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James Lamb · 01-20-2021, 12:41 PM

As someone already posted, with a 3.08 diff and the lock-up final drive on a 700r4 being .696, your engine rpms are probably not going to be high enough to prevent the transmission from constantly searching for 4th/lockup. Which in my experience very quickly = burnt out lock up solenoid.

I have a 1989 Suburban 4 x 4 with 3.42 diff. I run 31" tires, which run 48 revs/mile less than the stock tire size. At 55 mph my engine is turning over at 1427 rpm. In my case, I have a freshly rebuilt stroked 383 with a mid-range cam, so it makes enough torque that the engine is happy with this. Not sure it would like it much if I had a 3.08 rear end.

And as has been stated previously, the 700r4 is really a dog of a transmission. GM's answer to late 1970's oil embargo and high gas prices. I have had 4 of them in my truck, only one of which got ruined by someone (not me) doing something stupid. I've had bargain-basement franchise built ones and custom shop built ones with the heavier duty parts, none of them got much past 80,000 without issues. And I take care of my truck. Just not a good transmission.

If the current one ever craps out, I will bite the bullet and find a later electronically controlled unit and buy the wiring/ecu adapter kit.

01-20-2021, 12:41 PM	#17
James Lamb Registered User Join Date: Mar 2019 Location: Knoxville AL Posts: 74	Re: Trying to do the logical thing. As someone already posted, with a 3.08 diff and the lock-up final drive on a 700r4 being .696, your engine rpms are probably not going to be high enough to prevent the transmission from constantly searching for 4th/lockup. Which in my experience very quickly = burnt out lock up solenoid. I have a 1989 Suburban 4 x 4 with 3.42 diff. I run 31" tires, which run 48 revs/mile less than the stock tire size. At 55 mph my engine is turning over at 1427 rpm. In my case, I have a freshly rebuilt stroked 383 with a mid-range cam, so it makes enough torque that the engine is happy with this. Not sure it would like it much if I had a 3.08 rear end. And as has been stated previously, the 700r4 is really a dog of a transmission. GM's answer to late 1970's oil embargo and high gas prices. I have had 4 of them in my truck, only one of which got ruined by someone (not me) doing something stupid. I've had bargain-basement franchise built ones and custom shop built ones with the heavier duty parts, none of them got much past 80,000 without issues. And I take care of my truck. Just not a good transmission. If the current one ever craps out, I will bite the bullet and find a later electronically controlled unit and buy the wiring/ecu adapter kit.