trans skips second gear

[bippy93]

I've read lots of posts about c-4's skipping second gear. I went through
this with mine and thought I would let everyone know what was wrong with mine, maybe it will help someone else. Mine was a bit different than some
will be, because someone had removed some parts which made it tougher
to figure out. Throttle pressure is what slows the upshift process. A good
way to check and see if your modulator is working is to remove the
vacuum line. This will cause it to stay in 1st gear . Mine would not,
and I found that the pin and valve were both missing. After replacing
them. it still did not work. Next I checked the intermediate band adjustment,
a bit off but ok. I eventually found that the throttle pressure relief valve
was missing. This is the spring and valve assembly that falls out when removing the filter. After replacing this it shifts great. Hope this is not
to long and boring, but I felt like I was chasing my tail with this problem and
thought this might help others.


Ryan. ;D

 

[YellowBronc]

It most certainly will help. If anything is learned, it's to watch for these parts when you do a filter change or modulator change. Thanks for the info!

 

[Broncobowsher]

Ah, they joys of undoing someone elses handywork.

 

[ca1a73]

I do have this missing second gear problem, as noted in a recent post. I replaced my modulator and didn't notice a difference. I never checked my filter or checked for a missing pin and spring assembly, like you mentioned. Should i drop my pan and change these out?

 

[68ford]

When i had the vacum line off mine it would shift through the gears, just real fast, but it would go through all of them.

 

[bippy93]

Yes I would check and make sure your modulator valve and pin
are both there, if so, drop the filter and make sure the spring
and valve are there.

 

[bippy93]

With the vacuum line off, it should stay in first gear, no upshift. If it
does I would check the other parts I mentioned earlier. Let me know if
you have anyother questions.


Ryan. ;D

 

[ca1a73]

might think im crazy, but the filter is inside the tranny pan, correct?

 

[68ford]

i ran mine for years with no vacum line, it shifted all the gears, just really fast because without the vacum signal it senses no load to keep it in a gear for longer. that was also with no kickdown. so it had no idea how much throttle i was givin it, it just shifted like you were going slow down a hill giving it very little gas. ive been told they have centrifigal weights that rotation make them shift(like mine was) as a default, when the vacum signal goes away or something else fails.

 

[Hogback]

maybe that's what is wrong with mine. I bought my 75 a little over a year ago and it shifts very quick - way too quick, like its getting far too much vacuum. So i figured that's whats wrong. All the vacuum lines were screwed up so i have no idea which ones go where. Right now i have the transmission vac line going to the main vac thing on the back right side of the intake, BUT... ive had to restrict the amount of air going through the line so that pressure would drop enough so it will shift right, or at least close to right. So i could and probably do have something else wrong other than the vacuum. Any clues given that info?

 

[68ford]

if limiting the vacum made it shift better, you probaly need to just adjust it a little bit, or (this is crazy but works) put about a 1/2in diameter spring between it and the crossmember to help push it in, made mine work alot better, but mine had nothing but o-ring press fit holding it in, that normal?

 

[RIbronco]

There should be a bracket, that is fastened with the transmission drain pan bolts, to hold the modulator in place. The bracket is for a dual port vacuum modulator and will not fit a single port version. I ran into this when I had my C4 rebuilt.
If the modulator is not seated properly it would shift through all the gears very quickly, reguardless of throttle possition.

 

[68ford]

exactly my situation, i just was cheap and bought a spring. besides now its custom and coilovers are cool so i thought............ ;D

 

[bippy93]

Yes the filter is in the pan. Also the loss of vacuum to the modulator
is what tells the trans it has a heavy load situation and causes the
throttle pressure to raise keeping it in 1st gear. If it shifts with no vaccum
then you dont have enough throttle pressure. ;D