427/390 th 400 shifts fine at low speed, under load...will not!!!

[Bob S.]

After getting my car back from my mechanic, I find that it shifts through the gears fine at normal speed. Put the pedal to the floor, and the tach reaches around 4K and it hangs there! The tranny will NOT shift into second.

Brought her back in..they "suspect engine is not getting enough fuel" Mechanic is on his way out of town,and will address this later.

Important note: My 4BBL was NOT functioning properly. Secondaries were not opening. Accelerated and shifted fine. Now that a new carb, functioning properly is in, it's hanging and not shifting.

I'm not "comfortable" with what I'm being told.

As mentioned, I can drive the car with no problem, unless I put the pedal to the metal, and I'm stuck in first

Any help would be appreciated

Bob
(9944)

[Terry M.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, be sure you are getting vacuum to the vacuum modulator canister on the right rear of the transmission. I had one that clogged at the metal vacuum line right where it comes off the intake. also be sure that vacuum line is secured to the canister and the rubber hose is properly in place.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, be sure you are getting vacuum to the vacuum modulator canister on the right rear of the transmission. I had one that clogged at the metal vacuum line right where it comes off the intake. also be sure that vacuum line is secured to the canister and the rubber hose is properly in place.

Thanks, Terry...don't know why I'm being told it's fuel....strange, no?

[Paul D.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, it seems that you and your mechanic are concerned about two different things. You are worried about a trans that will not shift out of 1st gear at less than 4000 RPM at WOT and your mechanic (along with myself) wonders why that big block can't wind past 4000 RPM. I would imagine that is why he is mentioning fuel starvation. If the vacuum line checks out OK, take a look at the plastic gear on the governor. I seem to remember those giving trouble back in the day causing the tranny to not upshift, but I can't remember if it was always or just WOT. I also seem to remember a "feature" of the 400THM being that you couldn't force it to stay in 1st as RPMs built up even if you held the shifter in 1st, but I thought you would have to be a little higher than 4000 RPM. At WOT, your vacuum modulator may not be in play, and 4000 RPM may not be enough for the governor to force it to shift. In other words, your tanny may be OK, you just need to figure out why the 4000 RPM limit. My barely .02 worth. Chip.

[Anthony P.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

I would think that if it were the governor grear it would not shift at all. just my opinnion

[Edward J.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, If the car is nose diving when it hits 4 grand and will not shift ,that maybe it is running out of fuel.Since the turbo 400 is shifting normally under street driving conditions I suspect that the transmisson is fine.I would let your Mech. do his thing and ask him to check the fuel pressure and in-line filter.and even look in fuel tank at the sock on end of the fuel sender for being plugged. Fuel demand at that RPM is alot more than at cruise speeds.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Thanks, Edward....

Yes, he checked, visually in the tank...looked good, I have a new fuel filter on the way which I'll change myself. See what happens at that point.

Regards,

Bob

9944

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

I would think that if it were the governor grear it would not shift at all. just my opinnion

Thanks Anthony....

I'll take all the opinions I can get. She's running so fine, but for this issue. Not that I hot rod around all the time, but with 427 CID under the hood, and a wide open straightaway, well..sometimes you just gotta blow her out! No?

Best,

Bob
9944

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, it seems that you and your mechanic are concerned about two different things. You are worried about a trans that will not shift out of 1st gear at less than 4000 RPM at WOT and your mechanic (along with myself) wonders why that big block can't wind past 4000 RPM. I would imagine that is why he is mentioning fuel starvation. If the vacuum line checks out OK, take a look at the plastic gear on the governor. I seem to remember those giving trouble back in the day causing the tranny to not upshift, but I can't remember if it was always or just WOT. I also seem to remember a "feature" of the 400THM being that you couldn't force it to stay in 1st as RPMs built up even if you held the shifter in 1st, but I thought you would have to be a little higher than 4000 RPM. At WOT, your vacuum modulator may not be in play, and 4000 RPM may not be enough for the governor to force it to shift. In other words, your tanny may be OK, you just need to figure out why the 4000 RPM limit. My barely .02 worth. Chip.

Thanks, Paul...

I'll be changing the fuel filter as soon as the new one arrives, (Couple of days) and get into the vacuum hose and all...OK, so...Where exactly IS the Governor? (This is why I use a mechanic on most things)

Best,

Bob
9944

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, If the car is nose diving when it hits 4 grand and will not shift that maybe it is running out of fuel.Since the turbo 400 is shifting normally under street driving conditions I suspect that tthe transmisson is fine.I would let your Mech. do his thing and ask him to check the fuel pressure and in-line filter.and even look in fuel tank at the sock on end of the fuel sender for being plugged. Fuel demand at that RPM is alot more than at cruise speeds.

By the way....very nice Blue Coupe! Tell me, do you get that "Bump" when shifting from Park or Neutral into Drive or Reverse?

[Edward J.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, my 72 turbo 400 has a shift kit in it and does get a bump when shifting.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, my 72 turbo 400 has a shift kit in it and does get a bump when shifting.

Guess they all do. We've rebuilt the rear end, and it helped a bit, and it needed a rebuild, so that's good...What is a "Shift Kit?"

[Edward J.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Guess they all do. We've rebuilt the rear end, and it helped a bit, and it needed a rebuild, so that's good...What is a "Shift Kit?"

Bob, when I rebuilt my transmisson I installed a a shift kit which raises the presure in the transmisson to help with those light throttle quick shifts. so now it does not shift 1/2/3 so quickly under light throttle. and when you accelerate it has a nice crisp 1/2 and 2/3 shift. It may be possible that if your transmisson was rebuilt the shift kit was installed this was a common practice with the turbo 400 transmisson as they were dogs driving around town.

[Bob S.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, when I rebuilt my transmisson I installed a a shift kit which raises the presure in the transmisson to help with those light throttle quick shifts. so now it does not shift 1/2/3 so quickly under light throttle. and when you accelerate it has a nice crisp 1/2 and 2/3 shift. It may be possible that if your transmisson was rebuilt the shift kit was installed this was a common practice with the turbo 400 transmisson as they were dogs driving around town.

Yeah....Mine seems to shift fairly quickly at low speeds, but I'm fine with that. Then again, not being a mechanic, I may be wrong, and there just may be a kit in there. I'm happy with the "Around Town" performance, and I don't think we'll need to go into the tranny with this issue. Just want to get this one last thing straightened away.

[Paul D.]

Re: 427/390 th 400 shifts fine at low speed, under load...will not!!!

Bob, it's located under a cover on the side of the trans, but I don't think it's your problem. Tony stated he didn't think it would shift out of 1st at all if the gear was stripped and I think he is correct. Since it runs so well and shifts OK at cruise speeds, I think your mechanic is onto your problem. Does your 390HP have the Q-Jet Rochester on it? If so, It probably has a paper filter located in the carb inlet. If that's the one you are replacing, be sure to cut the ends off and unfold it. The residue inside can sometimes give clues as to other issues such as a rusty tank or sludge in the fuel. Chip.