700R4 torque converter lockup

[SirRobyn0]

I've got a 1984 C20, it's pretty much stock other than an edelbrock carburetor, and of course a geometery correction kit. Since this transmission is NOT electronic the TV cable tells it so much what to do. I bought this truck the carburetor was on it, but the geometery correction kit was not. The owner passed before it was completed, my point is I did not get to drive the truck before the change so I'm not sure exactly of the shift feel and lockup times before the carburetor was changed. I drove it two days after putting in the geometery correction bracket and setting the TV cable by the standard WOT / ratchet method. During those two days I felt that it was shifting up to soon and down to late. 1-2 under light throttle felt to slow and sloppy and I'd get into overdrive under light throttle at 35MPH. Reasoning that the TV cable is 35 years old, and may not have set correctly and knowing what an auto-trans should shift like, I tightened it up 4 clicks tonight before I left work, and then pulled over and reduced it to 2 clicks over the initial set point. I'm very happy with the shifting now. 1-2 is much tighter, and I get into overdrive at ruffly 43 - 45MPH under light throttle. My question is, is the transmission capable of locking up the torque converter in 3rd gear? I thought I felt it lockup in 3rd yesterday before I did fine tuning of the TV cable, but tonight it definitely did not lockup in 3rd. I wondered if perhaps I was simply getting into 4th and lockup at a low enough speed yesterday that I mistook it for 3rd. So can the 700R4 lockup the converter in 3rd?
Thanks,
Rob

 

[SirRobyn0]

I think I got this one figured out on my own. The answer is no I was not feeling 3rd gear lockup. The trans is capable, but there is a vacuum switch on the firewall that sends power to the TCC for 3rd gear lockup, which is disconnected. The switch is there as is the TVS but I'll need to get a vacuum delay. From looking at diagrams it's a 10 second delay. I'm thinking that to keep the trans from locking and unlocking excessively. So I likely won't get this until monday but I'll keep watching the thread over the weekend to see if anyone else has any thoughts. Then I'll report back with how this works out once I have it hooked up again.
Rob

 

[yevgenievich]

Most were wired to only lock up when in 4th. There is a 4th gear pressure switch inside that will let the circuit know when it is in 4th. But if your tcc control circuit is not working, that would need to be fixed.
By 1991, 3rd and 2nd gear pressure switches were added for some vehicles, but that was also a special valve body and a 5 pin connector

 

[gmbellew]

I believe my 1990 700r4 will lock in 2, 3, and 4.

 

[shiftpro]

Too high a gear ratio or larger tires will make OD very busy and create heat.
But that's not your issue, just mentioning it...
You're not running a 4x4 with big rugs so not an issue for you anyway.
I have seen kits to get lockup in 3th or 4th. Make it a 6 speed!

 

[RoryH19]

What is a good way to tell if the torque converter is locking up in 4th (OD)?
I would like to know the difference in how it would feel.
Trying to compare my 700R4 in my R10 to my 4L65e in my K5.

Thanks.

 

[SirRobyn0]

What is a good way to tell if the torque converter is locking up in 4th (OD)?
I would like to know the difference in how it would feel.
Trying to compare my 700R4 in my R10 to my 4L65e in my K5.

Thanks.

My method has always been to cruise in 4th and then touch the brakes, you should feel the TCC disengage. If you have a tach you should be able to see it on that. If you don't have a tach it's a little harder by feel if you don't know what your feeling for so you might run the test a few times.
Rob

 

[RoryH19]

My method has always been to cruise in 4th and then touch the brakes, you should feel the TCC disengage. If you have a tach you should be able to see it on that. If you don't have a tach it's a little harder by feel if you don't know what your feeling for so you might run the test a few times.
Rob

I added a tach to both of them so I can use that method.
Assuming the RPM's would go up after disengage?

 

[SirRobyn0]

Most were wired to only lock up when in 4th. There is a 4th gear pressure switch inside that will let the circuit know when it is in 4th. But if your tcc control circuit is not working, that would need to be fixed.
By 1991, 3rd and 2nd gear pressure switches were added for some vehicles, but that was also a special valve body and a 5 pin connector

I'm pretty well familiar with how early computer driven TCC systems worked such as the TH440 that GM used in their front wheel drives, not so much on a truck with no computer at all. On my truck it is most definitely locking up in 4th, so that is working, and maybe that is all there is. But my vacuum diagram shows a TCC switch on the firewall, which is plugged in on the electrical side, but has no vacuum line running to it. It is my understanding that some of these trucks used a vacuum circuit to engage the TCC in 3rd gear under certain conditions, but from what you said it looks like most trucks did not. Regardless of the function, I'm going to hook that vacuum switch up on Monday and see what happens. I have a fairly long commute and about 10 miles of it are on country roads where the truck spends most of the time in 3rd gear and having lockup function then could be beneficial. So we'll see what happens, I appreciate the info.
Rob

 

[SirRobyn0]

I added a tach to both of them so I can use that method.
Assuming the RPM's would go up after disengage?

Yes that is correct, but it will be less RPM change than a downshift. You maybe able to see it engage on the tach as well, but the easiest thing to do is cruse along in 4th gear and press the brakes, you don't need to push far enough to actually engage the brakes just press down slowly on the brakes and you should feel the TCC disengage and the RPMs go up 100 or so RPM before the brakes engage.
Rob

 

[RoryH19]

Yes that is correct, but it will be less RPM change than a downshift. You maybe able to see it engage on the tach as well, but the easiest thing to do is cruse along in 4th gear and press the brakes, you don't need to push far enough to actually engage the brakes just press down slowly on the brakes and you should feel the TCC disengage and the RPMs go up 100 or so RPM before the brakes engage.
Rob

Thanks, I'll give it a try.

 

[SirRobyn0]

Regardless of the function, I'm going to hook that vacuum switch up on Monday and see what happens.
Rob

Well, I had an awful day at work so at the end of it I simply hooked ported vacuum to the TCC switch so I could at least find out the function. As far as I can tell it does nothing, well actually I could hear it click in the cab when I stepped on the gas but other than that... I had read somewhere else that, that switch might actuate the TCC in 3rd gear under certain conditions. Elsewhere it was indicated that it would disengage the TCC under load in 4th. But I have no idea. Currently regardless of whether vacuum is connected to that switch or not the TCC will engage immediately when the transmission up shifts to 4th, and will stay engaged unless the brake pedal is pressed or the transmission downshifts. I guess my question at this point is that normal operation and what is that vacuum switch actually suppose to do.
Thanks,
Rob

 

[yevgenievich]

There were two vacuum switches, one for tcc and one for ignition on some models. It is also pupular with some builders to wire tcc to engage automatically with 4th gear via a 4th gear pressure switch. To sort it out, have to trace tcc wiring and verify current operation. On trucks it was common to only let 3rd gear tcc apply when shifter is set in drive. Reason is it is hard on a lockup clutch to hold while transmission is shifting.

Very likely yours has the auto engage wired in and there is just a 12v wire going to the trans from the brake switch. This way it will lock up as soon as 4th gear comes in and will stay locked until either brake or downshift happens.

 

[SirRobyn0]

There were two vacuum switches, one for tcc and one for ignition on some models. It is also pupular with some builders to wire tcc to engage automatically with 4th gear via a 4th gear pressure switch. To sort it out, have to trace tcc wiring and verify current operation. On trucks it was common to only let 3rd gear tcc apply when shifter is set in drive. Reason is it is hard on a lockup clutch to hold while transmission is shifting.

Very likely yours has the auto engage wired in and there is just a 12v wire going to the trans from the brake switch. This way it will lock up as soon as 4th gear comes in and will stay locked until either brake or downshift happens.

Mine has the ESC unit on the passenger side, and the TCC by the brake booster, or at least that is how the arrangement appears.
Thanks for that info. I honestly don't have a problem with it operating the way it does but I like the benefits of the TCC, so I'd like it to be all that it can be if possible. I'll look up the wiring diagram and do some tracing and see how it is actually setup vs what the wiring diagram shows.
Thanks again,
Rob