Emailed then about 5am this morning and they got back to me pretty quick.
My resistance from engine to black wire was same as probe to probe so I don't think it's a ground
"Hi! You can test the gauge with variable resistor 0-200 ohms, connecting red wire to power+, black to GND and resistor between white and blk wire. Turn on the power (12Vor 24V) and changing resistance from 0-200 ohms you should see the pointer go from 0-130psi. But, I think you may have a bad GND connection through the chassis from the sender to the gauge black wire. This is very common problem with VDO kit (sender & gauge) just because bad GND. You have it connected, but somewhere between this two it may be loose somewhere. Additional wire connected to the gauge black wire and the body of the sender in most cases solving the problem.
Have a good day!
Sincerely,
Shefik "
