1991 Toyota 4Runner 6 cyl Four Wheel Drive Automatic 224000 miles
I drive a '91 Toyota Hilux Surf (non-US version of a 4Runner) with only 224,000km on it. I have noticed that when the engine & transmission are warmed up, the whole vehicle shudders while cruising at 45-65mph while it is in O/D. The shuddering is similar to a washing machine with a tennis shoe in it, or almost fells like the shake you get when you've put a manual shifter to high for your road speed & it's dogging the engine. If I apply gas or manually disable O/D the RPM's of course increase but the shuddering stops. I think my problem lies in the tranny, but I wanted a fresh (and professional) view.
A few more details about my vehicle (photo attached): it is raised several inches, 35" tires with about 5/8" tread remaining rotated regularly, rear dif. pinion angle is ~3degrees & the tranny pinion is higher but I can identify the difference between the fast vibrations of the drive-shaft's U-joints and the larger shudders that concern me.
One more note: The transmission was a rebuilt by a company in NE who I won't name because they sent me a faulty unit that catastrophically failed within 2 weeks of installation (internal clutch disks actually shattered leaving me with 1-2 only, no reverse & no 4WD). After several conference calls between them, the Toyota dealer who installed it, and me, they finally accepted it back but then sent me this one with the serial numbers wiped out, so basically I'm on my own.
I usually do all my own work, including a complete tear down and rebuild of my motor, installing lifts, suspensions, and even fabrication my own custom parts... But when it comes to tying to understand how the fluid dynamics, planetary gearing and solenoid flow in an automatic transmission, I get migraines.