First of all this is an 1981 F100 with a inline six cylinder and a 4 speed manual transmission. Reverse was working fine yesterday and today it refuses to move backwards, no grinding or other noises. All four forward gears work fine. Where is a good place to begin to diagnose this problem?

 

Does the shifter go into reverse? and it just will not move?

 

I also had a 81 f100 with a 4 spd. One question is this the tranny were you have to push down on the shifter and go to the left? That is the kind mine had, but the problem I had was the shifter snapping.

 

Yes, it is that gearbox.
When shifted into reverse it is like being in neutral.

Could this be a linkage problem?

 

I have another question. Has it had any service lately? When I was putting that tranny in my old truck I had a neighbor helping me and there was a bolt on the left that he mistook for the filler, and it turned out it was holding something in place (I believe it had something to do with reverse, I am not sure though becouse I imediatly took the top off and put it back together (luckly nothing had moved). I could believe how he mistook that bolt for the filler if it wasn't for the one on the other side going in at an angle (like a filler).

If I remember right this style has the shifter on the tailshaft, and there are 2 Rods (linkages) that go into the tranny. My best guess is to look under there for these rods while someone inside the truck shifts through the gears, and notice how they move. If nothing happens in reverse, then it is likely in that shifter housing, if the rods move then it is most likely an internal problem.
Best of luck.

 

Heres some more info, the reverse lights still come on when i put the shifter in reverse, this leads me to belive that the linkage is still intact and working propely.
Can anyone confirm my reasoning?

I wont have time to look at it till this weekend so if i have to get a junkyard tranny, i want to have it already.

 

I believe the Reverse light switch is on the shifter housing, so the lights coming on could just mean that the shifter handle has gone into the right position, but it does not confirm that the linkages going into the tranny are doing their job. That is just if my memmory serves me right.