I have just bought several of the Lutron Dimmer switch from Home Depot. One that has on/off along with a sliding dimmer. When the switch is in OFF position, it has has little led that lit up servicing as a locator in the dark.

I'm simply just replacing these with an existing Leviton switch which is just an ON/OFF switch. The switch is controlling recess light on the ceiling.

Here's my questions:

The minute I put the dimmer switch, the line voltage change. Using voltmeter to test the line:
1. When dimmer switch is set to OFF. Voltmeter read 30.4 in AC
2. When dimmer switch is ON and the dimmer is slided to full brightness, the volt meter only read 90.1

But if I leave the line with Leviton switch, it's always 0 on OFF and 120.3 when ON.

I thought that the Lutron switch is defective, but all of them show the same behavior. Of course the light seem to work fine, even on the 90V line. But is the right behavior on all dimmer switches?

A call or email to Lutron tech support:
Lutron - Innovations in Lighting Control

Should help answer your question.

I doubt anyone here has actually performed the tests that you have done to confirm or deny the results you have found.

I suspect all is fine.