700R4 Lockup Torque converter control issues solved

[idahovette]

Can you lock up a 2500 stall converter in a 700R4?

 

[84-C20]

Ok so this did answer a few questions, like what that deelybobber is on firewall...and that with new $900 harness that has bubkiss for it.....ill never waste money on a named wire harness again......absolute waste of money.....sorry i should not get started on that topic. Anyways ill give you a breakdown of my setup....ill try to make it short. Ok its an 1981 body, single cab longbed 4wd k10 w/ 3/4 running gear from 1984 k20. Originally a 305 w/esc and i assume a th350 (3spd.) Now has blueprint 383, w/ built 700r4, 89 column (4spd) those are important details that i never know til its to late. So here is where im stumped, oh and its a 2500 stall. So the trans man, added a lockup in trans for the 4 pin plug, it has a +, -, or ground i believe ( ill snap pics and add later) i bought a 4 pin pigtail, but think it only had 3 wires.....so my first question is if not wanting to be responsible for flipping a switch ( i may forget its on or ? ) what would i wire the wires from 4 pin pigtail to ? And let confuse you and back up a bit, so the original setup (81, 305, th350) had a vacuum deelybobber like yours and also had a brake and clutch pressure pin near brake pedal, but somehow was also wired into a small box up on left side back of steering column frame called by pt.# tcc lockup relay, with doubled wire colors that i can only assume by what wires i could find of original harness go to brake, clutch switches and splice into transfer case wiring and possibly cruise control. Thats when i started getting confused and the dumb/expensive harness does not provision nor give any instructions, and the actual wiring diagrams all 20 something of same components all are confusing or do not detail any tcc relay or where it would be wired to.......... so with all this ive written, im sure ive only confused you more then you wanted....lol. any feedback would be appreciated and will give more precise information and pics next time i get a chance....thank you my friend.....sending pics of what i may have on phone, but probably not what you need for reference

Ok I'll do my best here. In the original setup it would have had to have had the TH350C The C indicated it had a lock up converter. A normal TH350 did not have a locking converter.

So one issue you have with the new transmission is we have no idea what solenoids are in the tranny or if they are hooked up inside. For the moment lets assume they are all there and hooked up.

My 700R4 is currently setup so it engages the converter clutch automatically in 4th, and manually with a switch in 3rd. I did this because I wanted control over it when towing, in 3rd. From the factory some 700R4's would only lock up in 4th, some in 3rd and 4th, some used the vacuum control in 3rd, and were always locked in 4th, others used the vacuum controller in 3rd and 4th. No matter the configuration they were wired to disengage when the brake pedal is pushed.

So first thing you need to decide is if you want it to lock in 3rd and 4th or 4th only. The simplest wiring would be always locked in 4th only. You could always start with that and then wire in a lock method for 3rd later. IDK you'll need to decide that. If it's currently got a manual lock up switch you could retain that to run the lock up in 3rd if desired.

Then after that I'll have to see if I can find my wiring diagram for it.

 

[84-C20]

Can you lock up a 2500 stall converter in a 700R4?

Well that's a really good question. I have no idea, in thinking about it maybe not? That's a question for one of the tranny guys to answer. and @ak4life we need to get an answer to that question before you proceed.

 

[Matt69olds]

Well that's a really good question. I have no idea, in thinking about it maybe not? That's a question for one of the tranny guys to answer. and @ak4life we need to get an answer to that question before you proceed.

You can get converters with lock up capabilities far beyond 2500rpm, just depends on the budget. The problem with high stall converters is as stall speed goes up, converter diameter usually goes down. As a result, the lockup clutch gets smaller, and torque capacity goes down. You can get small diameter converters with multiple clutch disc to get the capacity back up, but that means more fabrication, more labor, more expense.

 

[jbro1988]

Sorry to go off an old thread but this relates a lot to what I’m doing. I have a 84 k5 blazer that I just put a 350 in and had my 700r4 rebuilt. I removed all the old vaccine stuff to simplify under the hood but looks like I have the same vacuum piece on the firewall and I have 3 wires on the trans. I did find 2 wires that I believe go to the lockup plug that were either broken or burned up while replacing the motor. How does one go about bypassing the vacuum and also does anybody have a a wiring schematic that would help me identify the wires. Believe I had black blue and yellow at the trans plug? Thanks!

 

[Turbo4whl]

@jbro1988 Welcome to the forum. Your question is difficult to answer because GM made changes in the 700R4 almost every year they used it.

You may have already read somewhere that if the lockup isn't working, the trans can fail. The vacuum tip-in switch is very important. As stated in this thread some choose to lockup with a dash switch.

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See it can be confusing if the trans year is unknown. Most people never state what happens to the trans without converter lockup.

Lockup oil is sent with pressure to the torque converter. The overdrive planetary gear set is at the end of the trans. That same oil moves down the shaft and lubes the overdrive gear set. Without that oil, the gear set can fail.

 

[jbro1988]

@Turbo4whl Would it be possible to run a switch just for 4th gear lockup even though I have the 3 wire setup that’s for 3/4 lockup? The truck is mostly just an around town cruiser. I don’t wanna smoke my transmission. Or do you recommend I figure out that vac setup? Here’s pictures or what I think is left of that switch? Thanks

 

[Turbo4whl]

I would recommend fixing the tip-in vacuum and brake lever switches. If you dash switch it locked, engine stalls when you arrive at a stop. This is not good either. You will get tired of switching it on and off.

Early squares with locking converter, have two brake lever switches. One for the brake lights, the other for the converter. Later models, one switch to operate both. (more wires)

 

[grummy]

>>> You may have already read somewhere that if the lockup isn't working, the trans can fail.

And, we have heard from trans repair guys that this is untrue. I believe it to be untrue as well. If it never locks up, it just operates like any other torque converter trans with no lockup.

I fought with mine too. wiring made no sense, but also, its an '83 truck with a '93 Transmission out of a G20. I kept my vac and brake switch, but put a push button on the shifter stalk, connected to a latching relay that lets me turn it on or off. I put an LED behind the seat belt light in the gauge cluster to let me know when it is latched.

I did it this way, because my truck will ALWAYS lock in 2nd, and 3rd and OD. Nothing I tried would eliminate the 2nd gear engagement. I can leave it latched all the time, but it will go into second and immediately lock up which is a little annoying with the 2:73 gears. I still have some issue I believe to be vacuum switch related, where when driving 65-75mph into a heavy headwind in OD and locked up, I think it kicks in and out as I can feel a little jerkiness that also feels like a misfire, but I think it is the lockup because if I manually turn it off, the jerk goes away.

If I ever feel the jerk, I turn off the lockup because I would think any constant in and out of lockup would cause damage eventually.

 

[Turbo4whl]

@grummy

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This is the best I could find. The key shows converter and lube. (dotted red line). The lube is on when converter is locked. Forward to the converter in the shaft. Back all the way to the overdrive gear set.

 

[jbro1988]

Is there two separate vacuums I should be hooking up on the firewall or just that driver side one? I found I have two wires blue/yellow that seem to go to the same harness that were cut on the firewall and I found this green plug just inside the firewall.

 

[jbro1988]

I found this guy that had restored this harness out of one. It seems to have two vacuum mechanisms on it.

 

[Goldie Driver]

Is there two separate vacuums I should be hooking up on the firewall or just that driver side one? I found I have two wires blue/yellow that seem to go to the same harness that were cut on the firewall and I found this green plug just inside the firewall.

On my 1980 TH350C (lock up converter) that green plug is a test point for the converter. Since the color is the same I would assume the function is the same for your 700R4.

 

[bucket]

@grummy

The local trans shop that I used for years warned me that running a 700r4 unlocked for prolonged periods will kill it due to excessive heat buildup. They were correct and I've got over 70k of road miles with a temp gauge installed that proves it. My lifted 4x4 Suburban was undergeared for best use with overdrive. The converter spent a lot of time unlocked in 4th on the interstate due to inclines and/or headwinds. If it stayed unlocked too long, the trans started to get way too hot. The temp gauge told me when I had to back off the throttle a bit so the converter would lock again. Once locked, the trans temp would start coming down significantly.

Running it in 3rd gear (locked or unlocked) never got the trans hot.

 

[Ricko1966]

Read #3 first. And the easy fix is fix your lockup.