Just yesterday I replaced a defective modulator in my 1975 C4 that is in my 1970 Bronco. It would shift real hard, especially 1st to 2nd, chirping the tires. I knew chirping 35 inch tires with a locker on dry pavement can't be good on the drive-train.

It's very, very easy to replace. Some types are adjustable, rotate them one turn, drive it, and re-adjust. Mine is fixed (non-adjustable) with a one bolt bracket holding it in.

I also ran some tranny solvent through it, changed the fluid, filter and adjusted the bands. Everything else was in good shape.

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'66 Galaxie 500, 352, 4 bbl, convertible, auto, PS, 65 pink, stored in garage since '76. Other rides: built 1970 Bronco, 75 F250 HiBoy w/390, CJ5 lifted, 350sbc, Suburban tow rig, 07 BMW