I have a 1992 F-150 4x4 with a 302 that I swapped a junkyard ZF S5-47 manual transmission into back in 2006. It is not a daily driver but has seen at least weekly use since April, 2007. I have put about 20,000 miles on it since then. The transmission worked smoothly and noiselessly up until the past week when it started jumping out of gear.

The issue is that the transmission has started jumping out of all gears (2, 3, 4 and 5 but haven’t tried 1st) on a “coast” load. It doesn’t come cleanly out of gear. It makes a noise that sounds like the synchros are grinding. Once it is making this noise it won’t go into any gear with the clutch pushed in. The only way to get it into gear is to (1) slow down to under 15 mph [the noise vanishes and it can be put into gear by shifting with the clutch in] or (2) give the engine a little gas and without using the clutch, push the shifter into gear. Once in gear the trans does not make any unusual noise until the next time it jumps out of gear. If I pull it out of gear at speed and coast it does not make the noise.

I checked the engine and transmission mounts and they are tight and not excessively worn. The transmission is securely bolted to the engine. The trans is full of fluid. The clutch works smoothly and its master cylinder is full of fluid. The pilot bearing, flywheel, clutch disk, pressure plate, throw-out bearing and slave cylinder were all new when the trans was installed. The clutch pedal assembly was purchased used. I replaced a worn bushing in the upper pedal pivot last fall. Something about the geometry of the push rod between the pedal pivot and the master cylinder strikes me as odd but it seems to work. The transfer case has 144k on it but works noiselessly in all ranges.

Once I get a chance I will take off the shifter housing to look for excessive wear in the shift fork pads or to the groove in the sliding collar.

Is there anything else I should look at to determine why the transmission jumps out of gear and then makes noise? The sudden onset and the fact that it jumps out of all gears has me thinking (wishfully?) that it may not be internal to the transmission. I am grateful in advance for your advice.

 

worn bearings will make the trans fall or pop out of gear

 

Thanks for your response. Shouldn't there be some increased noise if the bearings were worn to that point?

 

Nope, just more play in them

 

I'm going to pull the trans. Let you know what I find. Thanks.