Transmission not shifting into overdrive!

[1985K10]

I didn't know there was a forum for '73-'87 Chevy fullsize trucks but I'm very glad to see there is! Hopefully I can get an answer to all my questions here. One question I have is about the transmission on my '85 Chevy 1/2 ton. It came factory with a 305 V8 and a 4 speed automatic transmission. I'm not sure if it's a 700R4 or 400R, I'm not too good when it comes to transmissions. My problem is that it won't shift into overdrive, it just stays in 3rd gear. I can't even shift it manually into 4th gear. Is there a vacuum or something that can be causing this problem? Maybe an electrical problem? Any help would be great! I'm trying to restore this truck and hopefully it can be my daily driver. Thanks for the help in advance.

 

[johnmc]

there is a switch on the brake pedal that un locks 4th over drive it could be stuck.

 

[chengny]

There should also be an O/D test connection dangling under the dash in that general area. A light green wire. The brake switch is normally open, so you should see 12V at the test lead when stepping on the brake pedal.

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[1985K10]

there is a switch on the brake pedal that un locks 4th over drive it could be stuck.

What does the switch look like? I'm going to try and look for it today whenever I get out of work. I'm also going to test the light green wire that chengny described. Hopefully it's nothing serious and I can get it figured out. I doubt it's something major.