C6 vs Vacuum Leak
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C6 vs Vacuum Leak
Hi all, among a few other problems I'm trying to solve at the moment, I have a C6 that has a very lazy 2-3 shift when under load and much more pronounced when the car has been running for a while. I was just wondering if anyone thinks that a vacuum leak could possibly be the cause of the shift problem. If there is a vacuum leak it may explain some of the other issues too.
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From: In my own world
vacum leak is a problem, but not in your case, a c6 won't come out of 1st if it has no vacum.pull the vacum line of the modual on the pass tailshaft, and see if trans fluid comes out, if so, replace the modual. pay careful attention to the painted stripes, and put one back that matches. if no trans fluid in the line, take a tiny straight screwdriver, and stick it in the modulater where the line goes, turn it in, or out to suit your taste in shiftng. a little at a time, and road test every time, is the key here. you can also adjust the new modual the same way
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I haven't heard of that adjustment before, how does that affect the shifts? Sharper/Softer or Later/Earlier?
Post Fiend
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From: Charles Town, W bygod Va
try asking in the transmission section. Clutch, Transmission, Differential, Axle & Transfer Case - Ford Truck Enthusiasts Forums
Transmission Rebuilder
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From: North Georgia
turn the screw out = earlier shift
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From: Virginia
If you have low vacuum, it will delay the shift. A good indicator of this problem is if the tranny will go ahead and shift if you suddenly lift off the gas pedal. This creates a high vacuum spike when the throttle blades suddenly close, and will usually make the tranny shift if it was waiting for a higher vacuum signal.
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From: In my own world
If you have low vacuum, it will delay the shift. A good indicator of this problem is if the tranny will go ahead and shift if you suddenly lift off the gas pedal. This creates a high vacuum spike when the throttle blades suddenly close, and will usually make the tranny shift if it was waiting for a higher vacuum signal.
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If you have low vacuum, it will delay the shift. A good indicator of this problem is if the tranny will go ahead and shift if you suddenly lift off the gas pedal. This creates a high vacuum spike when the throttle blades suddenly close, and will usually make the tranny shift if it was waiting for a higher vacuum signal.
Having said all that I now think the problem is still the 3rd/reverse band and/or its control circuitry (supposedly 'fixed' last week )
. I had about 900 kilos of gravel and cement on the tray today and a trailer worth another 900 or so and it wouldn't reverse (slightly uphill) in front of our house. It selects reverse, but no motion. Take off the weight and it will reverse OK untill the hill gets too steep and it will start slipping again.
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Cargo Master
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From: NJ
C6 will shift 1-2 without vacuum modulator conencted. You will not get 3rd without vacuum though. Quick and easy check is pull the vacuum line fmro the modulator off the intake and suck on it. You should be able to pull a vacuum and feel the modulator valve pulling in, you should also be able to put your tongue over the line after pulling vacuum and hold for several seconds, then feel a snap of air rushing in when you release this indicated the modulator is good and holds vacuum. If not replace the modulator.
Adjusting modulator for a later shift makes a softer shift, and for an earlier shift makes a firmer shift. Not what most people want to hear, but that's how it works on a stock valvebody.
Adjusting modulator for a later shift makes a softer shift, and for an earlier shift makes a firmer shift. Not what most people want to hear, but that's how it works on a stock valvebody.
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So which is true? Can't be both can it??
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OK, so here's another thought. Could the kickdown rod play any part in the slipping shifts? Obvoiusly when you floor it, the kickdown punches it back a gear, but what if you keep your foot planted, or very close to flat out? Could this stop it shifting properly?
Just a thought.
Just a thought.
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From: Virginia
Activating the kickdown definitely affects shifting. Also flooring the engine gets rid of the vacuum signal to the tranny, which also affects shifting. That's how the vacuum modulator works. High vacuum means the carb is closed, so there is little load on the tranny. Low vacuum means the carb is opened up and along with a low road speed(governor), tells the tranny to delay shifting to keep the engine revs high.
Theories of operation are all well and good, but if the tranny is slipping, it's time for a rebuild. No amount of fiddling with the controls of the tranny will help a slipping problem. A lazy shift could be a control problem, but if you have engine rpm flare up also when it shifts, and it's slipping in reverse, that indicates the clutch linings are worn in the tranny.
Theories of operation are all well and good, but if the tranny is slipping, it's time for a rebuild. No amount of fiddling with the controls of the tranny will help a slipping problem. A lazy shift could be a control problem, but if you have engine rpm flare up also when it shifts, and it's slipping in reverse, that indicates the clutch linings are worn in the tranny.
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Totally agree, but this transmission has just been 'rebuilt' twice by 2 different places. The level of incompetence in the automotive repair industry these days is pathetic. I need to know what is wrong so I can tell the next #$%kwit that touches it exactly what needs to be fixed. If I had the resources I would be fixing it myself.
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From: Texas
I've got the opposite problem. I've got a C6 on a 460 that shifts prematurely. Doesn't matter where i set the modulator, when I hit 3000 RPM I hear a squeal and it seems to force an upshift. Any ideas? It seems that it suddenly introduces vacuum from some source as if i took my foot off the gas and shifts. And then there's that terrible squeal everytime it does it.
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