C6 diesel question
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C6 diesel question
I just got a really clean 88 f350 diesel 4X4 from my friend with 90K original miles. It has a c6 trans. The problem is that it holds the gears and then shifts hard, almost like a shift kit. At low throttle it wont shift, you have to gas it. The earliest it will shift is at about 2500 rpm. There is no slippage at all. Any help would be apreciated.
Cargo Master
Joined: May 2003
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From: Earth
You might want to check your vacuum modulator. Pull the hose off and see if there is fluid in it. If there is, then it is shot. It also may just need adjusted as well. There should be a screw like adjuster in there, turn it counter-clockwise for the tranny to shift sooner (lower RPM), and clockwise for it to shift later (higher RPM). This would be the easiest thing to check first and see what happens. Good luck 
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New User
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Originally Posted by 53fatfndr
You might want to check your vacuum modulator. Pull the hose off and see if there is fluid in it. If there is, then it is shot. It also may just need adjusted as well. There should be a screw like adjuster in there, turn it counter-clockwise for the tranny to shift sooner (lower RPM), and clockwise for it to shift later (higher RPM). This would be the easiest thing to check first and see what happens. Good luck
The vacume modulator is new. When replaced it helped a little. Where would I find that adjustment screw? Thanks again also anyone know if there is a difference between the gas and diesel modulator? Maybe the wrong one was put on.
Cargo Master
Joined: May 2003
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From: Earth
For the adjustment, pull the hose off of the modulator and look inside where the hose was connected, you don't need to remove the modulator. Inside, you will see a little adjustable "screw" that has a slot in it for a screw driver. This is what you adjust.
As for needing 19 inches of vacuum, my warmed over 400 only has about 14-15 at idle and it shifts fine. I have heard of guys with much less vacuum that don't have problems either. But, more vacuum is better
As for needing 19 inches of vacuum, my warmed over 400 only has about 14-15 at idle and it shifts fine. I have heard of guys with much less vacuum that don't have problems either. But, more vacuum is better
Last edited by 53fatfndr; Jul 1, 2006 at 01:10 PM.
i ain't rite
Joined: Mar 2005
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From: Marlboro Mental Hospital.
Originally Posted by 53fatfndr
As for needing 19 inches of vacuum, my warmed over 400 only has about 14-15 at idle and it shifts fine. I have heard of guys with much less vacuum that don't have problems either. But, more vacuum is better
Cargo Master
Joined: May 2003
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From: Earth
Originally Posted by tjc transport
but that is a gas engine. the diesel uses a vacuum regulator valve mounted on the injector pump to works the trans. if you have less than 19 inches of vacuum, the valve will not operate properly, making the trans not shift till you let off the throttle.
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i ain't rite
Joined: Mar 2005
Posts: 65,591
Likes: 5,617
From: Marlboro Mental Hospital.
Originally Posted by 53fatfndr
Thanks
Originally Posted by 53fatfndr
I'm glad you posted that. I didn't realize the different requirements from a diesel to a gas engine.
the day we stop learning, is the day we turn into know-it-alls.
i have yet to meet someone that was a know-it-all, but i have met allot of people that thought they were.
like me, after i drink a few to many.
Last edited by tjc transport; Jul 1, 2006 at 07:13 PM.
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Post Fiend
Joined: Jun 2002
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From: Atlanta GA
TJC, you get a rep point for that post. I knew about the pump and such, but, did not know the diesel pump had to make that much vacuum for the tranny to shift properly. I will remember that for when I buy one
edit: I can't give one, I need to spread the love around.
edit: I can't give one, I need to spread the love around.