Join Date: Mar 2001
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Re: 89 e150 aod AND 35\" TIRES question
It's because you still have stock gearing with 35" tires. The gearing was specified for 31" tires for the trans to operate within the engines power band. The bigger tires lowers your engine RPM and at cruising speeds in overdrive you're lugging the engine to accellerate. That's why it keeps jumping down into third. On a flat stretch of highway, under no load it stays in OD, right?
The only real options you have are to put smaller tires on (33"), keep your trans in 3rd gear (don't use OD), or install lower gears (the most expensive route).
Consequently, earlier design OD transmissions were designed poorly with extremely small OD clutches that can't handle the in and out shifting you're experiencing. Your trans is overheating from the load you're placing on it and it will fail if you continue to drive like that. For the time being, until you decide what to do, keep it out of OD and change the trans fluid immediately.