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post #1 of (permalink) Old 03-31-2000, 01:32 PM Thread Starter
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HELP! C6 won\'t shift into 3rd Gear!

Hey guys: I got another problem. I helped my buddy swap his Manual Trans. for a C6 (in his 83 F150 4X4, I-6 Carbureted). It was a big job, but we got it done. The C6 Was from a Junk Yard but supposedly only had 100K Miles on it (but we don't really know the history of the trans. except that it came out of an 88 Bronco). The Truck shifts fine, except it won't go into 3rd Gear. All other gear positions work great. He installed a new vacuum modulator on the Trans and hooked the line up to the PVC Tee on the Valve Cover. Is that where the vacuum line should go? I thought that it might be designed to work off of full manifold vacuum. I appreciate any tips you can offer. Thanks.

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post #2 of (permalink) Old 03-31-2000, 04:23 PM
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Re: HELP! C6 won\'t shift into 3rd Gear!

Hook it up to a full manifold vacuum port. Third gear is completely controled by the modulator unless you have a fully manual vale body.

Did it go together nice and easy? You did something wrong.Will have to do it again.[img]/wwwthreads_images/icons/crazy.gif[/img]
post #3 of (permalink) Old 03-31-2000, 09:19 PM
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Re: HELP! C6 won\'t shift into 3rd Gear!

If it shifts normally from 1st to 2nd and won't go it to'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' 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.
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post #4 of (permalink) Old 04-03-2000, 02:30 PM Thread Starter
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Re: HELP! C6 won\'t shift into 3rd Gear!

Thanks guys. You were right, as I suspected. Full Manifold Vacuum made the difference. Its shifts like a champ now!

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