Hi Romuald
Welcome
Check your grounds,make sure you have a good ground to the engine,and a good ground to the block.
Nice truck,i just got a 88 Suburban back that was my mom and dad's years ago.It was an official veihical of the 1988 Winter Olympics.Mine is gas but that will change.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews