how do you change modulator on C-6 automatic?

[Banjo]

Got my engine running ok, still having odd problem with C-6 automatic in my 68 SWB. To give history, truck was running when belonged to buddy who blew engine. C-6 was fully rebuilt about 5 years ago, no known problems. Upon reassy alone, I jammed the torque converter and had to have transmission shop go through C-6, replacing 2 cracked gears and reman torque converter. Now the truck will take off in drive, but it won't shift out of 1st and when I manually shift, it appears to only have 2 forward gears. New fluid/filter, it's full, figured a modulator valve is about $20 or so and if I knew how to change, would try that before taking truck to transmission shop ($$$$).
Thanks, the old Banjo!

[DuckRyder]

There are two types,

Screw in

Push in.

The screw in is just that the whole thing screws in and out.

Push in has a retainer clip and just pushes in / pulls out once the clip is off.

Don't lose the rod that is behind (or stuck in) it.

Most of them are adjustable as well.

However, if you mean that if you pull the lever into 1st and then is doesn't shift to 2nd when you move the lever to second (ie manual shift) it doesn't sound like a modulator problem to me.

[Banjo]

I was probably unclear, I usually am! If I manually shift, I have 2 forward gears from what I can tell so far. Due to valve cover hitting part of Edelbrock linkage, have to put a 1 inch spacer under carb when I get off work today (truck at home) to get full throttle and be able to go through neighborhood without fear of throttle sticking/binding.
But, when I take off from dead stop in DRIVE, it'll stay in 1st gear until incredibly high rpm and as soon as I bump column shifter down into 2nd, it'll shift. Now, given I can't get up enough speed to run over 30 or so, because of being in neighborhood and fear throttle will bind, I can't tell (yet) how many rpm I'll be turning at highway speeds. But, as for now, it feels like only 2 forward gears, but I've yet to determine which is missing, 3rd or 2nd. I'm thinking 2nd.
I looked up modulator valves on partsamerica and since they're only about $15, that'll be first thing I change after I get throttle fixed, to see if that establishes automatic shifting through forward gears. Banjo.

[Wes]

It sounds to me like the governor weights are sticking. If the truck has been sitting awhile this may be the case. Once you get the truck safe to drive, take it for a good long run making sure the tranny oil is fully warmed up. I'm not a big fan of oil additives, but I have used Lucas trans additive and it works.

[RebelRider]

Hey Banjo,
Do you have the kickdown linkage hooked up from the carb to tranny?
The reason I ask is, my C6 is doing that also although it will finally shift out as speed increases enough. I have a new rebuilt w/new mod. Just haven't installed kickdown yet.

[DuckRyder]

Banjo,

If it has forward drive in manual 2nd you have second gear.

Depending on your rear ratio, you may not be going fast enough to reach 3rd.

Make sure you have manifold vacuum to the modulator (should be a steel line with only short runs of rubber at the manifold and modulator).

Make sure the kickdown is installed and properly adjusted.

If the modulator is adjustable you can make small adjustments to see if it helps or replace it with an adjustable one.

[Fake Name]

Umm- kickdown linkage? My '70 never had it from the PO. It has the vacuum line to the carb- am I missing something?

fn

[flyboy2610]

Yes. There should be a rod going from the carb to a shift lever on the side of the tranny. It kicks the tranny gears down when you get on the throttle, to provide additional acceleration for things like merging into traffic from a freeway ramp.

[jzjames]

Also consider that your vacuum line may be plugged in at the wrong location (at the carb vacuum port instead of at the manifold vacuum port) You may not ba getting enough vacuum out of the carb port - although I dont know that much about it - but these guys do:
http://www.network54.com/Forum/260730/

[Banjo]

Thanks to all who replied. I have brand new metal line from modulator to factory ported switch which screws into intake manifold behind carb, is has about 4-5 various size vacuum hookups, large one to power brake booster, smaller one to transmission modulator. No kickdown linkage hookup, never was in the 5 years my buddy ran it previously, so I know it would normally run and shift without one (naturally, they won't downshift when throttled without it).
Got my throttle linkage bind fixed, so was able to make full speed runs. When starting from dead stop, will take off in 1st if left in drive. Will NOT shift to second, despite high rpm. Pulling column shift down from drive to second results in instant shift. Will not automatically shift into 3rd or when column shifter moved to 3rd, despite high rpm. there is no 3rd.
Also, when slowing down, do not feel transmission gear itself down, so assume governor or is that the same thing as modulator?

[CH3NO2]

I had a C6 w/no 3rd gear.After rebuild still no 3rd,went through it a second time,still no 3rd.Finally found a ball that blocks off a drill passage in the pump had fallen out.So if you jammed the converter into the pump there may be some unseen damage.I would go ahead & replace the pump.

[Jake11]

Ck and see if the little valve in the case the modulator makes work
is free to move ok. They get stuck and no shift. On the rear of the trans
is the "govenor" that has valves in it that make it shift. They can get
stuck too. Gotta pull the tailhousing to get to it.

[Frankenstein]

I know that this is a very old post but I have a 71 with a 390/c6 that is doing the same thing and Im wondering what the problem was and if it was an easy fix. We thought that it was the tranny so we replaced the C6 and converter with another C6/converter and after a test drive it is still doing the same thing. The P.O. said that it was driving fine and one day he got in it and there was no 3rd gear. All the symptoms are just like the ones listed in the first post.

[Frankenstein]

It has to be something small and stupid, it is too weird that 2 different trannys,convertersand vaccume modulators have the exact same problems. I checked the travel of the kickdown rod and it is fully engaged when the carb is at W.O.T. just like on a Mopar or Chevy. It is a 2bbl Holley with a stock intake on a rebuilt 390. It has the stupid linkage trapeze thing on the back of the intake/firewall. I will be putting on a eddy intake and carb anong with lokar kickdown and throttle cables very soon. I am all out of ideas as what it could be, at this point I don't think that it internal.
I have searched for this problem and have come up with alot of people whos truck does the same thing but they never post what the fix was. Anyone have an idea?

[Jake11]

http://www.broaderperformance.com/Links-A.htm

Look around on here.