Tach vs Voltage Sense vs crank time

what i'm talking about is a dei unit, that has voltage sense to see if the car has started. there's no wire hookup, it senses through the constant power source that it is hooked to. i think that it works with crank time to see if the car has started or not. but i'm not sure if it works well. (the voltage sense settings are low or high). the dei tech site says that it senses voltage right before cranking, then right after cranking. because generaly a cars electrical system is 14volts while the moter is running, and 12 or less when it isnt. i guess my question is is it a 3rd way to do a remote start, or is it just a way to see if the car is running using crank time?

The DEI's instructions are lousy.  Here is how the three different modes work:

1.  Tach.  Like we expect, when the unit senses a certain RPM has been reached, it stops cranking the starter.

2.  Voltage sense.  The unit cranks the starter for a FIXED amount of time---default is 0.6 seconds, but you can adjust it.  If the unit "sees" the car's voltage rise, it realizes the engine is running, and therefore keeps the ignition alive.  If the voltage does not rise, the unit shuts the car off and tries again.

3.  I forget the name of this mode, but it says "sensing off" or something like that.  The unit cranks the starter for a fixed amount of time---again, 0.6 seconds default, and then makes no attempt to monitor the tach OR the voltage.  It simply assumes the engine has started, and leaves the ignition powered up for the next 12 or 24 minutes. 

So, even in voltage sense, the starter still cranks for that fixed amount of time.  In my opinion, no matter how you adjust it, it's going to either overcrank in the summer, or undercrank in the winter.  Undercranking will bring the customer back for sure, since the car won't successfully remote start.  Most customers aren't likely to notice overcranking at all, but I'd imagine it's not very good for the starter.

Of course, a newer car that runs perfectly, is likely to require about the same cranking time whether cold or warm---you might well get away with voltage sense.  If the car's old and junky and needs a tuneup to begin with, I'd try really hard to find a tach wire someplace.

Another tip:  Don't forget that these various modes can also be useful troubleshooting tools.  If you're having a difficult time getting your install to work properly, you might consider temporarily turning all sensing off, so you can more easily identify and fix any of the other things you did wrong.