I've searched for help with this often-covered problem, but have come up empty with a solution for my particular situation, so... if anyone can offer some suggestions as to what to check out next, I'd sure appreciate the help.I have '70 GTO with a 400 engine with a mild (not stock) cam and TH400 transmission. For the first 10 minutes of driving or so (starting with a cold engine), the trans shifts fine (in DRIVE). As soon as the engine warms up to about 170 degrees, the trans will not upshift out of first from a dead stop, and seems to free wheel when I let off of the gas. When in this situation, while driving along stuck in low gear, if I slide the shifter from D to D2, the trans will upshift into 2nd. If I then move the shifter back into D, it will shift into 3rd. When starting from a dead stop with the shifter in ratchet mode, the trans upshifts as it should when I ratchet the shifter, going into all three gears just fine.I have:* replaced the vacuum modulator* checked the vacuum at idle (at the modulator) - around 13 at idle, up to 20+ as accelerate* examined (looked fine) and updated the governor with a B&M kit, using middle-of-the-road weights and springsA friend of mine just rebuilt the trans. He said everything looked good inside, but questioned the number of balls, thought it may have been missing some, but reassembled it the way it came apart (but with new clutches, etc.). He suggested that I replace the valve body, so as my next move I'm considering picking up a TCI "TH400 StreetFighter® Series Valve Body 1965-up" - "Our manual/automatic valve body is designed for the street and strip performance car owner. It is the same valve body used in our StreetFighter® transmission series". I'd really appreciate some help here - I'm a shade tree mechanic, a trans greenhorn, trying to get things working right. Thanks!
have you tried reajusting the shift cable in the drive position?what is the color of the ring on the modulator? is it ajustable? i'd use the purple one for 13"of vac.
76 455/4spd TRANS AM69 GRAND PRIX 455/5SPD
Thanks for your response.* check the modulator and modulator tube going the OTHER WAY.. disconnect at the back of the manifold.. and use a hand vacuum pump.. think brake bleeder pump... verify that the vacuum DOES NOT LEAK down..# Did that already. No leak down with either the old or the new replacement modulator.* also.. verify that the vacuum ELbow or connection at the modulator is not getting hot and expanding from the heat of the exhaust...# Checked that out this afternoon. Went so far as to install clamps on each end of the short rubber hose. BTW, hose is in good shape, nice and firm, doesn't kink.* verify that the modulator tube connection to the intake manifold or back of the carb is not carboned up... or hooked inline or shares a vacuum source with something else..# Brand new, dedicated connection, just first putting the car on the road.* where is the vacuum for the modulator hooked up??? # To a fitting on the back of the manifold - a dedicated line for the modulator, full intake vacuum. The vacuum gauge measured 13+ (at idle) at the modulator end of the tube.* can you TEE into the modulator hose down at the modulator.. and monitor the vacuum reading.. you will want to do this with some RIGID nylon tubing or steel tubing.. for most of the run.. changes in manifold vacuum can collapse the hose when manifold vacuum levels are high.. and expand back to almost a normal shape when vacuum is low..# Kinda did this. Put the car on a lift, pulled rubber tube from modulator and attached it to end of vacuum gauge, started car, read 13 at idle, increased to 20+ when revving engine. But... thinking about it, I didn't hold the revs up and check the vacuum reading, I simply burped the gas pedal a few times and watched the reading rise and then drop back down. I'm thinking I need to hold the revs up for a while, to closer simulate driving conditions. Will do this tomorrow.* vacuum and FLOW of vacuum enough to make the engine RUN BAD when you yank the tube off the transmission modulator..# When I disconnect the line from the modulator with engine running, engine RPM increases* holds vacuum for 3 to 5 minutes when checked at the manifold end of the tube.. # Haven't done that, will give it a shot* you might examine how the governor fits in the side of the case.. some times the case wears and there might be enough leak past the governor when the fluid gets hot and thinner .. that it won't shift..# Need to do that yet.* you might also tear down the governor.. examine the piston for signs of impact damage.. where it fell to the hard floor and DINGED it..# Did that - pulled the governor and rebuilt with B&M kit, new middle-of-the-road weights and springs. Governor looked good, operated freely.============================================================================From what I've seen posted on this type of problem, I agree with your train of questioning - it would sure seem to be modulator related. I spent the afternoon today chasing down everything modulator related that I could think of, including replacing the modulator with a new one. I did all of my testing with the car fully warmed up and in "fail mode", driving it onto my lift to make the various changes. The only thing that I wish I had done this afternoon, and didn't, was to pull the modulator valve from behind the modulator and check it out (I think that's where the valve is, and that it should pull out, correct?)So, on the schedule tomorrow:- Monitor vacuum reading on modulator tubing in "fail mode" while sustaining driving mode RPM's- Pull/inspect modulator valve - Verify that line holds vacuum for 3 to 5 minutes when checked at the manifold end of the tubeThanks again for your response; I'll post results tomorrow night...
hellodid you check the linkage adjustment as ROWSLEY suggested? this a common problem.xxxxx"A friend of mine just rebuilt the trans. He said everything looked good inside, but questioned the number of balls, thought it may have been missing some, but reassembled it the way it came apart (but with new clutches, etc.). I'm a shade tree mechanic, a trans greenhorn, trying to get things working right."Other than possibly slipping, did it shift properly before rebuild. If so you are not missing any balls.
* have you tried reajusting the shift cable in the drive position?# Yep, no help. The shifter seems to be dead on in all gears.* what is the color of the ring on the modulator?# There isn't a ring on the modulator. * is it ajustable? i'd use the purple one for 13"of vac. # Yes, it is. I adjusted it both ways, no help. After the engine/trans warms up, same issue - stuck in 1st when shifter is in DRIVE.# Thanks! Dan
What I accomplished today:- The short rubber tube that connects the hard line to the modulator runs pretty close to the header collector. After reading your comments/suggestions yesterday, I thought that perhaps once the headers heated up the rubber hose might be softening from the heat and collapsing. I drove the car about 5 miles and, as usual, it shifted fine until the engine and trans heated up, then would not upshift out of 1st (and free wheeled) when in DRIVE. I returned to the shop, shifting manually, and replaced the 4" section of hose with a much shorter hose connected to a long looping hard line connected to another much shorter hose. Thus, I replaced the 4" hose with a long hard line that ran some distance from the header and was secured with a short rubber hose on each end. Road test results: no change from previously. Barely out of the driveway (engine/trans was still hot) and no upshift out of 1st and free wheeling. BTW, the 4" section of hose is very thick walled, and I plugged one end with my finger and sucked hard on the other end, and there was absolutely no movement of the wall what so ever. I pulled the long line and replaced it with the original, with a small hose clamp on each end.- With the original 4" rubber hose in place, I removed the rubber tube from the intake manifold and attached a vacuum pump. I applied 20+ vacuum, and it held it with zero loss. I could hear movement from within the modulator valve while doing this.- I attached the vacuum pump to the modulator (on the bench), and applied vacuum. The diaphragm moved smoothly as vacuum was applied.- With the engine/trans still hot, and the modulator removed from the trans, I then used a small needle nose pliers to attempt to pull the modulator valve from the trans housing. I tried a number of times, with a really good grip on it with the pliers, and the valve would not budge. Ultimately, I was able to grip securely, repeatedly twist and pull, and it finally came out. I don't have a micrometer, but checked out the valve with my digital calipers, and all looked good - no flat spots. I did notice what appeared to me to be an anomaly, a flat diagonal edge on one of the "lobes", although it appears to be a machined surface, so may be there by design. Here's a pic: http://www548.pair.com/danritz/modulator_valve.jpg~ Is this flat spot "normal"?~ Should the valve be that difficult to remove from the trans housing?My plan for tomorrow, with the trans cool, is to attempt to reinstall the valve, noting if it fits with the same tight clearances as it did when I removed it. So, after today's session, I think that I've determined that:- The modulator works.- The line from the intake manifold to the modulator does not leak.- The rubber hose connecting the hard line to the modulator does not collapse.Questions raised today:- Should the valve be so difficult to remove from the trans?- Is the flat spot on the valve normal?Another question: is the valve under spring pressure, pressing against the modulator, and, in operation, the movement of the vacuum modulator diaphragm allows the valve to be pushed outward by the spring?The only test on the agenda for tomorrow, for lack of anything else to try, is to see how easy/difficult it is to reinstall the valve into the trans housing.Thanks for your help thus far; any further thoughts are certainly appreciated!Regards,Dan Read more: http://forums.highperformancepontiac.com/70/9472534/the-general-discussion/th400-wont-shift-out-of-first-when-warm/#ixzz2bRa13YGw
- did you check the linkage adjustment as ROWSLEY suggested? this a common problem.# Yep, everything is right on, in all gears.- Other than possibly slipping, did it shift properly before rebuild. If so you are not missing any balls.# Yep except for slippage in high gear after it warmed up, it did previously go through the gears fine.Thanks!Dan Read more: http://forums.highperformancepontiac.com/user/02/reply/9472873/#ixzz2bRpdsYgq
hellothat valve should slide in and out easily.was trans hot when you removed it?if so see how it fits when trans is cold.if it fits when cold but not when hot there may be a crack in the bore.did your trans shift fine before the rebuild?if it did then it or the bore somehow got damaged or contaminated when he did it.try cleaning the bore carefully with a soft cloth.inspect bore for crack but they can be hard to detect.feel piston carefully for sharp edge.that is a machined mark intentionally put there at the factory but i do not know what it is there for, kinda weird, but i am not a trans guy, hate em, lol.VALVE OPERATION - At idle it is under around 14 hg of vacuum on a typical car. the vacuum pulls the nodulator piston back which allows the spring in front of the piston to push it backwards. This routes fluid to 3rd gear. when throttle is opened the vacuum in manifold goes down allowing the spring in the modulator to push the piston forward because the spring in front of the piston is lighter than the spring in the modulator.
- that valve should slide in and out easily. was trans hot when you removed it?# Yes, the trans was hot. I had just returned from a road test.- did your trans shift fine before the rebuild?# I'm pretty sure it did; all I remember is slippage in third gear. We had just completed the car, and were doing road tests, when we noticed the slippage and decided to rebuild it. There's an outside chance that this shifting problem was there and we didn't notice it due to the limited testing we did before pulling the trans.I'm with you - all I want a transmission to do is work. The less time I spend fussing with them, the better!Thanks for your help, I'll post tomorrow on the results of inserting the valve into the cooled trans.Regards,Dan
HelloIf it still sticks during installation measure how far it goes in before it stops. check lobe just entering trans and check bore at depth of first lobe carefully. There is maybe only .0005 clearance between the piston and trans. You can also put felt tip marker in bore and on piston. This will wear off wherever there is an interference fit. Sand offending high spot VERY lightly with 600 wet/dry paper with WD40 on it. If bore needs sanding simply wrap paper on a long socket about 1/4 size smaller then the bore. If you sand to much fluid will bypass piston and affect shifting. This would be no bueno, lol. If you sand the bore, clean the heck out of it including using q tips to get in channels to remove ALL the sand. VALVE OPERATION - At idle it is under around 14 hg of vacuum on a typical car. the vacuum pulls the modulator piston back which allows the spring in front of the piston to push it backwards. This routes fluid to 3rd gear. when throttle is opened the vacuum in manifold goes down allowing the spring in the modulator to push the piston forward because the spring in front of the piston is lighter than the spring in the modulator. the more vacuum it has the farther to the rear of the car it moves. Forward is 1st, all the way back is 3rd.
So... when I left off yesterday, the trans was hot, and I had an extremely difficult time removing the modulator valve from the trans using a needle nose pliers due to some sort of binding. With the trans cool today, I inserted the valve into the trans fairly easily, but found that when I pushed it all the way in, it would stick. I then basically polished the lobes on the valve using WD40 on 600 grit wet/dry sandpaper. I could then push the valve all the way in and slide it back out about 1/4" before it grabbed a bit. I wrapped a deep socket with WD40-drenched 600 grit wet/dry sandpaper and worked open the first bushing area inside the trans a bit, as that seemed to be where the holdup was. After a small amount of tweaking, the valve slid completely in and out without dragging or catching up. Feeling pretty optimistic, I took the car out for a road test and... the same problem showed up at the same point in time - once the engine/trans was warmed up, when in DRIVE no upshift out of first and free wheeling. I drove the car home (shifting manually), immediately put it on the lift, pulled the modulator, and attempted to pull out the valve with the needle nose pliers. The valve seemed to be stuck, as the pliers slipped off. I grabbed the valve a bit more securely, and it did then slide out. I was then able to again easily slide it completely in and out of the bore with no problem. I replaced the modulator and attached the vacuum line to it, and immediately took the car for another test drive, and was just out of the driveway in DRIVE, and I was again free wheeling stuck in 1st gear - same problem. I ran the car back onto the lift, left it running, pulled the rubber hose from the modulator, and did a vacuum check; it read around 13. So... WTH? One thing that I'm not too clear on: as vacuum is applied to the vacuum modulator and the piston is drawn into the modulator housing, what causes the modulator valve inside the trans to move outward from the trans? Hydraulic pressure that keeps the modulator valve pressing outward toward the canister? There's no spring behind the valve inside the trans, pushing it outward toward the modulator housing?Dunno what to try next.Suggestions definitely welcomed and appreciated!Thanks,Dan