My 1989 f150 4×4 has around 160000 miles on it. It’s a 4.9L inline 6 and a manual transmission . Borg werner 4 speed. It
won’t go into 3 or 4th gears. In the past couple weeks it has been idling fast when stopped at red lights etc. Around 2000 rpm in neutral. Then a couple days ago it wouldnt go into 4th gear then eventually 3rd as well. The clutch has been replaced recently as in the past year. Also shifting forks were replaced about a month ago . Im hoping it’s not my transmission . Any help would be appreciated. Thanks.
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Before you go worrying about replacing the transmission at this point, I want to asked you, did you look at all of the gearing? Since about the late 1980s, most U.S. SUV equipped with manual transmissions also used synchronizers. Though most people don’t think this is true, it is, believe me. The synchros allow you to upshift and downshift without worrying about matching engine/transmission speeds manually so the gears won’t clash.
Think of synchros as something like a specialized clutch for each gear. Working automatically, the synchros allow you to shift smoothly and quickly and without much thought.
The synchros are my first suggestion. Next, I’d suggest having the transmission rails for third and fourth looked at. This should work so that when you shift into third, the gears lineup correctly and likewise with fourth. If the rails won’t match up then it is unlikely your transmission will shift as well.
Then, of course, there’s the shift pawl itself. Is it lining up correctly so that your Ford is going into the correct gear? How about the gearshift line? Is it binding in the carrier?
You started out your diagnosis work quite well, however, there is still more to go.