Im still working on my 73 F100 and almost got all the bugs worked out finally. Trans shifts into forward and reverse good. Shifts from 1st to 2nd pretty good but it seems like its at extremeley low rpms. From 2nd to 3rd the engine rpms raise just like a normal shift but it stays raised for about 3-5 seconds and then goes clunk into 3rd. I dont feel any slipping since it jerks the truck when it finally shifts into gear adn goes along fine, just takes a minute for the shift to actually happen. Modulator valve? Band adjustment? Baby it till I can get another trans? The truck only has 51,000 miles on it. Tranny fluid is good and red and no burnt smell.

 

I think It has something to do with the vac modulator, but I'm sure someone with more transmission experience will chime in soon.

 

I second the modulator being the issue.

Make sure you have good vacuum going to it and that it is the correct one for your transmission.

I got a great deal on a truck years ago that when I test drove it with the owner it sped through the shifts and you were in 3rd by 35mph. I suspected the modulator, got a great deal on the truck, swapped out for the correct modulator and then it drove like new.

 

The modulator on it now is original as far as I know. It has two ports on it. I pulled one off and it had vacuum(the bigger line). I pulled the smaller line and it didnt have any vacuum, is it supposed to? The smaller line is t-ed in with the egr valve and other things.

Does the next modulator I get have to have the two ports? Does it need to have the same color stripe that my current one has? It looks like it has a red stripe on it.

 

Just thought I would add that it shifts through all the gears before 20mph. Its done shifting by about 15mph.

 

Was it working before?

Sounds like it's getting too much vacuum. Without vacuum it won't shift.

Make sure your lines are clean and not kinked - also your modulator could have failed or gotten trash in it.

I can't remember if it is only the transmission that determines the modulator or a combination with the motor. Might want to check with the parts dept.

 

I ended up buying the red striped double port modulator like it already had. Its also adjustable. I havent put it on yet, weather has been pretty nasty. Im still not clear if both ports are supposed to have full vacuum or just one of them?

 

The large connection must have manifold vacuum.
The small one is for altitude compensation and not really necessary.
You can plug or connect it.
There's a pin behind the modulator that can be lengthened which will raise the shift points.

Quite often on trucks the primary governor valve has a big honking weight that will cause this.
Try the modulator / or pin first.

 

Got a new modulator installed. Does the same thing still. As I posted before I don't feel any slippage when it goes from 2nd to 3rd it just shifts weird. The engine revs up a second then clunk it goes into third. Something sticking in valve body?

 

Have you adjusted the band?

10' lbs, back off one turn.

 

No, but I am goin to. Going to change fluid and filter also. I noticed in the haynes manual it says to get a new locknut after adjusting bands, do I really have to or will it be ok?

 

Crank the adjuster down to 10' pounds.

Remove the nut, clean it off, dab some form of glue onto the adjuster area close to the case.

Install the nut, back the adjuster off and tighten the nut.

Done deal.

 

I know this thread is old but I found it when I was researching my own issue. I have the same question about the locknut. When you say "some form of glue" what do you mean any examples of what you've used.

I wouldn't mind buying a new locknut, but not sure what exactly to ask for. Would it be the band adjustment locknut?

 

Would it be the band adjustment locknut?
Yup.

Any kind of gooey glue will work.