I have a 2001 cummins truck. I started the truck and let run for 10 minutes. Then turned on the lights and started driving. About 5 seconds of driving and the voltage gauge went all the way down and check gauges light came on. Turned off and back on and it was reading at about 10 volts on gauge. Then after about 2 minutes it went all the way down and check gauges light came on again. Is it the ECM?

 

It sounds more like an alternator or battery issue rather than the ECM. When the voltage gauge drops like that, it usually indicates that the alternator isn’t charging the battery properly or the battery is failing. I'd recommend testing the alternator output and checking the battery connections first. Corroded terminals or a weak battery can also cause voltage drops. If those check out, then you might want to look into the ECM, but I’d start with the basics!