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

Swearbody

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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!!!
 

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I would say the TCC is malfunctioning or the pressure switch is staying closed.
 

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Guarantee that you’ve got constant power being sent to the 4 pin connector at the transmission. I’ve got a manual over ride switch on my 82 so I can let the factory switches do their thing, I can manually unlock the converter and I can manually lock the converter as well. If I forget the switch in manual lock. The instant I hit 2nd gear, the converter locks and stays locked through overdrive. So, if your brake switch is good, it must be since the converter unlocks when you step on the brakes. I’d suspect a stuck vacuum switch or a possible short in the harness. Unplug the vacuum switch and see if the converter still locks. If it does, I’d check for power at the 4 pin connector at the transmission for power all the time the key is on. Shouldn’t. If you do then you need to find the wiring problem. If you have no power there, and the converter still locks even with the 4 pin connector unplugged. Then you’ve got internal issues. Possibly a stuck solenoid.

Ben
 

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Guarantee that you’ve got constant power being sent to the 4 pin connector at the transmission. I’ve got a manual over ride switch on my 82 so I can let the factory switches do their thing, I can manually unlock the converter and I can manually lock the converter as well. If I forget the switch in manual lock. The instant I hit 2nd gear, the converter locks and stays locked through overdrive. So, if your brake switch is good, it must be since the converter unlocks when you step on the brakes. I’d suspect a stuck vacuum switch or a possible short in the harness. Unplug the vacuum switch and see if the converter still locks. If it does, I’d check for power at the 4 pin connector at the transmission for power all the time the key is on. Shouldn’t. If you do then you need to find the wiring problem. If you have no power there, and the converter still locks even with the 4 pin connector unplugged. Then you’ve got internal issues. Possibly a stuck solenoid.

Ben
If im thinking about this right ,the fact that it unlocks when the brake is applied rules out stuck solenoid... right/wrong? Also it does not lock when 4pin is unhooked.
Mine is acting more as you have stated your does when you forget to switch off...like it is manually locked. Perhaps the plug itself is on wrong? the "clip" on the plug is broken off but I still lined up the tabs so that shouldn't be it... But then It was messed with by someone at some point since it was unhooked when i got it.

SO to be clear...if I unhook the vac switch it should default to open..a.k.a. tcc off even if the 4pin plug is connected
Also check for voltage at the plug when keyed on...Im betting the bank it has 12v when keyed on.
I would just do the same thing you did but I want it to unlock when Im passing in 4th without me having to flip a switch. My gears are low enough that it rarely sees enough throttle to downshift and if it did it would be in the +/- 4000rpm range instantly
 

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I would say the TCC is malfunctioning or the pressure switch is staying closed.
TCC is functioning as per any info I have been given. It doesnt make any noise,it locks and unlocks...just not when i want it to. 3rd and 4th feel great though with my gearing I only need 4th to lock...but that has nothing to do with the tcc functionality but instead the signals being sent to it. I mean by all means I could be wrong and im not saying I know..or I wouldn't be here today.
I think you may be right about the vac switch...Im betting it failed and thats why the 4pin was unhooked when I bought it. The guy I got the truck from could hardly tie his own shoes so I can see him just unplugging it and being done.
 

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Actually now that I think of it. I told you wrong information. You’ve got a stuck pressure switch on the valve body itself. On the truck side you have ignition power that goes through the brake light switch (closed with brake releases) that then goes through the vacuum switch (closed with high engine vacuum) that then goes to a temperature switch (didn’t pay attention to the year of your truck) that doesn’t allow converter lock up until around 140 degrees coolant temp. Then that should go down to the 4 pin plug.

So I theory, key on engine off, should have no power to 4 pin. Vacuum switch should be open due to no vacuum and also possibly temp switch not up to temp. Key on engine on and up to temp, should have power to 4 pin. That’s where the 3rd or 4th gear pressure switch comes into play. I’ve seen them both ways where only locks in 4th or locks in 3rd and 4th. If one of those is stuck closed, it’s along power to get to the solenoid at all times and once you hit 2nd gear. The valve body directs oil to the solenoid which is powered and locks the converter. That’s my updated diagnostic

Ben
 

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Im going to say you have a wiring issue and or a bad 4th gear switch.
If you pull the pan and post a pic of what switches it has that would help.

In your truck it should only engage in 4th. In 4th gear the 4th gear switch closes and complete the circuit to engage the torque converter meaning the switch is bad the wire shorted to ground or if the trans was replaced with a different year the wiring inside may be different GM used 23+/- different wiring configurations inside the transmission and in later years the computer controlled it.

the vacuum switch only disconnects the torque converter under hi load/ low engine vacuum. the vacuum switch has nothing to do with what gear the converter engages in.
 

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Actually now that I think of it. I told you wrong information. You’ve got a stuck pressure switch on the valve body itself. On the truck side you have ignition power that goes through the brake light switch (closed with brake releases) that then goes through the vacuum switch (closed with high engine vacuum) that then goes to a temperature switch (didn’t pay attention to the year of your truck) that doesn’t allow converter lock up until around 140 degrees coolant temp. Then that should go down to the 4 pin plug.

So I theory, key on engine off, should have no power to 4 pin. Vacuum switch should be open due to no vacuum and also possibly temp switch not up to temp. Key on engine on and up to temp, should have power to 4 pin. That’s where the 3rd or 4th gear pressure switch comes into play. I’ve seen them both ways where only locks in 4th or locks in 3rd and 4th. If one of those is stuck closed, it’s along power to get to the solenoid at all times and once you hit 2nd gear. The valve body directs oil to the solenoid which is powered and locks the converter. That’s my updated diagnostic

Ben
Im still not understanding the stuck valve diagnosis. Perhaps its a difference in vernacular. To me stuck implies that it does not cycle function and since its already confirmed that the tcc disengages when brake is depressed or 4pin connector unplugged, I am failing to follow the "stuck" diagnosis...or thats its the tcc solenoid at fault. It seems more to be a signaling issue but im still limited to my understanding of it.
I however did not know there was a temp sensor involved and need to check that one is present and functional. I do remember seeing what I thought was a oil pressure switch/sender that was right there next to the oil filter....perhaps that was a temp sensor. I know for a fact it isnt hooked up because the sender leaked and I plugged the hole. Considering that it was probably not yet 140deg when tested makes me think there is not a temp sensor on mine anymore.Perhaps there is a problem there but considering its place is last in line and voltage is present???
Good looking out on the info. I think with what ive learned from you so far I might have it fixed next time I get to it. Or Hell maybe ill buy a $10 floor mount dimmer switch and wire it to that. Seems to work for you and many others to have it toggled on/off. I figure a foot actuated switch keeps my hands where they need to be and keep me from having to lean forward....im worn and lazy. Thanks again!!!
 

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I agree that it's probably a transmission problem. I only say that because I had the same problem on my TPI Camaro and that lockup was controlled by the ECM. It would hold 1st gear out ok, then hit second and rapidly lock up and hit third and fourth gears. It was very annoying. I wound up having another trans built and I installed it, the car shifted and locked up properly after that.
 

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Im going to say you have a wiring issue and or a bad 4th gear switch.
If you pull the pan and post a pic of what switches it has that would help.

In your truck it should only engage in 4th. In 4th gear the 4th gear switch closes and complete the circuit to engage the torque converter meaning the switch is bad the wire shorted to ground or if the trans was replaced with a different year the wiring inside may be different GM used 23+/- different wiring configurations inside the transmission and in later years the computer controlled it.

the vacuum switch only disconnects the torque converter under hi load/ low engine vacuum. the vacuum switch has nothing to do with what gear the converter engages in.
Im in no shape to pull a pan yet otherwise id gladly do that. I can say it does not have a 4th gear external pressure switch like used on a monster trans kit. I dont know if the factory 4th gear switch you are referring to is internal or one you tap into externally. The trans was replaced by previous owner about 5 years ago so anything could have been done for all I know.
Im going to see if it has voltage when keyed on...if so im gonna look for a short and hopefully its just a broken and shorted wire. Maybe I nicked a wire when I reinstalled the engine...or any number of other things ive done recently. If I dont find a short somewhere ill probably resort to debauchery and just toggle switch it before the brake interrupt so it at least still switches off under braking(giving me time to remember to hit the switch-maybe)
Thanks for the 411
 

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I agree that it's probably a transmission problem. I only say that because I had the same problem on my TPI Camaro and that lockup was controlled by the ECM. It would hold 1st gear out ok, then hit second and rapidly lock up and hit third and fourth gears. It was very annoying. I wound up having another trans built and I installed it, the car shifted and locked up properly after that.
Hey man good to hear from you!! Man I hope not. In all honesty I doubt thats my issue though unless it just the tcc itself.. Ive driven the truck hundreds of miles and the trans seems solid. Ive put it through its paces and it hasnt mis shifted, held or hesitated a shift (other than when the tv cable wasnt adjusted properly- has been fixed), or overheated even running 3000rpm for 15-20 minutes with no lockup. The only problems arise once the tcc sees voltage, and then only because it shouldn't be getting voltage in 2nd or 3rd so im told. Otherwise it does A OK. Im gonna at least try a toggle switch and see how it does just switching it on in 4th. If that does not suffice ill keep running without lockup. It has a 2400rpm converter really doesnt seem to mind how ive been driving it. I just want to see a bit lower rpm at 70+ mph instead of 2.8-3k.
 

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I didn't mean to imply that the trans may be failing, just that it may be a problem that may need fixed with the pan removed.

I'm my case, I ended up replacing the trans because the torque converter let go and made a mess of things. It was completely unrelated to that annoying lockup issue.
 

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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
 

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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.
 

SirRobyn0

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Ok so a few months ago I posted about issue about my torque converter lock up control. I'm going to seriously summarize here but... My truck is a 1984 C-20 with 700R4 that is NOT computerized and it's pretty much a stock truck. It has brake switch to disengage the torque converter and a vacuum switch on the firewall. In the other thread we discussed how this setup should work, and basically the conclusion was that it would probably only lockup in 4th gear, and the vacuum switch should disengage the lockup when under enough load. But that it was common for the vacuum switch to be bypassed, so it would stay locked up anytime it's in 4th. Turns out that's all wrong.

So I've spend a massive amount of time going over wiring diagrams. The trouble is it seems like most GM cars and truck with a lockup converter had at least some primitive computer system, and mine doesn't. So eventually I did find a schematic, there's about 4 different ways they wired the non-ECM lockup converters. The options being 4WD, 2WD, 3rd & 4th lockup or 4th only. Yesterday I went through all the wiring and cleaned up all of the contacts for the lockup circuit. Took it out for drive and got a little surprise. This transmission will lockup in 3rd and 4th. even if the shift lever is left in overdrive it'll still lockup in 3rd. It's actually really impressive to me how well it works for having nothing more than a brake switch and a vacuum switch. So hopefully this post will help someone else that is struggling to get their lockup to work correctly. And now I've got a bit knowledge in this area too, so I might be able to help.

I'll also add from my research that it appears if the the transmission plug has 2 wires it's 4th gear only lockup, if it has 3 wires it's 3rd and 4th lock up.
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.
 

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