c6 wont engage with vac modulator hooked up

[redstone65]

I noticed my trans ('70 C6 hooked to a '74 360) would not up-shift while in Drive. So, I found the vacuum hose to the modulator wasn't hooked up. It had fallen down where I couldn't see it. I reattached it to manifold vacuum and then the transmission wouldn't engage at all in any position except reverse. D/2/1 were all dead. So I replaced the modulator and it did the same thing. All dead except Reverse.

I then decided to unhook the modulator line from the manifold (engine still running) and must have forgotten to put it in Park, because the moment I unhooked the line it lurched forward an inch or so. The e-brake was on so I wasn't hurt but it was still dumb. I had to grab some new underwear.

But, it did confirm that the modulator is the issue. With it disconnected from vacuum, the trans engages fine, but still won't up-shift. With it connected to vacuum, the trans won't engage in any position except Reverse.

Any ideas?

[davelj]

Was the line wet with trans fluid? That would indicate a bad modulator and the usual reason the line come off.

[redstone65]

Yes it had trans fluid in the vacuum line. That's another reason I replaced it, figuring it was bad. Why it won't engage now has me stumped. The only trans I've been inside of is a 4r70w, so the C6 internals are a mystery to me.

[DuckRyder]

Is the modulator adjustable?

Does the stripe color match the one you replaced?

Does the rod behind the modulator look manufactured somewhere other than someones back yard?

Is the kickdown lever hooked up, properly adjusted and does the lever on the manual valve move freely?

Hows the reverse engagement (hard, soft, normal)?

[cep62]

If it was sucking fluid out , check the level.

[redstone65]

Is the modulator adjustable?

Yes, it is.

Does the stripe color match the one you replaced?

The old one was so nasty that I couldn't detect a stripe on it. Transtar only had a white stripe that was a screw in, so I went with that.

Does the rod behind the modulator look manufactured somewhere other than someones back yard?

No it looks fine. Seems normal to me.

Is the kickdown lever hooked up, properly adjusted and does the lever on the manual valve move freely?

No, the kickdown lever isn't hooked up yet. I thought that only mattered when accelerating so I ignored it. Is it required for modulator function?

Hows the reverse engagement (hard, soft, normal)?

It's hard. Sometimes quite hard.

[DuckRyder]

What happens if you back the screw in modulator out say a turn?

Make sure the kick down lever isn’t stuck.

Harsh engagement hints at high line pressure, beyond that I’m out of ideas... I keep meaning to try to find my C6 book, if you don’t hear from me in a few days, give me a little reminder Via PM...

[redstone65]

Thanks Robert!

I'll do those checks and see what it does. I do have a spare C6 that I can swap in if needed. But, it's condition is just as unknown as this one. We'll see.

[DuckRyder]

Not finding book... it may never turn up...

The ford transmission forum has moved to tapatalk, which isn’t great in my opinion, but if you can tolerate tapatalk and the overall crustiness of the membership (IME) give them a shot, there a a number of people there who really know their stuff.

https://www.tapatalk.com/groups/fordtra ... m-f260730/

If you got a spare, buying an overhaul manual and a soft kit might be surprisingly easy...