Turbo 400 question

[dtkarst]

Got a 71 Longhorn BB with factory 400 pickup I've had for awhile that I've decided to tinker with a little. Runs and will drive, but not normal. I shift in reverse and it kicks backwards/engages like it should, then i shift to D and it doesn't kick in and engage like reverse does. I give it gas and i will slowly creep forward most of the time sometimes it doesn't. Doesn't make any abnormal noises, just won't move. I can then shift into L1 and L2 and it'll engage and drive like normal. Possibility that it could need gone through, not sure. I crawled under it and found the vacuum line is hooked up, but on the driver side, there are two wires (a black and an orange) with just the orange wire hooked up. I only see one prong on the switch..my question is, shouldn't there be two prongs on it? or is there a male end and a female end on it? Its been blizzarding all morning so i can't get a for sure look. But both connectors on the wires are female end. Could that be the issue I have? The oil is still clean and doesn't smell burnt. If i need to replace it thats what ill need to do but I'm just trying to exercise all options before i change it. thanks

[too much stuff]

I believe the wiring is for the kick down. Have you checked to make sure the fluid is full to the mark on the stick when hot?

[dmjlambert]

With the engine and transmission warmed up (from driving it for a little while in L2 perhaps, you should stop on a level surface and with the engine running and foot on the brake put the transmission in each gear for a couple seconds, and put it in park and leave the engine running. Then with engine running check the dipstick for proper fill. If it is under full or over full, I suppose that could cause trouble.

The electric connection is just the kick down, and the truck will drive like normal without it. The only thing it does is when you are going down the road and press the gas pedal all the way to the floor, it will drop to the next lower gear and that will make the truck accelerate quickly. Some people might call it passing gear. For purposes of getting the truck going, you can just disconnect it. I think that is the orange wire, you can confirm, with key on but engine not running, when you floor the gas pedal do you get 12V on that orange wire that connects to the vertical prong on the transmission?

The other missing electric connector prong was only on some transmission/truck years, and it is for transmission controlled spark. You don't need that.

This is what my transmission connector looks like, with just the kickdown vertical prong. I have not connected it yet, but plan to.

[dtkarst]

Fluid is full just a hair over full, nothing of concerned. I’ve checked and checked and checked the fluid on it.

[dmjlambert]

Some other folks who know more than me may need to chime in. I would probably test the modulator next. https://itstillruns.com/test-th400-v...-12179298.html

[AussieinNC]

TH400 trans ....

If you have no forward gear take up in D, but have forward gear take up in L1 or L2...the Low roller clutch has failed....

Its a one way sprag race style item that works in conjunction with the Forward clutch pack to provide 1st gear...see pic below...the rollers and springs have collapsed...common problem...

Modulators control clutch pack application pressures...

Kick down solenoids control only kick down valves...

Governors control shift points

[panhead59]

Great info. To bad but it sounds like rebuild time. Get somebody reputable to do it and you'll most likely never have to touch it again ! It's a 400 so it's definitely worth it.

[AussieinNC]

Yep...rebuild time....

Whoever you get to do the rebuild, make absolutely sure they replace both sprag clutch units with the highest count roller clutch units they can get...

Use Alto red clutch plates, kevlar band, and replace all support bushes....

They should also install a Towing level shift kit for better application pressures...

Once the trans is done correctly...it will outlive the truck...

[rjs53]

Have you checked to make sure the linkage is set up so that its shifting into drive complete? Just a thought but hate to see you pay a grand or more for something not needed.

[dtkarst]

Thank you all for the replies. I’ve tried additives which I usually never believe in and check levels, checked my linkages, etc. I’m afraid she needs pulled. Part of it I suppose, have a good transmission guy that we use on all of our old stuff so we’re in good hands. Thanks again for all the info, he described it the same way AussieinNC did so that’s probably what happened. That’s okay, plenty of projects to Keep me entertained