Trans shifting problem
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Senior User
Joined: Aug 2013
Posts: 133
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From: San Diego
Trans shifting problem
In the last 2 days my 73 F250 Camper Special 390, which has been running mostly well for the last year since I bought it, has developed a shifting problem.
When I shift into D, it takes off fine but won't change up at all. Shifting into 2 gives me 1st & 2nd gear, but shifting back to D drops it down into 1st again... wth? I haven't driven it at speed yet to test if it will eventually shift up.
I checked the trans fluid level (changed about 1000 mi ago) and it looks fine, fluid is in good condition too.
Any ideas?
When I shift into D, it takes off fine but won't change up at all. Shifting into 2 gives me 1st & 2nd gear, but shifting back to D drops it down into 1st again... wth? I haven't driven it at speed yet to test if it will eventually shift up.
I checked the trans fluid level (changed about 1000 mi ago) and it looks fine, fluid is in good condition too.
Any ideas?
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Check the vacuum modulator and the line that goes to it.
Sounds like either the modulator is bad or maybe the vacuum line has come off.
Check the line for kinks or other problems, make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad.
Sounds like either the modulator is bad or maybe the vacuum line has come off.
Check the line for kinks or other problems, make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal.
Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line.
When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.
Modulators are identified by the color of the stripe. Pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it. If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line.
When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.
Thread Starter
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Senior User
Joined: Aug 2013
Posts: 133
Likes: 0
From: San Diego
Done...
Mike, you nailed it! As if I expected any different having read a great many of your posts and responses....
Vac line to modulator was completely fuqued, hose all melty at the top end at the metal line heads towards the carb. And it was disconnected at the trans end.
I went ahead and replaced the vac unit anyway, since I was already on the ground, and it seems to be shifting very nicely now.
Thank you sir, much respect!
Vac line to modulator was completely fuqued, hose all melty at the top end at the metal line heads towards the carb. And it was disconnected at the trans end.
I went ahead and replaced the vac unit anyway, since I was already on the ground, and it seems to be shifting very nicely now.
Thank you sir, much respect!
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Hotshot
Joined: Apr 2009
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From: Stanley, VA
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