700r4 locking up in 2nd also locking too fast. Vac switch?

[AuroraGirl]

Has the OP hooked up a pressure gauge to the port on the trans? I would be wondering if your TV Cable is adjusted right, if the fluid is DEX III or DEX VI (not anything else, unless a custom racing ATF but it sounds like yours is a standard unit?), and what your fluid looks like on the dipstick. Im wondering if the internal pressures arent just enough they overcome seals or force their way into the tcc solenoid chamber or whatever it uses. basically, if your immedetialy following a shift it to me would make sense the line pressure is high and slamming into the gear with high pressure its passing anything and everything and applying, but its fluid path is such that it bleeds off elsewhere in 1 and r. Idk. Or that the TV cable is being screwy with its adjustment and combined with this other stuff I mention to give the unique behavior. I dont think failed trans, failed torque converter, etc.

hows the fluid cooler lines? right diameters and free flowing?

 

[AuroraGirl]

SO as stated my 700r4 is locking up in 2nd and too early in 3rd-4th. I didnt even know they could lock up in 2nd. I havent been able to get to it since april due to being ill so i could have done something wrong hooking it all back up but im leaning towards other things.
1) it was unhooked when i bought the truck leading me to believe it had problems before
2) it has a 2400rpm converter I installed when it was built last year( the truck not the trans)- trans was replaced with a "new" unit in 2016 from what I can tell. there is a date on the trans case/label.
The tcc works obviously... Also the brake pedal switch functions to release the tcc so it doesn't die when stopped or bog slowing down on the brakes. It does however immediately lockup after each shift.
If you hammer down on it and get it to downshift it immediately goes into lockup after each upshift

I do not want an aftermarket controller or any "bypass" switches. Im leaning towards this being the vacuum switch being faulty. ANY input is appreciated.

Also long time no see to those who ive spoken with before. Treatment is going well and I finally got the 3.90s and truetrac installed. Time to get those highway rpms back down to manageable. Im running around @ 3000rpms a little too much,LOL. Also raced a new-ish f150 the other day from a 50 roll and was quite happy that he was still behind me when I backed off at around 4500rpm in 4th( No speedo gear change yet,LOL) @4500 im guessing I was doing north of 120mph...if locked up the math would be 145+ at 4500 so yeah...120ish.
Anywho thanks all!!!

Is your truck Federal emissions, by chance, and do you know what its factory setup was? No electronic carb or anything right? The replacement trans, can you tell us what it was for specifically, like what it was specced for. A 1984 700r4 had more than one way to operate the TCC solenoid and that means your specific trans we should verify what it was for and what your truck would have architecture for it.

ported vacuum, a coolant vacuum switch, a vacuum switch on the firewall it sounds, and a brake switch

 

[Matt69olds]

GM used several different methods to control lockup. Based on your description, I’m betting you have a stuck pressure switch. For whatever reason, the trans thinks it’s always in 4th gear. I’m pretty sure a 700 is hydraulically incapable of lockup in 1st gear, that means as soon as the trans makes the 1-2 shift, with the electrical circuit messed up it will lock.

You need to drop the pan and check the function of the pressure switch. Another possibility is the 4th gear pressure switch wire is pinched between the filter and valve body, showing a ground all the time.

 

[Itali83]

There are a few different ways that GM setup the TCC clutches. But if it's locking to soon in 2nd & 3rd, I'd be looking at the vacuum delay valve before the vacuum switch, that will definitely cause it to look to soon, but otherwise operate ok. Next would be a defect vac switch, sticking closed all the time. This is the vacuum switch mounted to the firewall by the brake booster.

I did a post a while back on the system like you have where it can lock in 2nd, I'll see if I can dig it up for you and see if it helps you with ideas.

Youre forgetting that there are pressure switches on the valve body. They have final say on when converter locks. Converter should never lock in 2nd.

Ben

 

[AuroraGirl]

Youre forgetting that there are pressure switches on the valve body. They have final say on when converter locks. Converter should never lock in 2nd.

Ben

I think it depends on the year. I did some reading and it looks like some units, mostly early on, not later, were able to in 2nd. Im guessing it was something that put too much lugging or too frequently went on, off which is only going to add wear and heat when realistically you dont often stay cruising in 2. plus for bigger heavier things you start to put a lot of force on that thing and locking up so soon is just asking for trouble. Im guessing cars may have been quicker to jump on that.

 

[Itali83]

I think it depends on the year. I did some reading and it looks like some units, mostly early on, not later, were able to in 2nd. Im guessing it was something that put too much lugging or too frequently went on, off which is only going to add wear and heat when realistically you dont often stay cruising in 2. plus for bigger heavier things you start to put a lot of force on that thing and locking up so soon is just asking for trouble. Im guessing cars may have been quicker to jump on that.

Not trying to sound like a know it all….,, BUT,, 700’s came out in ‘82 for the trucks. I’ve got an 82, rebuilt the transmission. It had 3rd and 4th gear pressure switches. I’ve rebuilt an 85 700r4. Had 3rd and 4th pressure switches. Had my 87 700 rebuilt same deal. Swapped an 88 700 in the place of another had the switches. Same with the 89 I had to do a pan gasket on. See a trend here? Hydraulically, yes, it’s possible to lock converter in 2nd. But it was never meant to at least in our trucks. SO, once again, it has to be a stuck pressure switch or wires are chaffed together in the pan bypassing the pressure switch.

Ben

 

[AuroraGirl]

Not trying to sound like a know it all….,, BUT,, 700’s came out in ‘82 for the trucks. I’ve got an 82, rebuilt the transmission. It had 3rd and 4th gear pressure switches. I’ve rebuilt an 85 700r4. Had 3rd and 4th pressure switches. Had my 87 700 rebuilt same deal. Swapped an 88 700 in the place of another had the switches. Same with the 89 I had to do a pan gasket on. See a trend here? Hydraulically, yes, it’s possible to lock converter in 2nd. But it was never meant to at least in our trucks. SO, once again, it has to be a stuck pressure switch or wires are chaffed together in the pan bypassing the pressure switch.

Ben

I dont take offense at your comments, Im only talking about the possibility because the OP indicated he bought a "new" 700r4 but I asked for the numbers because there is the possibility there is a spec for some california backwater where a c10 without power steering had a 700r4 with 2nd lockup and also had hydroboost. (Hypothetical but just trying to express why I mentioned) I think knowing what vehicle that 700r4 is setup to run in is the first thing to know when comparing operational behavior. I do think something is wrong tho, it should respond to load and coast and before shifts etc or whatever they did vacuum hydraulicly.

 

[SirRobyn0]

Youre forgetting that there are pressure switches on the valve body. They have final say on when converter locks. Converter should never lock in 2nd.

Ben

Of course your right about the pressure switches, and I certainly did not mean to imply that my previous info was all inclusive. When I was having trouble with my truck I had a serious struggle find out how the system was suppose to work, so I posted what I discovered.

I honestly have no idea if they are suppose to lock in 2nd, what I do know is that they are capable of doing it.....

I'm by no means an expert on the lock up system on the these trannys just posting what I've discovered because like I said when I was having issues, no one seemed to know anything.....

 

[AuroraGirl]

Of course your right about the pressure switches, and I certainly did not mean to imply that my previous info was all inclusive. When I was having trouble with my truck I had a serious struggle find out how the system was suppose to work, so I posted what I discovered.

I honestly have no idea if they are suppose to lock in 2nd, what I do know is that they are capable of doing it.....

I'm by no means an expert on the lock up system on the these trannys just posting what I've discovered because like I said when I was having issues, no one seemed to know anything.....

sharing is caring when its wholesome

STDs are not caring to share, but thats unwholesome, for example.

Thats why you dont borrow your ford to someone or leave it in the parking lot to share your gasoline with someone who you havent met yet... then they brake the filler neck. Good times. damn ford stds.

 

[C10MixMaster]

You must be registered for see images attach

the switch with the blue connector is the 4th gear switch if that switch is bad (stuck closed) or the black wire attached to it is shorted to ground the trans can go into lock up any time there is power to the transmission.

 

[Matt69olds]

Some of the early 700 did have a 2nd gear pressure switch. I have no idea what applications used the switch, but they did use it in some transmissions. The pressure switch is near the TCC solenoid.

 

[Swearbody]

I’m not saying the converter solenoid is bad. We’ve diagnosed its not bad by stepping on the brake pedal and having it unlock. I’m saying there are pressure switches on the valve body itself inside the transmission that are supposed to prevent voltage from getting to the converter solenoid (also inside the transmission) until either 3rd or 4th gear depending on the style. That’s where I think your problem lies. Not the converter solenoid itself.

Ben

Right on. So one of them may be faulty or getting voltage it shouldn't

 

[Swearbody]

There are a few different ways that GM setup the TCC clutches. But if it's locking to soon in 2nd & 3rd, I'd be looking at the vacuum delay valve before the vacuum switch, that will definitely cause it to look to soon, but otherwise operate ok. Next would be a defect vac switch, sticking closed all the time. This is the vacuum switch mounted to the firewall by the brake booster.

I did a post a while back on the system like you have where it can lock in 2nd, I'll see if I can dig it up for you and see if it helps you with ideas.

Is the vac delay valve the little check valve looking thing in line to the vac switch. How would i test that. I took it off and it seemed like air only went through it one way

 

[Swearbody]

That's from the other thread I was mentioning. Not sure if it really helps much. Let me know if you think it would help, I can see if I can locate the wiring schematic at work tomorrow.

Yeah it helps. Im pretty sure ive got 3-4 lockup setup. I have the vac switch and delay valve,brake switch with no external 4th gear pressure switch. Its an 84 as well with no tcm,ecu etc. Pretty sure its 3 wire as well. I plan to mess with it tomorrow and see what else I can find. I cant find a replacement vac switch anywhere...even online. Im gonna try a few things that will help track down the culprit tomorrow.

 

[Ellie Niner]

FWIW- My 1986 will only lock the TCC in 4th gear if there's no vacuum to the switch. That happens right after the 3-4 shift at 45+mph. I'm having some issues with vacuum lines and delay valves, so have had the switch connected directly to manifold vacuum. In that case, the TCC will engage in 3rd after I'm above 45 or 50mph, and will in 2nd if I'm holding it in that gear and exceed 45-50mph (fairly easy with 3.08 gears). Without the delay valve, the TCC will lock a bit sooner under light loads and unlock a bit earlier if manifold vacuum drops. My semi-educated guess is that the delay valve is there in part to keep the TCC from locking and unlocking rapidly under on throttle/off throttle driving conditions, though I need to check the color code on my delay valve to see what the actual time delay period is. Either way, it only fine tunes the lock and unlock a bit, and doesn't contribute hugely to the road speed at which things happen in my application.

Mine is Federal emissions with no computer (save for ESC).