C6 shifting odd
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Elder User
Joined: Feb 2007
Posts: 690
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From: Madison Ohio
C6 shifting odd
What tells the C6 to upshift?
My 89 F250 4X4 with a C6 is not happy to upshift if I give it a LITTLE bit of peddle. It was rebuilt however it was done like 4 years ago and sat on the shelve or in the truck until now.
Its almost as if as soon as I let off the gas it shifts? I check the kick down and its not even pulling the cable until about 3/4 wide open. But I can take it down the road and wind 2nd out with the motor hitting 3 grand with out it grabbing 3rd? 1st to 2nd is meh about the same. I can get it up to 15 mph and let off the gas and pop 2nd? I'm not sure I will grab 3rd with out letting off the gas.
Does it just need to get driven for a few miles or do I have a problem?
My 89 F250 4X4 with a C6 is not happy to upshift if I give it a LITTLE bit of peddle. It was rebuilt however it was done like 4 years ago and sat on the shelve or in the truck until now.
Its almost as if as soon as I let off the gas it shifts? I check the kick down and its not even pulling the cable until about 3/4 wide open. But I can take it down the road and wind 2nd out with the motor hitting 3 grand with out it grabbing 3rd? 1st to 2nd is meh about the same. I can get it up to 15 mph and let off the gas and pop 2nd? I'm not sure I will grab 3rd with out letting off the gas.
Does it just need to get driven for a few miles or do I have a problem?
Lead Driver
Joined: Aug 2006
Posts: 5,670
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From: Sand Lake, MI
Check the vac modulator and vac supply line to it, if tests good verify engine vac reading is up to spec.
Vac line starts out rubber, changes to metal tube, then back to rubber again down at the modulator.
Vac line starts out rubber, changes to metal tube, then back to rubber again down at the modulator.
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Elder User
Joined: Feb 2007
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From: Madison Ohio
I did not check at the modulator its self. At the tree pulling off the next port I get a steady 18. Its new hose and metal tube going down. I know he replaced the Modulator when it was rebuilt but its now 4 or 5 years old could it have gone bad?
Lead Driver
Joined: Aug 2006
Posts: 5,670
Likes: 12
From: Sand Lake, MI
I know he replaced the Modulator when it was rebuilt but its now 4 or 5 years old could it have gone bad?
Doubt it just sitting, usually causes motor to noticeably smoke when a modulator fails internally, sucks trans fluid right up the vac line. Small chance it stuck/froze up, cheap/easy enough to try a new one.
"18" is fine, new or not I'd pull a vac on the modulator line and test for leaks just to be sure it doesn't have a crack or pin hole someplace.
Might be worth running a line in the cab, say everything else checks out, monitor the vac readings while driving, just to be sure something odds not going on once up to full running temp.
Doubt it just sitting, usually causes motor to noticeably smoke when a modulator fails internally, sucks trans fluid right up the vac line. Small chance it stuck/froze up, cheap/easy enough to try a new one.
"18" is fine, new or not I'd pull a vac on the modulator line and test for leaks just to be sure it doesn't have a crack or pin hole someplace.
Might be worth running a line in the cab, say everything else checks out, monitor the vac readings while driving, just to be sure something odds not going on once up to full running temp.
Elder User
Joined: Nov 2010
Posts: 957
Likes: 2
From: Graniteville
Some possible culprits:
- Faulty Vacuum modulator
- Improper assembly of the valve body
- Improper installation of a shift kit
- Sticking valve in the valve body
- Possible problem in the govenor on the tailshaft
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Elder User
Joined: Feb 2007
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From: Madison Ohio
It holds vac. I thought I could see the shaft of it while still in the trans. HA not! I was going to pull it out here just have to find something to plug the hole with first.
whats the diff from the purple or green Modulator?
whats the diff from the purple or green Modulator?
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Elder User
Joined: Feb 2007
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From: Madison Ohio
OK update I think the wrong one is on the trans? or its bad. I have a OLD C6 here out of a 85. I pulled the mod off it. It starts to react to vac around 6" and is fully pulled back around 15" the one I just pulled out of the 89 start to react around 15" and is fully pulled back at about 23"
Parts store as the Green? one that they say is common. Mine had a black maybe a purple strip on it. for $17 and change Think I'll try the green one.
Parts store said the color will determine the shift points. and the purple will cause a later shift? Not sure if this is true as the amount of info I know about trans I could fit in a coffee cup! LOL
Parts store as the Green? one that they say is common. Mine had a black maybe a purple strip on it. for $17 and change Think I'll try the green one.
Parts store said the color will determine the shift points. and the purple will cause a later shift? Not sure if this is true as the amount of info I know about trans I could fit in a coffee cup! LOL
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Elder User
Joined: Feb 2007
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From: Madison Ohio
OK picked up the Green striped Mod. Got it home and bench tested it. Started to retract pin at 11" and at 20" it was fully retracted. Popped it in and took for a quick ride ( almost out of gas at this point LOL ) Shifted MUCH MUCH better. However I forgot what it was like to drive a C6 with 4.10 and stock tires 2K RPM at 45 Mph ... Yea low gas mileage here I come! LOL Might be time to look for another ZF here.
Thanks for the help!
Thanks for the help!
FTE Legend
Joined: Mar 2009
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From: North Bay Ont Canada
you are checking the vaccum diaphram & it sounds like it is not leaking. But the actual modulator VALVE is in the tranny case right in front of the short pin that is between the diaphram & the valve. To check to see if the valve is sticking pull the mod & the pin out of the way & pull the spool valve out of the tranny case (with a small screw driver), it should move in & out freely. Gigger
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