viper 4105 wont learn tach

good afternoon. ive searched this forum and others but i cant seem to find out a solution to my issue. i have a 96 dodge cummins that i installed the viper 4105 remote start on. im only using this unit for the remote start and door locks, no alarm. when the RS is in tach mode or voltage mode it wont stay running for longer then 30 seconds or so. i checked the shut down diagnostics and the viper flashed three times or low voltage/ rpm in tach mode or voltage mode. ive tried to start the truck and hold the valet switch within 5 seconds as the directions state but the led never comes on to say the tach is learned. i have verified that im getting 1.94-1.97 volts to the pruple/white tach wire at the viper box when at idle. i am able to get the truck to start and run fine by using cranking time so i know the connections are all good and the RS functions like it should. unfortunately being an older diesel the weather drastically changes the crank time needed to start the truck so i cant rely on one set cranking time. i must have it in tach mode for it to be reliable in the future. any input on this would be appreciated

What is the tach wire from the 4105 connected to on the truck? What are you using for the TACH signal? 1996 Dodge Ram, 1500...2500?When all else fails, Read the Instructions

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the tach wire is connected to a grey/black wire that my guage runs off. the wire is in the bundle that goes under the steering wheel. i verified with a multimeter that its the same wire its a 3500. ive watched the voltage fluctuate with the rpms using a multimeter so i figured i had the right wire

alright i must have the wrong tach wire. i disconnected the wire i taped into and the tach still worked fine so unless its a wire coming from the tach and not going to it then its the wrong one. im having trouble locating the correct tach wire at this point

From Bulldog Security: TACH GRAY/BLACK or BLACK/ GRAY (AC) @ IGNITION COIL http://www.bulldogsecurity.com/bdnew/vehiclewiringdiagrams.aspx Also, set DMM to AC not DC.When all else fails, Read the Instructions

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More info: Tachometer gry/blk, blk/gry AC ignition coil Also located as gray/blue at large plug through firewall on drivers side at highest point of firewall, also gray/blue at cluster. http://www.readyremote.com/main.asp?action=select&yr=1739&product=Remote Start&make=Dodge&model=RAM#When all else fails, Read the Instructions

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thanks for the info. it was actually a grey/blue wire for the tach feed. hooked it up and all is good. thank again