700R4 LockUp Troubleshooting

[sick472]

My K20 has the 700R4 and I cannot tell if the lock-up function is working. It's either so slight that I cannot notice it, comes in at the same time as 3rd or 4th, or it's not happening. The transmission was just rebuilt at a good shop. It happens to be an 88 or 89 model transmission.

I know it has 12 plus volts to the "A" pin at the transmission (with the motor running). The brake switch cuts that power like it should and the vacuum pod switch starts to supply that power at 4" Hg (meaning there is no power coming through it until it sees at least 4 inches of vacuum). I hooked a volt meter up this morning on the drive in to work (to the test connector) and the "A" pin 12 volts is constant from starting the engine through all gears. The only exception is when the brake is applied. The "B" pin has power to it too even though I have not been able to monitor it as the truck is moving through the gears.

Side Note: I could not get the vacuum switch to open (cut power) under a heavy foot at highway speeds like I would expect, but that should not affect it locking up, but rather effect it unlocking. 4" Hg of vacuum may be too low and my stock rebuilt motor may not drop that low, IDK.

Does anybody know at what vacuum the vac pod is supposed to activate?

Either way, I cannot seem to feel it lockup going through the gears or unlock once the brake is tapped. One curious thing I have noticed is that my test light will not light up at the test connector, but my volt meter reads 12 plus volts.

I believe I have one of the two posted wiring diagrams. I'm not sure what to look for regarding the EGR Bleed Solenoid, but other than that the diagrams fit my system. And, I'm not sure which diagram is correct for inside the transmission itself.

Any tips on what to do next?

I will be consulting my transmission shop, but thought about checking here first. Thanks in advance!

[pjmoreland]

If you have a tachometer, you should see the engine drop a few hundred RPM when the TCC locks. You should also be able to hear the change in RPM. It sounds like a gear shift. Once it locks, the RPM becomes very stable and shouldn't change much when you make slight increases or decreases to your accelerator pedal.

Do you have factory wiring for a 700R4 in your 1985 K20? If you do, then I believe it is configured to work with a normally closed 3-4 switch inside the transmission. Your 88/89 transmission probably has a normally open 3-4 switch instead. This could be what's causing it not to work. You should be able to determine which type of switch you have, as follows:

1) Unplug the TCC harness connector from the transmission
2) Measure for continuity between pin B on the transmission connector to the case of the transmission

If you have continuity, then you have a normally closed switch. If no continuity, then it is normally open. You will need to know exactly how your truck is wired in order to know how the transmission needs to be wired internally.

Here's a good description of the various ways 700R4 transmissions were wired internally over the years of production:

https://www.thirdgen.org/forums/carb...c-control.html

Would you be able to post a zoomed in version of the schematic you posted on the left above that shows the transmission internal wiring so it is legible?

[pjmoreland]

Is there a wire connected to the D pin on your transmission? Your 88/89 transmission will likely need the D pin to be connected to ground.

[pjmoreland]

If your truck is wired like the second schematic you posted, then I believe it is designed to lock the TCC in all gears, and the vacuum switch unlocks the TCC during acceleration in gears 1-3, but the TCC will just be locked all the time in 4th gear. This arrangement would need a normally closed 3-4 switch, as shown on the schematic. The internal wiring in your 88/89 transmission won't support this type of functionality because one side of the 3-4 switch is connected to ground instead of being connected to the TCC solenoid. If this is correct, then I see two options:

1) Drop the transmission pan and rewire the transmission to be like an 85 transmission. You will probably need to get a new 3-4 switch that has two terminals as part of this change so it will no longer have one side of the switch connected to the transmission case.

or...

2) Make some modifications to your truck's harness. This option would result in different behavior of the TCC. You could make the following changes:
-Remove the wire from the B terminal and tape it off
-Install a jumper between the B and D terminals
This assumes you have a normally open 3-4 switch, which you would need to confirm. This arrangement will make your TCC lock up in 4th gear only, and only with high vacuum. You will need to have a vacuum delay valve in the vacuum line that feeds the vacuum switch to reduce the amount of on/off cycling that can happen on hills. You probably already have one of these there if everything is stock.

[sick472]

Thanks for the quick response PJ!,

No tach in the truck, but I am 99.9% sure the lockup is not happening. It does have all the original wiring for the 700R4. The only exception is that there is no EGR solenoid to be found, but I think the wires are there. There is a third pin in the connector (in the "D" position) of the original wiring and a pin in the transmission to correspond to it. The close-up, below, closely resembles what I have. Maybe, with the exception of the two case grounds . I am going to make sure there is a wire for the "D" pin in the harness this evening along with the continuity check you mentioned. I believe this "D" wire would have gone to the EGR solenoid. My transmission man has suggested that I ground the "D" pin wire as I am going down the road to see what happens. That sounds kinda iffy to me just because I haven't wrapped my head around how it works just yet, but I am glad to hear you say similar.

IF grounding the "D" wire while in 3rd or 4th gear causes the lock-up to function...would it need to be on a switch to disable it or could it just be grounded permanently? I don't want a switch to forget about and cause any damage.

The reason I ask is that the vacuum pod has not shown itself to go open yet under a low vacuum event and the circuit will likely be live upon deacceleration (aside from braking). I wonder if the "D" pin was used for a computer to switch the ground wire open and closed?

[pjmoreland]

The B pin on your transmission is likely connected to one terminal of the 4th gear switch, and then the other terminal of the 4th gear switch is connected to the transmission case (ground). If your truck wiring matches either of the two schematics you posted, then the B wire is providing 12V, which is what you measured. This is a bad situation because the 4th gear switch would short the 12V straight to ground when the switch closes. Seems like this would blow the fuse, unless the 4th gear switch burned up instead.

Connecting the D pin to ground shouldn't hurt anything. The 4th gear switch would not be utilized though. It would likely cause the TCC to lock up in all gears, except when released by the vacuum switch during acceleration or when the throttle is fully closed, assuming the vacuum switch is connected to a timed vacuum port. That's questionable though if your vacuum switch is stuck closed. In that case, the TCC will be locked up all the time, even at idle. It wouldn't be good to have the TCC locked at idle in first gear. I imagine your engine would die as soon as you shifted into first.

Disconnecting the factory B wire and installing a jumper between B & D on the transmission is similar to just grounding D. The difference though is that it is grounding D through the 4th gear switch. That way the TCC locks up in 4th only. In my opinion, this would be superior to just grounding D.

I have been searching wrecking yards recently for these vacuum switches, and I have found that vehicles from the mid-80s often have a second switch connected to vacuum that looks like the TCC vacuum switch, but it isn't exactly the same. I don't remember what this other switch is for, but it has a larger body than the TCC vacuum switch. Is it possible your truck has two switches and you are testing the wrong one?

Here's a little video I made that shows what the TCC vacuum switch looks like. It also shows the vacuum delay valve I mentioned earlier.

https://youtu.be/McejhiEMph8

[pjmoreland]

Sorry, I just edited my post above to change "3-4 switch" to "4th gear switch". I was using confusing terminology.

[pjmoreland]

If you have been testing your vacuum switch at idle, and it's connected to a timed vacuum port, then the switch should not be closed in this scenario. You'd need to rev your engine a little bit and leave it revved for several seconds to give the vacuum delay valve time to supply vacuum to the switch. The switch should then close at this point.

[sick472]

Between the TCC Control Sticky you linked and all the good info you are supplying...My head is spinning a bit. It's starting to sink in though.

Just for the record...

I am checking the proper Vacuum Switch and mine closes at the 4" like your video suggests. I did it just like you did with a might vac except I did not do the test with the delay in line...very cool. Thanks for the video.

My vacuum switch is connected to manifold vacuum, however. I will have to consider moving it to ported vacuum source on the carb.

This next question will really help it all sink-in faster...

What pins on the transmission correspond to which pins on the harness connector? I wish the diagram noted that...

[pjmoreland]

They have the harness connector flipped relative to the transmission connector in the diagram. You can tell that based on the empty fourth dummy pin location.

A = 1 (12V from brake switch through closed vacuum switch that powers the TCC in gears 1-3)
B = 3 (12V source provided from brake switch that powers the TCC through the 4th gear switch)
D = 2 (probably connected either to EGR stuff, or nothing)

[pjmoreland]

Based on your first schematic, the EGR bleed solenoid and its wiring have no effect on the TCC lockup with the original 1985 700R4. It's a separate circuit. If you've got that wiring, it could complicate things though since it used the D pin in the original transmission, and now the D pin in your current transmission serves a different purpose.

[sick472]

Last night's tests...

1.) There is no continuity between the "B" pin and the case. This indicates that I have a N.O. 4th gear switch...correct?

2.) I did take the truck for a drive and grounded the "D" pin wire after I was in 3rd gear and no lock up was evident. I stopped messing with it at this point, but noticed that I did not have any power to the radio this morning. Grounding that wire may have blown a fuse. I'll check that this evening.

NOTE: all of my truck wiring colors match the first diagram, at least the wires on the outside of the transmission.

According to the 3rd Gen link you supplied, with a N.O. 4th gear switch I should look into jumping pins "B" & "D" by merely cutting the two wires near the transmission connection and connecting them together...Correct?

[sick472]

I Just got off the phone with my shop and they "believe" I have one of the two wiring types shown in the attachment, but he does not remember a Temp Switch. The fact that the fuse may have been blown due to grounding the "D" wire indicates a bad solenoid which he is willing to swap out.

The type #14 wiring would be easy to set up correctly by grounding the "D" pin once the solenoid is fixed.

The type #15 may be a candidate for the "B' to "D" jumper, but I cannot say for sure due to the possible absence of the temp switch which makes me skeptical that this diagram even closely resembles what I have. I'll have my guy figure out what I have when he is swapping out the solenoid. It would have been nice if he had done this to begin with, but hindsight...

[pjmoreland]

Sounds like your transmission guy will be able to get you squared away. I'm a little concerned about the wire you have connected to B right now. It's got 12V on it from the brake switch. If your transmission is the newer style with the B pin connected to a 4th gear switch to ground, then it's not going to want to have 12V connected to B, as this would result in a short circuit to ground when the 4th gear switch closes.

It does sound like you've got a N.O. 4th gear switch.

The newer style with the temp switch should work fine with B and D wires disconnected from the harness and jumped together at the transmission connector.

[sick472]

I see your point about the hot "B" wire. I plan to do a continuity test between pin "A" & "D". If there is continuity, I likely have type 15 and the hot "B" wire would be a problem...no continuity and the "B" wire is a moot point.

I am betting I have something like the #15. It would make since that a fuse would have blown with the hot "B" wire.

Who knows at this point, but at you have helped me learn how these things work...much obliged!

[pjmoreland]

I think it is fun to try unraveling these types of puzzles. It sure is satisfying when you get them to work. I had no idea there were 23 variations of the 700R4 wiring. That's crazy!

[sick472]

Here's a twist to the puzzle which I will be communicating to my transmission guy.

Last night's test...

I cut the hot "B" wire and grounded the "D" wire. Everything functioned well right into 4th gear. There was no lock-up that I could tell all the way through 3rd gear, but forth gear hit and it locked up within a 3 mph increase as I was accelerating (around 70 mph). Probably in less than 2-3 seconds after it went into 4th gear. BUT, it had a bad shudder/vibration that was felt and heard. It felt like an out of balance tire. I immediately disconnected the ground and the shudder went away. Upon reconnecting it...the shudder came back without fail over several trials in 4th gear....no lock-up in 3rd whatsoever while toggling the ground. The ground was switched in the cab so I could toggle at will.

I am at a loss until I can get my guy to drop the pan and define how it is wired internally (he apparently did not pay any attention to that when he rebuilt it - uuuhg!). I don't know what to think about the shudder as I would have been happy with lock-up in 4th only.

[sick472]

My guy is concerned that I may not be sending enough power (volts or amps) to pin "A" and has suggested that I run a new 12 volt wire (fused at 10 amps) from my battery to pin "A" and then run a switched ground wire from pin "D" to a good ground. This would serve as a test to prove that the transmission's electrical components are good (or not) by eliminating any possible shortcomings of my wiring that is external to the truck. The ground switch would be closed after entering 3rd gear for observation of the lock-up function through 4th gear.

Does that sound right?

[pjmoreland]

Originally Posted by sick472

Here's a twist to the puzzle which I will be communicating to my transmission guy.

Last night's test...

I cut the hot "B" wire and grounded the "D" wire. Everything functioned well right into 4th gear. There was no lock-up that I could tell all the way through 3rd gear, but forth gear hit and it locked up within a 3 mph increase as I was accelerating (around 70 mph). Probably in less than 2-3 seconds after it went into 4th gear. BUT, it had a bad shudder/vibration that was felt and heard. It felt like an out of balance tire. I immediately disconnected the ground and the shudder went away. Upon reconnecting it...the shudder came back without fail over several trials in 4th gear....no lock-up in 3rd whatsoever while toggling the ground. The ground was switched in the cab so I could toggle at will.

I am at a loss until I can get my guy to drop the pan and define how it is wired internally (he apparently did not pay any attention to that when he rebuilt it - uuuhg!). I don't know what to think about the shudder as I would have been happy with lock-up in 4th only.

That sounds like progress! I wonder if the transmission was rewired to have the 4th gear connected between the D pin and the TCC solenoid. That would explain why the TCC didn't lock up in 3rd. You should be able to leave your manual switch on all the time, and it will lock up automatically in 4th gear only.

[pjmoreland]

Originally Posted by sick472

My guy is concerned that I may not be sending enough power (volts or amps) to pin "A" and has suggested that I run a new 12 volt wire (fused at 10 amps) from my battery to pin "A" and then run a switched ground wire from pin "D" to a good ground. This would serve as a test to prove that the transmission's electrical components are good (or not) by eliminating any possible shortcomings of my wiring that is external to the truck. The ground switch would be closed after entering 3rd gear for observation of the lock-up function through 4th gear.

Does that sound right?

Seems like a reasonable test. Performing the test will at least motivate your transmission guy to take a closer look if it doesn't make the shudder go away. Hopefully it does make the shudder go away though.

[pjmoreland]

Describe the shudder in more detail. Do you think there is any chance the TCC is locking and unlocking over and over? I have experienced this when I didn't have a time delay in my vacuum switch. Do you have an inline vacuum delay valve in the hose to the vacuum switch? I have found a couple of those delay valves in wrecking yards recently, and both of them were stuck open. A few blasts of carb cleaner through them fixed them up. It should be easy to blow through it in one direction and difficult in the other direction.

[pjmoreland]

Here's what the vacuum delay valve looks like inside. The little orange thing is a one way check valve that gets gummed up and won't close all the way. The silver spot is a restricted orifice that lets the vacuum build slowly.

[sick472]

The shudder vibration is very much like a out of balance tire at highway speeds. The sound is comparable to that also...something like an out of balance meaty truck tire. The sound is low in tone and not much louder than the sound of my motor (quiet mufflers), leaky window seals, and the off-road tires that it wears, but has a frequency of a tire rotation at highway speeds. If it is locking and unlocking back and forth...it's doing it real fast. There is no clank or clatter noises, just a fast whoop, whoop, whoop that will remain until the ground is opened. The power during this shudder is questionable. It still maintains speed, but giving it more throttle just drops it into 3rd. I think the shudder disappears once the down shift occurs. I don't remember as I was more interested in toggling the ground open to make it go away and not self destruct. The vibration that comes from the shudder can be felt in the steering wheel and this makes the already loose steering feel even more loose...once again like an out of balance tire or sloppy tie-rod end.

The vacuum delay is installed and has been blown through both ways and works as you described. I will check it's orientation, but I think it is installed with the light blue portion towards the vacuum switch.

[pjmoreland]

Ok. I doubt it could lock and unlock that quickly.

[sick472]

Originally Posted by sick472

...run a new 12 volt wire (fused at 10 amps) from my battery to pin "A" and then run a switched ground wire from pin "D" to a good ground. ...

I wired the transmission as stated above and it is still acting the same way.

No lock-up until right after it goes into 4th gear. I can make it go into 4th around 55 mph with a very light throttle on flat ground and the shudder is much, much worse than it is with a heavier throttle and at 65. If I ground the circuit between 70 and 75 mph, lock-up engages and the shudder is barely noticeable, but still there. Any added throttle at the higher speed and it down shifts and lock-up disengages and the light shudder disappears until 4th comes back in.

It's time my guy drops the pan.

Thanks for hanging with me on this PJ! I wonder if the TCC solenoid is faulty...IDK.