C6 Trans shifting Question
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New User
Joined: Aug 2010
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C6 Trans shifting Question
I have an 88 F250 4WD truck.
Automatic C6 tranny will not shift when in D I can shift manually from 1 to 2 but when back to D for 3rd, it's like D is 1st only. I've replaced the vacuum mod, haven't adjusted it, replaced fluid, pan gasket, filter. If I start in D, it winds up and will not shift, if I let off the gas and back on, it just revs as if its in neutral until it slows down enough to go back to 1st.
Fluid was clean, not burnt, no metal in the pan.. I replaced the VSS and Vac Mod all with the same result. I am wondering if I have a linkage problem or possible valve problem.
Any ideas?
Automatic C6 tranny will not shift when in D I can shift manually from 1 to 2 but when back to D for 3rd, it's like D is 1st only. I've replaced the vacuum mod, haven't adjusted it, replaced fluid, pan gasket, filter. If I start in D, it winds up and will not shift, if I let off the gas and back on, it just revs as if its in neutral until it slows down enough to go back to 1st.
Fluid was clean, not burnt, no metal in the pan.. I replaced the VSS and Vac Mod all with the same result. I am wondering if I have a linkage problem or possible valve problem.
Any ideas?
Post Fiend
Joined: Jan 2005
Posts: 8,402
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You should have around 18in Hg vacuum at the modulator on the trans.
If a diesel and you do not have that amount of vacuum then the vacuum pump may be bad or the VRV on the injector pump may be bad.
However check the condition of the pipe from trans to engine first if vacuum is bad.
If a diesel and you do not have that amount of vacuum then the vacuum pump may be bad or the VRV on the injector pump may be bad.
However check the condition of the pipe from trans to engine first if vacuum is bad.
Thread Starter
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Joined: Aug 2010
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That did it!!
Thanks guys for your valued and much appreciated input. It was a vacuum problem. I had none at the Modulator and once I corrected that, this thing runs like a dream. Few more little cosmetic things and all will be well.
