Th400 wont go into 3rd

[chris3353]

Are you confident you are getting a vacuum signal at the module? If yes, do you have a vac gauge? do you know what it is? Can you tee into this hose/line for a test drive?

This is the lowest hanging fruit, low/no vac signal= no shift

 

[Ricko1966]

Are you confident you are getting a vacuum signal at the module? If yes, do you have a vac gauge? do you know what it is? Can you tee into this hose/line for a test drive?

This is the lowest hanging fruit, low/no vac signal= no shift

To add to this though, if it's the modulator it would also have a very delayed,harsh first,second shift.

 

[Beast496]

No third gear almost always ends up being the direct clutches used up or low pressure which will be the same result. If that is the case, it's just a matter of time before you lose reverse as well. I'd put a pressure gauge on it and see what it reads for line pressure at idle and through the gears at rpm, especially when third gear should be applying (~45 mphish). That will tell us what it is doing for sure. My money is on direct clutch pack.

 

[Tyger13us]

I know that if the vacuum signal is weak it wont shift like it should, the the "modulator valve" is stuck or plugged up it wont shift right, Not the modulator it self. the modulator sound like it good, but it pushes the Modulator VALVE in the case back and forth.
If its really stuck, the only way it can shift is threw the governor.

I would hold it to the wood and run threw the gears, 1-2 worked 3rd will shift just when the motor is going to blow up,
the governor valve will have enough pressure on it that it will BANG shift to 3rd.
If it will never shift to 3rd no matter what and just free wheel, no engine load, ya got something plugged up in the valve body or a pan full of clutches.

 

[NickTransmissions]

I recently purchased this 1983 K10. It has a new 6.0 LS motor with a TH400. The truck will not go into 3rd gear. Sometimes I think it's trying to grab 3rd when the tranny is cold, but slips right out. I feel I have done all the recommended suggestions on YouTube with no results:
1. Changed the fluid
2. Adjusted the Vacuum modulator; had no change in either direction, so I changed it. Still no changes in shift strength or point when it shifts.
3: I removed the governor and found the valve was stuck. Disassembled and cleaned. Still no change. I ordered the kit to adjust the weights. I put the heaviest weights on and was going to adjust if I noticed a change, but it made absolutely no difference.
4: I verified the shifter was aligned correctly.

I am at a loss of what to try ot look at next.

Not sure if you have already identified the cause but based on a quick review of the thread, I going to suspect your direct clutch pack is shot. Drop the pan and see if there's any burnt clutch material at the bottom...If so, yank it for overhaul...Install a 4L80E direct drum w/34 element HD sprag and have the direct clutch piston machined down for an extra .090 steel as well as a factory spec friction. If high RPMs (i.e. WOT pulls in 1st and 2nd at 5500 RPM or more, install high rate return springs in both the intermediate and direct clutch return spring assemblies and a HD retainer plate for intermediate...If going north of 6500 regularly, do the HRRS and drill a .055 auxiliary bleed in the direct drum to further mitigate risk of clutch drag and centrifugal apply. If no high RPMs and engine power less than 650 or so, just go back with the stock return springs and add a bonded piston to the direct drum - five friction plates and five steel plates will be sufficient.