odd trans shifting problem.
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Freshman User
Joined: Jul 2007
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odd trans shifting problem.
ok, well im sorta figuring out this truck as i go. i got it for cheap, 6.9 diesel with 3 spd c6 (87) when its warm the shifting is a little off. sometimes its like it doesnt want to upshift or downshift when its suppose to. well the other day when it was really cold, 20ish degrees (first time driving it in that cold weather) it took a few mins of driving to get it to shift through all the gears. and when it was warmed up, it seems like the trans doesnt want to downshift when its suppose to. I know it has a shift kit, so i dont know if that is the reason its off or not.. i just ordered the trans modulator, so in a few days i will see if that helps. in the meantime, what else could be wront with it? fluid is good, the truck sat for a few years before i got it, but it had fluid in it and i already changed the fluid/filter before driving it.
ideas?
ideas?
Post Fiend
Joined: Nov 2004
Posts: 5,638
Likes: 1
From: Manitoba
You should start from the bottom and work up......... do a band adjustment, Park neutral position switch adjustment, then gear lever adjustment, vacuum test and VRV adjustment. Checking/replacing the modulator is only 1/4 of the work.
Once all this is done then troubleshooting will be relivant if there is still a problem...
If you need the how too's for this stuff click on my name email me and I can send it too you.
Once all this is done then troubleshooting will be relivant if there is still a problem...
If you need the how too's for this stuff click on my name email me and I can send it too you.
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Freshman User
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well i think i may of found the issue. i finally found a vac gauge and i tested the vac pump and it pretty much read zero. when i started it it moved not even a 1/16 of an inch on the gauge..so.. this will cause the issues with no vac/no shifting correct?
Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
No vacuum in the system will cause the C6 in these trucks to shift like you are flooring it all the time. So if it does upshift it will likely be high in the RPMs and feel very positive.
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Freshman User
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yeah im just going to buy a new pump. pensacolal diesel has them for 104 with no core needed..tried to get the pulley off, and talk about flimsey..so i have to get one of those. the old one was rusted on there so bad i dont think it even had a bolt on it and it still stayed on.. and yes it feels like no power brakes, im not sure what it would feel like with power brakes since this is always how its been. and it does have high rpms.. and all the other signs of it being a pain in the ***. im almost 100 percent certain the vac pump is causing my problems.
Post Fiend
Joined: Nov 2004
Posts: 5,638
Likes: 1
From: Manitoba
You need a special puller for the pulley removal and replacement.... don't try prying it off or hammering it back on....... cuzz if ya do you'll screw up one or both parts......
Vac pump pulley puller is same as power stearing pulley puller.
Vac pump pulley puller is same as power stearing pulley puller.
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Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
I lost vacuum a few times when I still had the C6 transmission and it did the high RPM hard shifting and lack of power brakes as well as no cruise control or other vacuum accessories. In my case it was the alternator belt blowing apart.
I agree that its likely a bad vacuum pump.
You could also try a local wrecker to try and locate a cheaper vacuum pump.
I agree that its likely a bad vacuum pump.
You could also try a local wrecker to try and locate a cheaper vacuum pump.
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Lead Driver
Joined: Nov 2003
Posts: 6,900
Likes: 3
From: Campbell River, B.C.
I think the only difference is the pulley that is installed on them since later IDIs had a serpentine setup. Just bring your old one to compare against but it should be the same.
Postmaster
Joined: Apr 2008
Posts: 4,802
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yah, those seem to be a wear item - mine went out, I borrowed one from a buddy while his truck was down, tried finding me a replacement for cheap but I couldn't, so I converter to hydroboost and put my old pump back in and gave my buddy back his good pump. He now hes full vacuum in his truck, and I have just enough to work my HVAC and cruise, but they don't take much and I have reserve canisters for both of them anyways.
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