2002 volvo s60 rs problem

The vehicle has an Avital 4113 r/s. The car randomly under (wont crank) and over cranks (grinds the starter) without any obvious pattern. Sometimes it works great. We have power and ground cables directly from the battery, Verified all connections including key sense, tried both virtual tach AND tried running a tach wire from the purple coil wire. The unit wont even recognize the tach signal even though I verified AC voltage all the way to the start module. In desperation I even tried switching with a known good module. The unit runs great once it does start the car and all other functions work awesome(keyless, trunk pop). Has anyone else had this problem?

Have you tried voltage sense? How hard is is to get to a fuel injector?I drink current, eat ohms, and bleed voltage

Thanks for the reply. We have tried voltage sense and the car under cranks unless we turn the crank time really long.  We havent tried a fuel injector yet. It acts like a bad wire connection except it OVER cranks as often as it under cranks, maybe more.

It sounds like the starter doesnt know whether the vehicle is running or not via a tach issue. I would try a non common color at the injector and see where that takes you.I drink current, eat ohms, and bleed voltage