TH700-R4 Questions

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Hi, new poster here so let me know if I'm out of line... Okay, on to business:

Yesterday I bought a 1985 GMC Sierra Classic, 350c.i./5.7L, TH700-R4, 4x4, NP-208, stock ride height & suspension.

Whenever I am accelerating, the acceleration is smooth until 3rd & fourth gear. When hitting 3rd, the transmission seems unsure and sometimes switches between 2nd & 3rd rapidly before staying in 3rd. Once I'm in third gear the truck lurches & lunges until RPM gets above 2,700 or so, as if it was backfiring (Though it's not). Same thing in 4th gear (OD).

Also, it seems to switch to third gear a little early and kind of bogs down the engine, though I've never had a 700-R4 before, my other 1980 GMC has an SM465.

Any help would be GREATLY appreciated! thanks.

P.S. great site

[VileZambonie]

Check the adjustment of your TV cable. It sounds like it's set too loose. When you feel it bog it's because not only is it upshifting too early but then the converter clutch engages.

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Alright, I will check the TV cable when I get home tonight. How will I know when it's adjusted correctly? Will I have to just test-drive it every time? P.S. Could the lock-up converter be engaging too early? I thought it was only supposed to "lock-up" after 45 MPH. Mine seems to lock up @ 35.

Thanks for the help once again!

[Blazin]

Don't quote me but if I remember correctly you disconnect it open the throttle wide open pull the cable to meet it and attach. Like I said its been a long time and I watched a buddy do it to a customers truck that he had rebuilt the trans. in.

[Lt.Del]

It seems like the converter is locking up too quickly.  The converter lock up is controlled by certain sensors.  I believe one is tell it to lock up only in high gear.  One senses the vacuum or distributor.  One switches the lock up off when applying brakes.   I am no expert in trannys, but have put a th350c in my 79 and when I flip my manually controlled switch on to lock it up, if it is too early, it makes the truck start to bounce and lurch and whatever, just like a manual tranny when throwing it in high gear too soon,.

[VileZambonie]

The converter clutch engages on your trans via pressure switch in the trans. The converter clutch can apply in 3rd or 4th gear. The circuit is interupted by an increase in TV pressure, Brake switch on input, or low vacuum switch. Make sure the TV cable is set correctly and then if it's still locking up too early check the low vacuum switch. The worst thing about a lot of these trucks was the 700R4 paired with 3.08:1 Final drives - it's definitely better with 3.55 or numerically higher