'90 Supreme won't turn over

I have a '90 Supreme that won't turn over when I try to start it. The lights and everything still work, and a boost doesn't help so I know it's not the battery. It used to help if I rolled the car a little bit back or forth, even just a few inches. Now, nothing. I was told it wasn't the starter. Maybe it's something in the shifter?

Any advice would be appreciated.

Thanks

Jennifer

First, check the remote positive terminal. Follow the battery positive terminal and one goes near the fuel pump relay/fuse. It usually has a red cap on it and looks like a bolt. The positive cable is attached in there and tightened down by a nut.
If the nut is barely tight, you might have light and so forth but would strain when trying to start the vehicle. So make sure that the remove positive terminal nut is tightened up good.

Next, if the remote positive terminal nut is tight then engage the emergency brake. Turn the ignition switch to ON, then move the transmission gear to Neutral, then try to start the engine. If the vehicle starts in Neutral then possibly the Neutral/Safety switch which is located at the side of the transmission needs adjustment.

Lastly, if starting the engine in Neutral does not work then make sure that the emergency brake is engaged. Directly supply the starter with power by using a large screw driver or you can get a cheap $9 remote starter switch. You connect one end of that switch to the battery positive terminal and the other to the starter and you press the button to supply power to the starter. The starter should turn over the engine. If that turns over the engine, then the starter solenoid may need to be replaced first and if not the whole starter needs to be replaced.