Re: HELP! C6 won\'t shift into 3rd Gear!
If it shifts normally from 1st to 2nd and won't go it to 3rd.....it's hard to say. Most shifting problems either involve the vacuum modulator or the govenor hanging up. In your case you state that it shifts ok between 1st and 2nd ..... .....but does it really? What's the shift points at a normal take off from stop? At a mostly closed throttle it should be at a fairly low speed. If you take the vehicle over 65-70 mph in Drive.....does it shift into 3rd? If so then the problem is NOT in the bands/clutches/accumulators/pump/or valving .......that is, if it shifts into 3rd eventually some how the trans is "seeing" W.O.T. and is shifting accordingly. This implies a bad vacuum modulator (which can't be the case if you have normal 1-2 shifts.......let's see here.......(it has been since the mid 70's since I've touched one of these things so...bear with me) didn't some trans use the vacuum modulator only for the upshift........but relied on an electric (throttle actuated) kick down? I'm sure that this was the case for the FMX and perhaps the later model (70x ) C4's? If this was the case however you'ld have some loose wire hanging around and would wonder where it went......which, since you didn't mention it, you don't....so that's probably not it. Sometimes the electric acuator is constantly engaged.......the 1-2 upshift would be normal......the the "kick down" would never let it out of 2nd until 65-70 mph.
Tap the vacuum directly off the manifold as stated.......blow by windage may lower the perceived manifold vacuum at the higher load/engine speed that you see above 20-30 mph if you are using the PCV hose, preventing the upshift.
Figures don't lie ....... but liars sure do figure.