Ground TCC lockup?

[K10-Kansas]

Hi all. , Thanks in advance for any help on this.

Does the torque converter lockup pigtail need to be grounded on a 700r4 in a1971 K10 with a carbureted Blueprint 350? The truck is otherwise stock with no modern additions.

[pjmoreland]

Vehicles originally equipped with a 700R4 included external logic circuits for determining the appropriate conditions to lock or unlock the torque converter clutch. These methods ranged from a simple manifold vacuum switch on early models to electronic speed based controls on the later models. There were quite a few different ways 700R4 transmissions were wired internally, as shown in the documents posted in the following thread.

https://67-72chevytrucks.com/vboard/...15#post8974715

The electrical connector on the 700R4 has three pins. Typically, two of these pins are for applying 12V and ground to lock the torque converter clutch. The third pin is typically a signal from the transmission to indicate to the external logic circuits that the transmission is in 4th gear.

The simplest of control systems would be a toggle switch on your dash that sends 12V to the transmission to lock the TCC when so desired. Typically people will replace their brake light switch with one that has a second set of contacts that can be used to cut off this 12V from the toggle switch to unlock the TCC automatically when you hit the brakes. Also, it is typical to use the ground provided by the fourth gear switch when in fourth gear to provide the ground for the TCC. This makes it so the TCC can only be locked in fourth gear.

One step up in complexity from this basic system is to include a vacuum switch in the circuit along with the toggle switch, brake switch and 4th gear switch. The vacuum switch senses manifold vacuum and unlocks the TCC when accelerating. This arrangement can have some undesirable side effects. One is that the vacuum switch can cause rapid on-off-on-off cycling of the TCC at certain throttle positions. A time delay relay can be added to the mix to reduce this effect.

These approaches still lack any sort of speed-based control, which will lead to lugging of the engine since the TCC will remain locked as you slow from higher speeds. This can be mitigated by leaving the transmission in 3rd gear or by turning off the toggle switch when not on the highway. Various manufacturers offer speed-based controllers for the 700R4, and this is what I would recommend.

https://www.summitracing.com/parts/bmm-70244

I built myself an Arduino-based controller that controls the TCC based on speed and manifold vacuum, but this combination is not available commercially. A vacuum switch, however, could be installed in conjunction with the B&M speed-based controller.

[K10-Kansas]

Originally Posted by pjmoreland

Vehicles originally equipped with a 700R4 included external logic circuits for determining the appropriate conditions to lock or unlock the torque converter clutch. These methods ranged from a simple manifold vacuum switch on early models to electronic speed based controls on the later models. There were quite a few different ways 700R4 transmissions were wired internally, as shown in the documents posted in the following thread.

https://67-72chevytrucks.com/vboard/...15#post8974715

The electrical connector on the 700R4 has three pins. Typically, two of these pins are for applying 12V and ground to lock the torque converter clutch. The third pin is typically a signal from the transmission to indicate to the external logic circuits that the transmission is in 4th gear.

The simplest of control systems would be a toggle switch on your dash that sends 12V to the transmission to lock the TCC when so desired. Typically people will replace their brake light switch with one that has a second set of contacts that can be used to cut off this 12V from the toggle switch to unlock the TCC automatically when you hit the brakes. Also, it is typical to use the ground provided by the fourth gear switch when in fourth gear to provide the ground for the TCC. This makes it so the TCC can only be locked in fourth gear.

One step up in complexity from this basic system is to include a vacuum switch in the circuit along with the toggle switch, brake switch and 4th gear switch. The vacuum switch senses manifold vacuum and unlocks the TCC when accelerating. This arrangement can have some undesirable side effects. One is that the vacuum switch can cause rapid on-off-on-off cycling of the TCC at certain throttle positions. A time delay relay can be added to the mix to reduce this effect.

These approaches still lack any sort of speed-based control, which will lead to lugging of the engine since the TCC will remain locked as you slow from higher speeds. This can be mitigated by leaving the transmission in 3rd gear or by turning off the toggle switch when not on the highway. Various manufacturers offer speed-based controllers for the 700R4, and this is what I would recommend.

https://www.summitracing.com/parts/bmm-70244

I built myself an Arduino-based controller that controls the TCC based on speed and manifold vacuum, but this combination is not available commercially. A vacuum switch, however, could be installed in conjunction with the B&M speed-based controller.

Oh, wow, so could I wire an indicator to that 3rd pin to let me know in the cab if lockup is engaged or not?

I forgot to mention that I did wire up a 4 blade brake switch activated by the brake pedal and have that running to a vacuum switch that has an adjustment screw. I don't know if I wired any of it up correctly but I did do a lot of research to do it. But it's my first time doing anything like this so I'm unsure about it.

When you say "...Also, it is typical to use the ground provided by the fourth gear switch when in fourth gear to provide the ground for the TCC. This makes it so the TCC can only be locked in fourth gear..."
Does that mean that the ground from the brake switch would run to the ground pin on the TCC pigtail?

This is all a bit confusing so I'm still unsure if I need to ground the TCC pigtail or how behavioral would be a

[pjmoreland]

Fourth gear and lockup are two separate things. Fourth gear is the overdrive gear, and lockup is the clutch in the torque converter. The third pin on the transmission just indicates when the transmission has shifted into 4th gear. It simplifies things if you only ever lockup the TCC when the transmission is in 4th gear, so you can use the signal from the 4th gear switch to control when lockup is allowed.

I suggest wiring the second set of blades on your 4 blade brake switch to a 12V source that is on only when your ignition switch is on. Your stock fuse panel should have a provision labeled "Ignition Unfused" on it for this purpose. Install an inline fuse (1A should be sufficient).

With a basic setup like this, I highly recommend installing a toggle switch on your dash to disable lockup when cruising around town or in situations where the engine is struggling due to the lockup being engaged. Otherwise, you will get frustrated by the behavior (at least I did) and have no way to control it.

String the 12V supply through all of your switches (brake, toggle, vacuum) and then connect it to the TCC+ "A" terminal on the transmission. This will provide voltage to the TCC solenoid if all of the switches are closed.

To complete the circuit, the TCC- "D" terminal on the transmission needs to be connected to ground. My recommendation would be to connect it to ground via the 4th gear switch, which is inside the transmission and wired to the "B" terminal. You really need to do it this way because you don't want the TCC locking up in the lower gears. You accomplish this connection by installing a jumper wire betwen the "B" and "D" terminals on the transmission connector. There are variations from one transmission to the next on how the 4th gear switch is wired inside the transmission, and some 4th gear switches are normally open (N.O.), whereas some are normally closed (N.C.). Using the 4th gear switch to provide the ground path for TCC- requires the 4th gear switch to be of the N.O. variety, and one of its terminals must be connected to ground inside the transmission. TCI sells a kit that includes a N.O. switch and instructions for installing and wiring it properly. You will need to determine which kind of 4th gear switch you have and how it is wired inside your particular transmission. This is why I included the link in my previous post that shows all of the ways 700R4s were wired internally over the years. Use a test light or multimeter to confirm you have no continuity between the "B" terminal and the case of the transmission. This isn't a foolproof test though. If you do have continuity, then you probably don't have the right kind of switch, and it will likely need to be replaced.

You should be able to confirm functionality of your TCC circuit fairly easily without an indicator light. If you have a tachometer, then you should be able to see when the TCC locks, as the engine will drop by a couple hundred RPM. You should also be able to hear the RPM drop. You should also be able to see/hear when the TCC disengages. To disengage, if you don't have a toggle switch, then depressing your brake pedal slightly with your left foot should de-energize the TCC solenoid, assuming you have the TCC power routed through the brake switch.

Here's a basic wiring diagram.

[K10-Kansas]

Thanks a ton. I really appreciate the great info.

So I can easily test the TCC functioning by noting the tachometer display showing a drop of a few hundred RPMs while getting up to speed on the highway.

I can also see the TCC functioning by tapping the brake lightly which would show an increase of a few hundred RPMs on the highway?

Also, maybe with a wire tester I would see the TCC A pin 12v turn off when stepping on the brake pedal while parked?

Before doing this, I looked up a picture of a relay diagram for TCC, a vacuum diagram for TCC, and the brake switch and tried to blend them into 1 diagram. This is what I came up with. I haven't tested it yet to see if anything functions correctly.

[pjmoreland]

I will study your diagram more closely later. A couple of quick thoughts:

1) Where exactly are you getting 12V? Looks like you are getting it from your brake lights. Hopefully you are getting it from the orange wire that comes from your brake light fuse. This source will be hot even when the truck is off. I recommend getting it from Ign. Unfused on your fuse panel.

2) is that a time delay relay? If not, then you don't need a relay.

3) Your indicator light will be on all the time with it wired that way. You would need to connect the two wires of the light to the TCC+ and TCC- terminals of the transmission for it to light when the TCC is locked.

4) What is your plan for connecting the B and D terminals?

[K10-Kansas]

Originally Posted by pjmoreland

I will study your diagram more closely later. A couple of quick thoughts:

1) Where exactly are you getting 12V? Looks like you are getting it from your brake lights. Hopefully you are getting it from the orange wire that comes from your brake light fuse. This source will be hot even when the truck is off. I recommend getting it from Ign. Unfused on your fuse panel.

2) is that a time delay relay? If not, then you don't need a relay.

3) Your indicator light will be on all the time with it wired that way. You would need to connect the two wires of the light to the TCC+ and TCC- terminals of the transmission for it to light when the TCC is locked.

4) What is your plan for connecting the B and D terminals?

1) The stock harness does have an orange wire running from the bulk head to the the brake switch. I also have an accessory wire running to pin 30 on the relay.

2) It's not a time delay relay. I had used a relay to take the place of the fact that I didn't use a cruze control brake switch. I'm not sure if that is an okay setup though.

3) By dash light I mean it's the stock brake light on the dash instrument cluster.

4) If I connect B and D does that cause the transmission to only lockup in on the highway right? So I should connect B and D?


I just put a wire tester on the A pin, turned the ignition key to on (not starting but just 1 click tot he right), then pushed the brakes and I did not get 12v to the A pin.

[pjmoreland]

Ok. I understand why you are using the relay. You are using it to reverse the logic of the standard brake switch.

If things are wired correctly, then you should have 12V at pin A when not pressing the brake pedal, and then the voltage should go away when you do press the brake pedal.

I'm heading out to a wrecking yard at the moment. I will look at how you have the relay wired this evening.

Jumping B and D together may work, but it depends on how your transmission is wired internally. Do you know what year and model of vehicle it came from originally?

[K10-Kansas]

Originally Posted by pjmoreland

Ok. I understand why you are using the relay. You are using it to reverse the logic of the standard brake switch.

If things are wired correctly, then you should have 12V at pin A when not pressing the brake pedal, and then the voltage should go away when you do press the brake pedal.

I'm heading out to a wrecking yard at the moment. I will look at how you have the relay wired this evening.

Jumping B and D together may work, but it depends on how your transmission is wired internally. Do you know what year and model of vehicle it came from originally?

Oh, i had that backwards. I see it should get 12v when not pressing on the pedal. So does that mean if i don't figure this out, i could just drive around for a few weeks with no power running to the transmission and it would never lockup? Like the only problem I'd have for a few weeks would be getting bad gas's mileage on the highway?

Its a brand new transmission from the factory. I had a shop put it in but at that time, every inch of wiring in the truck was a spliced, diced and jumped mess. I pulled every inch bumper to bumper and put in an American Auto wire 67-72 classic update harness myself and it came out great. Except fory add-ons like trying to figure out the vacuum switch, brake relay, and TCC pigtail.

[pjmoreland]

Ok, I took a closer look at your wiring diagram. The way you have your relay wired looks good. Verify that you can hear the relay clicking when you press the brake pedal.

As mentioned earlier, the way you've got the dash light wired won't work. It will just illuminate when you press the brakes. If you want it to illuminate when the TCC is locked, then you will need to connect the two dash light wires to the A and D terminals of the transmission.

With your current setup you should have 12V on terminal A of the transmission with your engine running and with your brake not pressed. Also, you will need to connect the vacuum switch to manifold vacuum for this test. If you connect it to the timed port on the carburetor for this test, then there will be no vacuum at idle, so the vacuum switch won't close.

The last piece of the puzzle are the B and D terminals. I'd say the odds are pretty high that all you have to do is install a jumper between them. It would be nice to confirm though. Do you have a test light or a multimeter with an Ohms setting? If you have a test light, then clip it to the transmission case and touch the probe to terminal B. Does it light? Hopefully it doesn't. A multimeter should show infinite resistance for the same test. If it shows zero Ohms, then you've got the wrong kind of 4th gear switch inside the transmission.

It shouldn't be an issue to drive around without the TCC working. You might want to leave the transmission in 3rd gear though, as more heat will be generated in 4th with the TCC unlocked.

[K10-Kansas]

Originally Posted by pjmoreland

Ok, I took a closer look at your wiring diagram. The way you have your relay wired looks good. Verify that you can hear the relay clicking when you press the brake pedal.

As mentioned earlier, the way you've got the dash light wired won't work. It will just illuminate when you press the brakes. If you want it to illuminate when the TCC is locked, then you will need to connect the two dash light wires to the A and D terminals of the transmission.

With your current setup you should have 12V on terminal A of the transmission with your engine running and with your brake not pressed. Also, you will need to connect the vacuum switch to manifold vacuum for this test. If you connect it to the timed port on the carburetor for this test, then there will be no vacuum at idle, so the vacuum switch won't close.

The last piece of the puzzle are the B and D terminals. I'd say the odds are pretty high that all you have to do is install a jumper between them. It would be nice to confirm though. Do you have a test light or a multimeter with an Ohms setting? If you have a test light, then clip it to the transmission case and touch the probe to terminal B. Does it light? Hopefully it doesn't. A multimeter should show infinite resistance for the same test. If it shows zero Ohms, then you've got the wrong kind of 4th gear switch inside the transmission.

It shouldn't be an issue to drive around without the TCC working. You might want to leave the transmission in 3rd gear though, as more heat will be generated in 4th with the TCC unlocked.

Yup I hear the relay clicks.

I followed those 2 wires running to the dash. I didn't connect them to create a TCC lockup indicator. They are just a part of the stock setup. The white one runs to the steering column. The blue one runs behind the instrument cluster and is label brake light.

Oh I didn't know the transmission had to be running. Yeah when I started the truck the A pin reads 14.5 volts. Seems kind of high.

I don't see a vacuum connection on the manifold.

My multimeter shows OL between the b and d pin.

I'm not sure what that tells me.

Thanks for all your help on this.

[pjmoreland]

It's not a matter of having the transmission spinning. It's a matter of having your engine producing vacuum to close the vacuum switch. 14.5V at the A pin sounds just right with your engine running. That's the output of your Alternator. You're on the right track. Now you just need to figure out the 4th gear switch, and, if it's correct, add the jumper between the B and D terminals.

[K10-Kansas]

Originally Posted by pjmoreland

It's not a matter of having the transmission spinning. It's a matter of having your engine producing vacuum to close the vacuum switch. 14.5V at the A pin sounds just right with your engine running. That's the output of your Alternator. You're on the right track. Now you just need to figure out the 4th gear switch, and, if it's correct, add the jumper between the B and D terminals.

Great thanks. So since my multimeter shows no continuity from the B to the D pin, that means I should connect the B and D wires together?

[pjmoreland]

Originally Posted by K10-Kansas

Great thanks. So since my multimeter shows no continuity from the B to the D pin, that means I should connect the B and D wires together?

That is a strong indicator that you have a normally open 4th gear switch, which is what you want. The only way to know for sure would be to drop the pan and trace the wires inside the transmission. Probably not worth the trouble. If it were my truck, I'd just go for it and connect the B & D wires together. I would also put a 1A fuse in the voltage supply line just in case...

[K10-Kansas]

Originally Posted by pjmoreland

That is a strong indicator that you have a normally open 4th gear switch, which is what you want. The only way to know for sure would be to drop the pan and trace the wires inside the transmission. Probably not worth the trouble. If it were my truck, I'd just go for it and connect the B & D wires together. I would also put a 1A fuse in the voltage supply line just in case...

Great, would that fuze go between the vacuum switch and A pin? Or between the 87a relay pin and vacuum switch?

[pjmoreland]

I'd put the fuse on the wire connected to terminal 30 of the relay.

[K10-Kansas]

Originally Posted by pjmoreland

I'd put the fuse on the wire connected to terminal 30 of the relay.

Gotcha, thanks, so on the wire connecting the cab bulkhead to the relay 30 pin. Thanks, I'll do it tomorrow.

[pjmoreland]

If that wire is passing through the bulkhead, you could install the fuse on the interior side, if that is more convenient.

[K10-Kansas]

Originally Posted by pjmoreland

If that wire is passing through the bulkhead, you could install the fuse on the interior side, if that is more convenient.

Yeah, the wire between runs from the interior cab bulkhead the the relay I installed under the dash. Perfect thanks.