AS the title says I have a 1991 GMC Safari van. 2wd. 7004r transmission. The overdrive has failed and there is no reverse. Is there a way of scoping this type of tranny for diagnosis? Are there any shifter solenoids or modules that could possibly fail? I smelled an electrical burning odor the day it failed.

http://autorepair.about.com/library/ts/at/bl-at-24.htm

I'd say the burning smell was probably clutches burning that you smelt. I doubt its a solenoid and even if it is the clutches are probably burnt already so its too late.

Usually when shift solenoids fail on automatic transmissions they do so because the vent won't close back which bleeds away pressure from the pistons that supply force to hold the clutches. Not enough pressure and they slip. Once they slip they are burnt.

Of coarse it could be a band or I guess they have a sprag for overdrive, who knows. Check the fluid if it has the nice burnt smell to it and has went to a gray color I'd say get ready to have it rebuilt.