I've had a Sandel SN3308 EHSI and a Garmin GNS530 panel mount moving-map GPS in my plane for over 5 years. They've never quite worked right, thanks to the botched installation by Eagle's Nest Aviation in Ukiah, CA. The most recent problem: the Sandel/Garmin wouldn't stay in GPS mode; they would automatically switch back to VLOC.
I spent over $3000 having a different outfit rewire the panel, and it finally seemed to all be done right. However, there was one small, show-stopping issue: if you selected the GPS from the SN3308 (or from the GNS530), after two seconds it would go back to VLOC. It didn't matter if it was tuned to an ILS or not (if it were tuned to an ILS, you wouldn't get it out of VLOC mode at all, presuming the GNS530 was configured to auto-switch to the VLOC if an ILS is tuned in).
After getting Sandel on the phone, we went through some configuration checks. Turned out the GNS530 wasn't properly configured. I wish I could remember specifically what it was, but it's several pages into the configuration (which you get to by holding down the Enter key while powering up).
There's an LOC / GS / ARINC page. On it is a setting that can be something like ARINC 429, or several other things. It should be ARINC 429 Graphics w/Int (or something like that).
If I get a chance to look at the plane again, I'll get more details and update this post.