1986 2wd 720KC Auto Tranny Issues

[720noob]

Hey all, I got some tranny issues that I need help with.

 

Issue: It doesnt like to go into reverse for a few secs, then it finally goes in. It doesnt like to shift outta gear til the engine is revved high and kept there then it shifts, When it shifts it doesnt slam gears it goes in smooth. After driving on the freeway it shifts pretty much like it should. It bogs down when it shifts, but i think thats a carb issue.

 

I thought it might be the vaccum modulator, but im not sure since im really new to the 720 game. I am mechanically inclined, so replacing some parts or the whole thing isnt a big deal to me. Just need some help in the experience dept. Im going to assume its the L3N71B Trans, but as the name states......720noob.

 

Any advice would be greatly appreciated.

 

Id like to keep the auto. maybe a 5spd conv down the road but auto for now.

 

Could i direct swap the L4N71B in its place without any issues??

 

Thanks again for all your help in advance.

 

Chris

[datzenmike]

Check the obvious things. Fluid level must be in the run range on the dipstick and definitely not over full.

 

Fluid should be bright RUBY RED and see through clear. If cloudy, brown or burnt (or worse black) it and the filter should be replaced.

 

Sluggish shifting into gear could be low fluid or a dirty filter. If you have never had your tranny fluid changed in the last 30K miles then it's due.

 

An L4N71B from and early D21 or KA 200sx would work. I have an L4N71B and it's slightly longer than the stock 3N71B but the driveshaft may fit ok. The Z series and the KA motors had the same bolt pattern and tilt so it should work.

 

One thing check the output spline. Mine was from a six cylinder Maxima and the spline was larger. I don't know if they all are or just the larger motors like the VG.

[720noob]

Would you recommend having the trans flushed through a shop? Or is the conventional "driveway drop the pan " method good enough. I know by flushing it should clear out everything. Im unsure if the pan drop does a good enough job at clearing out all the old fluid. I have no idea as to when the fluid was changed last since I just got the truck. The guy before me hadnt done it either, But he didnt have it for very long either.

 

If i drop the pan I should be able to get to the filter that way correct?

[datzenmike]

Trans shops will tell you anything is wrong with it to get your money. Yes get a filter kit and drop the pan. Replace the filter and I think there is a brake band adjustment. Refill and pull the return cooler line off and idle the motor. This will flush out the old oil in the cooling lines too. When it comes out clear you're done. Check the level and add to full line.

 

 

What color is the fluid right now?

[720noob]

Its red-ish, Its not the deep red as new tranny fluid should be.....i can tell its old but it doesnt smell burnt or look burnt.

[datzenmike]

Well that's good. Auto tranny is good for about 100k miles under (very) normal driving. Just think of deep fryer oil. The heat eventually breaks it down and it can't do it's job. Hot weather stop and go, heavy loads and the worst, TOWING will shorten the life of it. A 30K change is better and the ATF is cheap. It also allows you to replace the filter and set the band adjustment to compensate for wear.

[720noob]

Thanks again for all your help mike. Ill do the service and let you know if the problem continues.

 

Ill be sure to get up some pics of the ser vice and my truck once i can.

[datzenmike]

I have the '84 FSM if you need specs for the band adjustment.

[720noob]

I have the '84 FSM if you need specs for the band adjustment.

 

That would be awesome! Thanks again for all your help.