791xv Cranks but no start

When you add a new remote to an alarm with remote start, it is added with default programming. It sounds like you need to go into the programming and set it up for voltage sense and the crank time and whatever other options may have been changed in the alarm's programming on initial install. Thats why tach sense is so much better than voltage sense. If you have access toa bitwriter(if its a DEI alarm), you can transfer the original programing by selecting "0" when it asked which remotes that you want to have the programming apply for. If not then you gotta count some chirps and reprogram the alarm for that remote. If you're not real experienced in this, I would take it to a dealer.     It could take you an hour and them 5 minutes.