What is happening? Modulator value issue

[RealJakeMalloy]

My guy changed the modulator valve on my 1976. The first new valve caused the truck to shift weird during slow acceleration almost always around 20mph. The best way I could describe it is that it feels like if you were driving a 5 speed and didn't take your foot off the gas while shifting. The car just kinda revs up briefly.

He then bought a second new value - the one with a green stripe for the C-4. Same exact issue.

What is odd is that there was no issue with the original valve - it was just old. And no - I don't have it. I threw it away never thinking a new one would be worse.

Any idea what is going on?

 

[Yeller]

The modulator is adjustable it has a screw adjustment in the end. There are also different ones, did the original have a green stripe?

PSA: if your transmission shifts properly and the modulator is not leaking internally, do not “replace it because we are here” often the replacement will not function exactly the same. I know I’m too late to the party and it’s not helping today lol.

When I worked in the transmission biz we kept every used modulator we could find, if we replaced one often it took a lot of trial and error getting it where it shifted like it used to.

 

[RealJakeMalloy]

Thank you. Honestly, I don't recall if the original one had the green stripe or not. The two new ones were one non green and then one green (which we thought was the real issue). I think it just needs to be adjusted?

 

[Yeller]

Possibly it just needs adjusted. Honestly it’s been so long I don’t remember what the different stripes signified, but they do signify some differences in transmissions and shift feel

 

[RealJakeMalloy]

Yeah - you think it could be a matter of just trying different ones? It is a C-4 transmission.

 

[RealJakeMalloy]

What is the common adjustment? In other words, is there a specific direction or amount of tweaking?

 

[Yeller]

There is a screw in the vacuum port, counterclockwise is a later shift and clockwise is an earlier shift, if my memory isn't failing..... I would make big adjustment, make it make a significant change, then fine tune from there.

 

[RealJakeMalloy]

Thanks - turns out what I am trying to describe is a shift flair - it is intermittent. Will make an adjustment and see what happens.

 

[stupidboy]

The green stripe is for cars and yellow for bronco and trucks.

 

[Tiko433]

I remember I adjusted the length of the pin also

 

[RealJakeMalloy]

Are you sure about the yellow vs. green stripe? I have been reading a lot of recommendations that indicate green is correct.
I made a mistake and when my guy took out the original we threw it out. We bought a new one (not green stripe but not sure if it was yellow) and we had the flair. We then bought green but the flair issue persists, although intermittent and mostly when I am feathering the accelerator and always at 20 mph. Usually, if I gas it hard it won't happen.

 

[DirtDonk]

Don’t have any help on the transmission, but for parts in general, you just ran across the bane of our existence.
Never, and I mean NEVER throw an old part away unless you absolutely positively 100% know for sure that it’s no longer any good.
Like if it’s a literally melted into a slag heap.

You absolutely cannot throw new parts at a problem anymore. Doing so may literally create new problems where you didn’t have them originally.
Making diagnosing an existing issue much harder.

 

[Pa PITT]

Man I'm with Dirt Donk on not tossing old parts... CARBURETOR CORES. NO 1.
bACK WHEN i HAD MY PARTS STORE ... We tried to keep almost all carbs & most starters & ALternators ... Just in case my customers came back after a few months wanting their old part back.
Stash um back Guys .

 

[RealJakeMalloy]

Trust me - lesson learned!

Man I'm with Dirt Donk on not tossing old parts... CARBURETOR CORES. NO 1.
bACK WHEN i HAD MY PARTS STORE ... We tried to keep almost all carbs & most starters & ALternators ... Just in case my customers came back after a few months wanting their old part back.
Stash um back Guys .

 

[sprdv1]

There is a screw in the vacuum port, counterclockwise is a later shift and clockwise is an earlier shift, if my memory isn't failing..... I would make big adjustment, make it make a significant change, then fine tune from there.

You so good..