using a tach reference the remote starter outputs a signal to the starter to start cranking.  The remote start then monitors the tach input and shuts the starter output off when the frequency on the tach wire is near the preprogrammed value.  The system should also time out if the car doesn't start after X amount of seconds but from what I hear that isn't always the case.

In voltage monitor mode the unit outputs a signal to the starter for a preprogrammed amount of time.  At the end of that time period the output shuts off and then the unit monitors the voltage coming in to the remote start unit and tries to determine if the alternator is producing voltage.  If it is cold outside or the motor needs a tune up it's very possible that the car won't start in the short time the output is on.  If you increase the output time there is a good chance in the warm months the starter will drag a bit.

DEI's virtual tach, from my understanding, constantly monitors how long it takes the starter to crank when started with the key.  It stores this information and uses it to determine how long the starter should stay on.  It's really more of a "smart" voltage sense system then a virtual tach system - but the smart people at DEI know that the name "virtual tach" will impress many, many more people then "smart voltage"

Hooking the tach up on some units provides added benefits such as over rev protection that automatically shuts the unit down if the engine accelerates.  Voltage monitoring systems don't have access to actual engine RPM data so they can't do this.  This is a "nice" feature for older people who may struggle getting in and out of their vehicles and are concerned they may step on the accelerator pedal while trying to get in.  That is the only reason I can think of for having it, but it is still a nice feature.

To make working with tach signals easier I strongly recomend getting a meter that will read frequency.  You can a Craftsman one that works well for ~$30 at Sears.  Use the frequency setting to measure the suspected tach wire and you can, without a doubt, determine if the wire is a tach wire or not.  Many people use the AC setting on their meter, but the issue here is that the tach data is not an AC signal, it's a time based signal.  AC meters don't measure time between pulses they measure average potential.  A frequency meter will actually measure the time in between the pulses and will be 100% linear from one RPM to the next.  When I used to use my meter set to AC I never new for sure if I had the right tach, it was always a "uh, I think it's this one" feeling.  Once I started using frequency I knew everytime, without a shadow of a doubt, that I had the right wire.

With frequency you can also verify the tach signal by starting the car, monitoring the frequency (say 11hz) and then doubling the RPM of the car (say it idles at 1000RPM, hold the motor at 2000RPM).  This will cause the meter to read 22hz proving that it is the tach signal.  Finally, what I think is the best reason, is because if you really want to, you can mathematically calculate the frequency of a tach signal and prove that it is or isn't the right signal.  With AC voltage you're just looking for numbers that change, you can't quantify them in any way.

Now that that's all said, the only car I ever didn't hook tach up on was my wife's 2004 Honda Accord.  I did the install outside at Christmas time and didn't feel like messing with it (I had Honda goop all over me and wasn't about to spend another 20 minutes on the car).  Since then the car has became my daily driver and I still haven't hooked tach up on it.  In the Winter when it's cold it won't start on the first attempt but it usually does on the 2nd.  As the car gets more miles on it I'm expecting it to eventually quit starting in the cold months at which point I will run the tach wire.