VDO sender/hobbs

Hello,

912ULS on Evector SportStar w/ Mitchell oil pressure gauge:

I just had to replace my VDO oil sender 360-430 (old one would peg on start up, then normal after 2-3 min)  

After reconnecting the g and wk wires to their terminals on the new sender, my oil pressure now pegs to max when the master power is turned on.  If I re-connect the old sender,  the gauge reads zero as expected with master power on. 

I thought maybe the new VDO sender needed a dedicated ground.  So I added a dedicated ground wire at the sender,  the oil pressure gauge works as advertised but now the Hobbs meter does not run.  

I assumed this would be a simple VDO replacement.  There are only two wires.    Gauge either pegs if I keep original wire connections, or gauge is normal with a dedicated ground, and Hobbs no longer works. What am I missing?