So - I am having some issues getting an aftermarket digital oil temp gauge
to work.

The sender connects to a threaded adaptor that replaces the oil relief plug
- which I imagine many of you folks likely have in place on your beloved
vans.  First question - if this is your set up, have you had to ground the
sender since the bottom of the block is aluminum?  When I jumper a ground
to the sender the gauge reads -18C which equates to 0 Fahrenheit...(I opted
for Celcius over Fahrenheit).  Without the jumper to the ground, the gauge
reads full scale - 181C or 300 F.  Note - This is just with the ignition on
without any temp being generated from the motor.

When I run the motor with the sender jumped to ground and the sender wire
attached to the gauge the gauge just stays on -18C and there is no
fluctuation - so somewhere in this equation the resistance isn't dropping
as the oil temp rises....which is how I believe things work with temp
gauges.

If I drive the van without the sender grounded - the gauge at full scale
(181C)  begins to slowly decrease as the motor warms up - but continues to
drop until it reaches -18C and then stays there - it doesn't set on a
temperature.

This gauge is from Intellitronix (M9108) and came with a sender.  I had
originally installed a GW sender but then exchanged that earlier this week
thinking it might be the issue as it may not be matched correctly with the
gauge.

When looking at GW (as a resource) they don't seem to mention grounding the
sender - nor did the instructions I received for the gauge and sender -
however I imagine they typically sell these units to vehicles that don't
have an aluminum cases and the sender grounds directly to the motor.

Any thoughts appreciated as I am running out of ideas here.

Bruce