Long time no see everyone! Got home from college a few weeks ago and finally got the time to fiddle around with my 4th gen Tbird, got a used Mirror 105 motor in it with a mystery cam after the previous rebuilds some of y’all might remember from a while back. Anyways, that’s sorted out and the transmission still doesn’t have 3rd gear. Vacuum pressure at the manifold is definitely low (not quite sure how low since we were using a harbor freight gauge an import guy I know has). We went through it for leaks and blocked off every line that wasn’t needed by the engine itself to minimize odds of a leak slipping through as well. Currently I’m thinking that what I need is a special “low vacuum” modulator like the purple stripe one used on the later C6’s, but I can’t find one for my 1966 C6. Manually switching gears proves I only have 1st and 2nd, and the motor is about ready to qualify some pistons for the space program at 60mph despite my best efforts to force a 3rd gear shift. Now given that this car used to be a daily driver, and I want it to be a daily driver again, 3rd gear is very important with the 45-55mph speed limits around town. The lack of vacuum modulator is going to be a real killer to testing if that’s the issue near as I can tell unless there’s a guy out there who makes them off web, and the only other guess I’ve got since reverse still works is a stuck valve body. As such, I’m wondering if replacing it with a manual valve body would remove the need for the modified vacuum modulator?
In the course of 12 years, I went through 3 vac. modulators in my '32 (302/C6). You could always tell when it was on it's way out...there would be a long 'pause" between 2nd & 3rd. Also, when put in reverse, there would be a second or two before it went into gear. Would just replace it with another OEM modulator and good to go...for a few years. Never did figure out what was causing their demise. A good flush is always a good thing too...especially if its been sitting a while.
I believe so given that we did not touch the transmission since I bought the car, and 3rd worked fine until we tore the motor apart in 2017 the first time. Haven't had a motor stay good since then long enough to need 3rd until this one, so I have no clue if it stopped somewhere in between now and the most recent motor. Hell she made it from Florida to Canada and back on the same transmission setup it has now, all that’s changed is the motor
Here is a picture of what it looks like with the tailshaft housing removed. Google around a bit to get an explanation of servicing.
You have to remove the 4 bolts that hold it on the tailshaft. Once it's removed you can access the governors from the inside by removing a clip. Once the clip is removed be careful the spring doesnt fly out, the piston should slide out easily. I think the small one is the secondary governor that controls the 2-3 upshift timing. It should slide freely in the housing with a coating of atf on it. On re-assembly torque the 4 bolts to 12-16 ft. pds.
Get a hand operated vacuum pump.... They are cheap. Connect a hose to it that is long enough to reach inside the car. Pump around 18-20 inches of vacuum and see if it shifts then. This will also tell you if it can hold a vacuum. If it shifts.... Great! If not..... You know you have problems.
Are you sure the mechanical linkage is adjusted correctly per the shop manual? Things may not be adjusted right if the engine has been out 'n in. -Dave
We had some linkage issues in a prior iteration that have resulted in us checking the adjustment once the motor is in every time we’ve done it so far. I had not thought about this, I’ll have to give it a shot
That's a GREAT suggestion...I never thought of that. If it does shift at 18-20 inches, try it at lower vacuum. I always timed my 302 with a vacuum gauge and my max vacuum was 16 inches and would time it at 15. 15 inches of vacuum was enough to operate my C6 modulator.
Have you checked out the VTCI Forum www vintagethunderbirdclub.net or a message board that focuses on C6s? You should have a "green Dot" dual-range Cruise-O-Matic version that starts out in 2nd if in the "green Dot" position versus the 'D' position. X2 on checking the governor and linkage adjustment.
I have an... interesting relationship with the vintage tbird forum. It is the green dot C6, that’s how I know I don’t have 3rd when I try to manually engage it. Will recheck linkage and try governor if the vacuum is not the issue. On another note, we rechecked the vacuum and timing with that gauge today and I think we were hitting 14” max at the manifold, hooked up in the same place we have the trans line hooked up at.
Might look at your kickdown link, if it is stuck in the down position at the trans it will never get 3rd. Unhook the kickdown link at the small lever on the trans, push the lever down and it should spring back up a little bit. Don't think you would want to run a manual valve body in a cruiser like yours.
I would pull the pan drain the converter. change valve body to a 67 model to get away from the green drive. and be certain the governor is not stuck. Use Ford Rotunda trans fluid.
What’s the difference between the 66 and 67+ valve body? Do I need to make any modifications to install it?
So new update, tuesday it started giving us neutral in second and second in third. Or at least we think it’s second since we can’t really go above 40 without worrying about overrevving the motor. Vacuum doesn’t net us another gear, and our linkage is fine. Currently thinking that replacing the modulator, cleaning the governor, and a fluid and filter change is the way to go, otherwise I probably will just drive it until the trans goes and do an OT swap with a tboned Grand Marquis. Does this seem like a solid plan?
When you pull the trans pan if it has more than a small amount of black thick residue. changing the fluid will not fix anything. The only way that crud gets in a trans pan is from the clutch lining material. pulling that trans pan and changing fluid is iffy. you still have dirty fluid in the converter trans lines and oil cooler. the ford filter is a screen its easy for stuff to pass thru it and jam the valve body & governor. I take the filter medium from a turbo 400 and sandwich it between the ford filter and valve body.
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