I have a 74 250 with a 390 and a C-6, the tranny seems to be fine but doesn't shift like I think it should. The previous owner said that he had done something to make the tranny shift at lower RPM, I guess that he adjusted the Vac. Modul. . I did a search but couldn't find what I was looking for.

The 74 C-6 had only one Vac. modulator is that correct? Where is it located, I found what I suspect is it near the side of the pan that has the funny shape. Also how do you adjust the screw, which way do you turn it to shift at higher RPM? Would anyone know the stock setting or do I just turn it until the tranny shifts better? Thanks for the help.

Jake

 

The c6 only has one vac modulator. And it should be in the back right corner of the pan, with a hose running up towards the engine.

I'd assume that if you wanted to make it shift at a higher rpm... that you'd turn it to the right, and left to make it shift at a lower rpm.

HTH.

 

If you take the modulator off there should be a short rod behind it, the rod controls the part of the shift point and the gov does the rest. I can't remember exactly, I think the longer the rod the later the shift and the shorter the rod earlier the shift. Better confirm this with someone who keeps up with the C6's, been a long time since I tinkered with one them.

 

If your C6 does have the adjustable modulator NEVER EVER adjust it more than one full turn in or out. In ( clockwise) make's a later, harder shift.

 

Thanks for the help everyone. I am going to try and get a look at it this weekend, and see what I can do.


Jake

 

Mine holds second but wont shift into third it did before it sat for a month after I drove it for a 200 mile trip no problems.I changed the vacuum modulator hose and swithched vacuum points didnt change a thing.I pulled and gutted the tranny no visible problems.Now after reading Im thinking.Its a short rod never turned the vacuum modulator and I read that the vacuum loss on these C6z are usually the Mod o ring kick down and band adujustment.I would make sure these are sealed nice before you do what I did.Just got off the Phone and even though we totally cleaned the valve body it still could be that.Any more ideas for my problem or similiar things

 

So is there a scew inside the modulator that you turn? Do you take the vacuum hose off and stick a small screwdriver in there?

Thanks

 

Yes there is a small screw inside the vaccum valve. Remove the hose to get at it. Remember, no more than 1 turn either way. It's best to go a 1/4 turn at a time. Screw it in for firmer, later shift's.(clockwise)

 

I there a similar trick for an AOD? Mine shifts 1-2 way too early. AOD? Won't kickdown 3-2 untill rpm gets low.

 

Yes there is. What appear's to be a passing gear rod or cable at the throttle body. This is not for passing gear. It control's line pressure's. This will also change your shift point's. It shoud be done by a mechanic with a set of line pressure gauge's. But you can try by slightly adjusting the small adjusting screw. Remember to just slightly adjust.

 

Is it possible that my modulator is not adjustable? I crawled under last night, took the vacuum hose off and tried sticking a small screwdriver in there but could not engage any slot. I shone a light in the hole and could see the end of the thing but there was no slot.

Can you buy a adjustable modulator?

 

Nearly all replacement modulator's are adjustable. If you are in doubt ask the salesman.