Lost all gear drive except 5th. i have removed the top of the box and all gears and shifters seem fine.

Hello. It seems the 'shift fork' is either 'misaligned' and/or 'bent' in 3rd, 4th, 5th and REVERSE. It's, also, possible those gears are 'stripped' on their shaft(s); making driving that truck, in those gears, impossible. Good luck and keep me informed; if you want to.

try to change oil in the gearbox

can i assume you pulled the side plug on tranny and the level
is right there? or gone way down, ever do a 50k service? to it?

M/T tranny (not auto) what engine? the m./t varies by HP of ENGINE.
If its really in 5th ( i bet not)
if the RPM rises fast that is called clutch slip
yes, clutch bad.
if not in 5th actual rpm will rise fast too. slip,
if the other gears wroke like 4th at wide open throttle up hill
gee the chutch is good.
so its not going in to 5th (for lots of reasons)
but 1 reason , is easy.

-AX 4/5/15 transmission
this tranny has a shift tower. (that thing on top that the shifter lever
fits in to)
most tranny that fail to actual shift into gear
is caused by a $1 plastic part failure
so pull the shifter off top of the shift tower.
and look, is the plastic buffer parts all bad, missing, cracked or
destoryed, sure they all end there... all.
i can also remove the shifter, lever.
and shift to 5ht by hand, and see that it DOES.
it either does, or does not.
if it does, the lever parts are bad.
that simple.
see part 61 (im guessing which of 5 tranny it is , )
http://cdn.liveoffroad.com/images/Aisin_AX5_Transmission.jpg

check the drain plug magnet for metal bits. Mostly you can remove the rear extension housing with out removing the gear box and you will find a small gear on the cluster shaft ( bottom shaft) and a larger gear on the out put shaft ( tail shaft) . Check that the shifter fork is actually moving the larger gear or gear dog into 5th gear (Fork loose on selector rail) or check that there are still teeth on the smaller gear as excessive high speed under load will rear out the teeth

Well, I've have heard of this problem before but I've never seen it for myself. I wrote a opinion here just the other day concerning this problem in a 1990 bike.

This is unusual. I'm afraid that if your transmission is doing this, you are going to have to do at least a partial teardown of the transmission.

I could be that the shifter cam is not rotating fully to put the 5th gear clutch dog into gear. This would cause just the tips of the lugs to be driving the final drive gear. And, it could jump out and back in from time to time.

Now, why the shifter cam is not rotating fully is the question. If the rest of the gears seem to be shifting fine, I wouldn't think it was the shifter pawl but it could be.

If it were mine, I'd remove the exhaust system and whatever else needed to gain good access to the the transmission top cover. Remove the cover and raise the rear of the bike off the ground. Now, you can shift the transmission and watch the actions of the shifter cam. Pay particular attention to the 5th gear shifter fork. See how much "play" the fork has once it's shifted into gear. You may spot something wrong there and be able to repair the problem without pulling the transmission out of the frame. If not, you may have to pull the entire transmission or at least the trap doors with the shafts to inspect the final drive gear. You may wind up replacing the final drive gear if the clutch dog has been jumping in and out of gear.

One other thing. Have you checked your front belt pulley? I've seen the splines in the pulleys wear to the point that they just strip out and won't pull the bike in any gear. Check that before you start tearing down the transmission. Usually when this happens, (excessive wear on the front pulley) it is accompanied by a small transmission fluid leak. If you ride the bike and park it, it'll leak a spot about the size of silver dollar. If you wipe the spot up, it won't leak again until you either ride the bike again or start it up. Then, it'll leak again. The transmission fluid is leaking past the "quad seal" behind the pulley.

Good Luck
Steve

remove gear stick and check its in properly, if you removed the gear shifter quadrant box bolted to top of gearbox you more than likely put it in while a gear was selected, remove and make sure gear box selectors are all lined up in nuetral then refit quadrant box and then gear lever.

Sounds like a piece of linkage came loose. If it were an internal tranny problem, doubtful that the stick would be loose. Examine all linkage for something very obvious.

4th gear is driven straight thru the middle of the gearbox. All the other gears are driven via the lay gear. I'm not 100% familiar with your particular box but it seems like a key on the front gear of your layshaft is sheared. This can be brought on by towing heavy loads in 5th. hope this helps.

I've had a similar problem, and it was the rubber bushings in the shifter link; as some particular ones age they get sloppy and 5th gear seems to be the one that suffers. Others will age and disintegrate, then you won't be able to shift at all. If all the other gears are fine (no grinding, judder, popping out of gear or other untoward behaviour) I'd look to these rubber bushings, and replace them all while you're at it. They're replaceable at much less cost than a tranny rebuild, and are most likely that cause, i.e. after rebuilding tranny, you still won't get into 5th because the shifter won't have been rebuilt, just the contents of the gear box!

Check the "Bentley Bible" aka the Saab 900 Official Service Manual, costs about $40 from Amazon, for info, it's an invaluable resource!

16 valve manual: http://www.amazon.com/Saab-900-Valve-Service-Manual/dp/0837603137

8 valve manual: http://www.amazon.com/Eight-Official-Service-Manual-1981-1988/dp/0837603102

Get the appropriate manual for your car and enjoy the Saab!

You need a new 5th gear- luckily this is easy to replace- I did it myself following online directions for 5th gear replacement. The 5th gear is not in the box with the other gears- its off to the side so you can access it very easily by removing the left front wheel. Just don't let the idler assembly come apart, lots of tiny parts and springs...
The 5th gear can ware out prematurely if you rest your hand on the shifter- do not rest your hand on the shifter in the future.