Have a rebuilt early C4 with 50kms on it in a 66. The shifts between 2-3 at around 60kph are cycling (sometimes called shuttling) back and forth when the slightest load is applied to the box. It also does the same thing around the 80kph mark under similar load. If you accelerate through the range past these points it holds it in third as it should. Hover around those lower speeds and it cycles / won't pick a gear. Fluid levels are ok and have tried softening and hardening the shifts via the adjustable vacuum modulator. Vacuum Line to manifold is new and the vacuum around 15 at low revs. Ford manual doesn't cover this scenario, a few bits on the web about it happening with C4's but none with a resolution.
Have checked the kickdown as well. Have put a new vacuum mod in it for next test run but I don't think that will solve the issue. The valve body might have been tampered with by PO, if you put it in 2nd it doesn't shift to 3rd but when in drive runs 1,2,3 like some Transgo kits.
Anyone had a C4 do this before?
C4 Auto cycling between 2-3 shift
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C4 Auto cycling between 2-3 shift
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