TH400 not shifting 2-3 at WOT

[critter]

So here are the symptoms. At part throttle I can shift my TH400 manually with no issues. Shifts are firm and immediate. At wide open throttle the transmission will shift from first to second manually but when I manually shift to the stop for third, nada, nothing, it just keeps winding out in second gear. I tried launching in second and going WOT and shifting it to third. It stayed in second. Then I put it in drive and launched at WOT. It stayed in first. No gear shift at all.



So it points to line pressure issues or the governor. Anyone have suggestions? I'll be calling the shop that built it tomorrow.

[MrQuick]

Modulator? No vac? Should be hooked up to manifold vaccum.

I believe I ran into the same issue with it hooked up to a ported vac source.

[MonzaRacer]

should shift by governor at around 5500 unless its been modified. I have a spare if you might be interested.Ill dig it up if you want.

[critter]

Yes, at WOT with no vacuum present the modulator controls the shifts. I think I have valve body issues or a stuck governor. I'm calling the shop who built it today for their take on it. Monza, thanks for the offer. I'll see if I can resolve it and hopefully they'll make good.

[Jake's Performance]

Yes, at WOT with no vacuum present the modulator controls the shifts.

No it doesn't, at WOT with no vacuum doesn't do anything except keep line pressure up.

[critter]

Well I said that ALL wrong. I shouldn't post before having coffee. Jake is right. The modulator works when vacuum is present. At WOT there is no vacuum and the governor takes over. Sorry for the confusion.

[critter]

I got home this afternoon and tried a few quick fixes. I swapped the governor for a known good one. I checked the fluid level, it was a tad high so I drained a little to get it spot on. Neither try helped. I did some more driving and I noted that, even at part throttle, if I push it to third gear manually it won't shift until 3500rpm.

[Jake's Performance]

Pull the modulator out, and then pull the modulator valve behind it, feel for sticking.

[critter]

Will do. Thanks, Jake.

[critter]

So at the request of several folks I yanked the modulator and checked the valve behind it. Guess what? No spring at all on it. I pulled the one from my other transmission and dropped it in, sure that I had the cure. I drove it. No change. So then I tried the one other thing on my list. I unplugged the kick down switch at the throttle. It still wouldn't shift manually to third under 3000 rpm but I could now manually shift it to third above that RPM.

So, what should I do? Leave it as it is and go to Norwalk for Pontiac Nationals or get it to a transmission shop? I'm worried that, if they left that spring out, what else did they leave out?

[Jake's Performance]

The spring is not stock, they didn't use them from GM. That is a part of a shift kit to make part throttle shifts earlier and usually makes it way to early on a car with decent rear gears.

I'm guessing you have a sticking detent valve or 2-3 shift valve.
Time to drop the valve body.
Give me a call in the shop tomorrow. You can fix this in an afternoon. It's definitely VB related at this point.

[Jake's Performance]

Chris,
I got your message but tried to call the number back and it is no good?

Jake

[critter]

Jake, sorry. I'll try to call again tomorrow. I'm not sure what I said. If the coffee hadn't kicked in I might have had you calling Mongolia or something. ;)

Here's where I got to tonight. I dropped the pan and pulled the solenoid. I checked it off a 12 volt source and it seems to open and close properly when voltage is applied and released. I did dig into my factory service manual and locate the detent valve assembly. I didn't have time to drop the valve body but I did stick a screw driver down in there and the valve would compress and return with the spring pressure behind it.

One thing I did notice tonight. In that there are 2 wires going to the kickdown plug on the outside of the case. There's only one plug blade there. I did some reading in the FSM and the 1974, PQ code TH400 had another wire that ran to what they call pressure switch assembly on the accumulator in the valve body. Any ideas on that one? From the looks of it I don't think it affects anything. The FSM wiring diagram seems to show it running to a gauge for cars equipped with rally gauges.

[Jake's Performance]

Chris,
Some units had a pressure switch on the 3rd gear accumulator.
It was used for the TCS on the early 70s cars for emissions. Advanced the timing in 3rd IIRC.

Could be the 2-3 shift valve.

If you need a VB I can send you one already built up.

[critter]

I yanked the valve body. No sticking on the 2-3 but I removed it anyway and polished it all with a Scotch Brite. I drove it today but there's really no change. It seems to have a mind of its own at WOT.

I'm going to call another local shop tomorrow since I have a week to be ready for moving out to Norwalk for Pontiac Nationals. Jake, I appreciate the offer of the valve body. I didn't want to go to a whole lot of trouble here since the 4L80e is going in when I get back.

[critter]

So after dropping the valve body in the TH400 and checking the 2-3 valve I slapped it all back together. It still didn't seem right, not wanting to shift into third. I did notice that the fluid level was a tad high so I drained some out today. I just got back from a test drive. The manual shift from 2-3 at WOT didn't work again but I was short shifting it. So I decided to slap it in drive and let it eat. When the rpms pushed north of 5000 I started to get nervous but I figured I'd either blow it up or find out what's going on. Darned if it didn't pull a perfect shift at 5500 rpm, which is where I normally shift it manually. I did it 2 more times to be sure. Dead on 5500.

The best I can figure this governor was set up that way. Works for me.