Hi,

I`m wondering if there`s some way to improve the operating of the transmission in my truck. The original transmission has probably been a TH400 --- the sticker in the glovebox says it came with a 3speed trans. Some previous owner must have replaced it with a 700R4. The truck actually has a 4speed with a converter clutch now. There are two switches to turn on and off the converter clutch in 4th and 3rd gear manually, and it works fine so far.

The problem is that the transmission will shift as it wants to even with the converter clutch still locked, which is probably a very bad thing to do. I`m trying to operate the converter clutch manually just the way it would be operated automatically as best as I can. But I have no reliable way to predict downshifts like from 4th to 3rd when driving up an incline. I would end up driving with either the converter clutch unlocked most of the time, generating lots of heat, or I`d be running too much risk of the transmission shifting with the clutch locked. It`s not so much of an issue on flat terrain, but I`m in Hill Country ...

Is there any way to improve this setup, like automatic operation of the clutch, or is there a way to keep the transmission from shifting at all until I allow it to shift? I wish the truck had an manual tranny ...

With a properly set up 700R4 the lockup clutch locks and unlocks automatically. There is a series of switches to facilitate that. There is a Throttle Position Switch (TPS), a brake switch, and a couple switches inside the transmission. There were many different variations so it is difficult to give an exact description. As I recall there is supposed to be a switch on the valve body that unlocks the clutch during shifts.

The transmission can also be modified to operate the lockup clutch hydraulically. This is something you would have to talk to a transmission guy about. I have never done that.

Your best bet is probably to wire up the proper wiring. I'm guessing the switches on the valve body are in place so you can probably still use those. You'll have to get into it before you can tell exactly what you have though.

Hm, I`m guessing that the truck doesn`t have all the required switches like the TPS and the brake switch because they were not needed for the original TH400. When the TH400 was replaced with the 700R4, they probably didn`t bother to install these switches and just hooked up the ones for manual operation.

The transmission was recently rebuilt, and the guy at the shop that did it told me that there`s only one wire in the transmission, going to the solenoid that controls the clutch. He said they only found out that one of the switches locks/unlocks the clutch in 4th gear and that they don`t know what the other switch is for since it only seems to apply another ground connection. I`ve been checking these switches with a voltmeter before the rebuild and found that the one for the lockup in 4th supplies 12V to a wire while the other one doesn`t seem to apply any voltage. The switches seem to share the ground connection; both of them have to be turned on to engage the clutch in 3rd.

But then, it may not be true what he told me. They were even so stupid not to refill the transfer case with oil, and now they refuse to stand up to their warranty. Stay away from National Transmission in South Austin ...

Sounds familiar. I will have to do a little research. I don't remember exactly how that is supposed to be wired. Been a little while since I worked on one.

Apparently you do have the correct switches. I remember those. I don't remember how they are supposed to be hooked up.

Even if your truck doesn't have the brake switch or the TPS you can make it work quite a bit better than it does now if you wire up the correct switches on the valve body. I just have to look up how they are supposed to be wired.

By the way, I'm not too surprised they are not honoring their warranty. Frankly I'm surprised they even gave you a warranty with that set up like that. Not that it is right, but it doesn't surprise me.

Sounds familiar. I will have to do a little research.


Thanks, that would be cool :) I will have to deal with all the wiring anyway: find out what wire is for what, put in some fuses, fix some connections, run a wire into the cabin to supply accessories. So far, only the battery connections have been fixed.


Apparently you do have the correct switches. I remember those. I don't remember how they are supposed to be hooked up.


Yeah, they work. Actually, there are two switches on the floor on the drivers side. One of them is connected to the switches that are in the dashboard. The idea must have been to have the switches in the dashboard to generally turn the locking on/off and using the switch on the floor with the left foot to engage/disengage the clutch while driving. But for one thing, I somehow don`t like it, and for another, the floor-switch is sticky so that I`d have to replace it.

The other switch, I don`t know what that one is for. Maybe I`ll find out.


Even if your truck doesn't have the brake switch or the TPS you can make it work quite a bit better than it does now if you wire up the correct switches on the valve body. I just have to look up how they are supposed to be wired.

Cool --- I just don`t want to break the transmission by having it shift with the clutch engaged. Hm, I`ve been crawling under the truck today to find the sending unit for the oil pressure gauge and noticed that there seem to be two wires going into the transmission. That`s probably a ground connection and the 12V for the solenoid?

By the way, I'm not too surprised they are not honoring their warranty.


They will have a hard time to get out of that. Shortly before the transmission was rebuilt, the fluid in the transfer case was changed, and I still have the bill for that or might get it reprinted. So there was definitely oil in the transfer case. But when the drain plug on the transfer case was opened about 1100 miles later, about a cup of oil came out, and there was no visible leak. I`d have seen it leaking on the floor since I kept looking, and the truck is pretty high. Moreover, their warranty explicitly covers things that are attached to the transmission --- they do exclude the driveshaft and a few other things from that, but they do not exclude transfer cases. We got a fight at hand, but I don`t see how they would get out of it. It`s not even a matter of warranty since they just failed to do their work in the right way.


Frankly I'm surprised they even gave you a warranty with that set up like that. Not that it is right, but it doesn't surprise me.

Yeah, I wondered about that, too.

you need to find another shop that knows how to actually do stuff. all you need is a pressure switch to turn on the lockup whenever your trans shifts into 4th gear. i've not done it but my trans buddy probably would know.

I don't like manually operated switches for the lockup clutch. I like automatics that are really automatic. Shifting a manual transmission is kind of fun. Flipping switches while driving is not.

all you need is a pressure switch to turn on the lockup whenever your trans shifts into 4th gear.

That works, but that isn't the only way to do it. You can make it lock in 2nd and 3rd as well and that helps improve fuel economy quite a bit.

Hi,

I`m wondering if there`s some way to improve the operating of the transmission in my truck. The original transmission has probably been a TH400 --- the sticker in the glovebox says it came with a 3speed trans. Some previous owner must have replaced it with a 700R4. The truck actually has a 4speed with a converter clutch now. There are two switches to turn on and off the converter clutch in 4th and 3rd gear manually, and it works fine so far.

...

You'd be way ahead to find a junk vehicle with the correct hookup parts. Your original setup had a VRV on the injection pump to send a variable vacuum signal to the TH400 3 speed trans. Now that you have a 700R4, you need a TV cable hooked to the fuel injection pump for it to work correctly. You also should have the other stuff, e.g. the brake-light switch hooked to hitting the brakes disconnects the lock-up converter, and if you have 4WD - engaging it also disconnects the converter lockup, etc.

The main thing though, is the TV cable.

In regard to the TPS switch - that is mostly used on C-code engines to control the EGR vavle solenoid in conjunction with the lock up torque-converter.

If you cannot find the proper parts, repost. I have several junked 6.2 diesel trucks out in my field that have 700R4 setups.

I'm guessing this truck already has the TV cable installed. If it didn't the tranny would probably have smoked by now, not to mention the fact that it would slip, shift at the wrong time, and in general not function correctly. The main things besides the TV cable are the TPS and the brake switch, niether of which is very hard to find.

Thanks for your input, I appreciate that very much! Monz is right that having to flip switches while driving is not exactly fun, so it`s really good news that I might be able to change that with your help :)

As to the TV cable, I think that has been installed. The transmission shifts up and down as it should, you only have to manually operate the clutch. I will have to read up on this and then check if there`s such a cable.

Applying the brakes does not disengage the converter clutch --- another thing that I don`t like. I`ve been thinking about installing a relay driven by the brake light switch to turn off the clutch. But once I`d release the brake pedal, the clutch would lock up again. I didn`t try to engage the clutch in 4wd, but since operation is totally manually, I guess I could.

Hm, in a normal setup, how would be determined when to engage and to disengage the clutch? From driving the Tahoe, I figure it mainly depends on the load (throttle position), on vehicle speed and on engine speed (hence oil pressure in the trans) --- besides a switch on the brake pedal, temperature and eventually a switch on the shift lever of the transfer case. But that was all controled electronically by the PCM. Without the electronics, the same factors still apply, but how are they taken into account?

I`ll try to find parts on a junkyard. I was told about one that has some Suburbans sitting on their lot in different states of being taken apart. I need a flywheel cover, and maybe I can pick up parts needed for improving the setup of the transmission.

This turns into some kind of long-term project. The wiring is a mess, paintwork is needed, a freeze plug is leaking a little, and a hundred other things need to be improved. The good thing is that I have the time to do it :)

After a little research here's what I came up with. There are a million different ways to set it up.

The simplest thing to do is make it lock in second, third, and fourth. Simply supply 12V to the solenoid and it will do that. I am sure that is what it does when your switch is in the "on" position. It doesn't unlock during shifts. Is shifting with the converter locked causing problems? I know it probably downshifts a little more distinctly but I don't think it should be too harsh on the upshift. I've driven trucks set up like that and it doesn't seem to be an issue.

Another simple way to do it is set it up like 4DR4X4 suggested and make it lock only in fourth gear. It is easy to do, but exactly how you wire it to do that depends on the year of the transmission. Late model transmissions had a two-wire lockup clutch solenoid that you can wire to a grounded fourth gear switch. That is a pretty nice setup. Earlier models used a single wire grounded solenoid that can be made to do the same thing but it uses a different pressure switch which is wired into the + side of things. You will have to drop the pan to see what you have on your transmission.

Thanks a lot!


The simplest thing to do is make it lock in second, third, and fourth. Simply supply 12V to the solenoid and it will do that. I am sure that is what it does when your switch is in the "on" position.


There are two switches --- red and blue --- and a sticker on the ashtray which reads (all in capital letters):


"must have lock-up in 4th gear or trans. failure
red lock-up on only can achieve 3rd gear L-up by using both switches
use for towing
do not downshift while in 3rd L-up"


The red switch switches the converter clutch when in 4th gear. The blue switch switches the clutch when in 3rd gear. The switches are connected to each other so that the blue switch does nothing when the red switch is off. The red switch seems to supply 12V, the blue switch seems to supply ground.


It doesn't unlock during shifts. Is shifting with the converter locked causing problems? I know it probably downshifts a little more distinctly but I don't think it should be too harsh on the upshift. I've driven trucks set up like that and it doesn't seem to be an issue.


Exactly, it doesn`t seem to unlock but can shift back and forth between 4th and 3rd --- a little more distinctly like you say. But when picking up the truck after the transmission was rebuilt, I was told that shifting with the clutch locked would be like shifting a manual with the clutch locked.

Shifting with the clutch locked is what I`m trying to avoid. Is that really something I don`t need to worry about?


Another simple way to do it is set it up like 4DR4X4 suggested and make it lock only in fourth gear. It is easy to do, but exactly how you wire it to do that depends on the year of the transmission. Late model transmissions had a two-wire lockup clutch solenoid that you can wire to a grounded fourth gear switch. That is a pretty nice setup. Earlier models used a single wire grounded solenoid that can be made to do the same thing but it uses a different pressure switch which is wired into the + side of things. You will have to drop the pan to see what you have on your transmission.Hmm, I wouldn`t know what to look for. I`ve seen the 4l80E in the Tahoe a couple times with the pan off when changing the oil: looks like a block of metal that dips into the pan, with a filter on the bottom and a few wires on it.

Even if I could make it so that the clutch is locked only in 4th gear, won`t it shift out of 4th with the clutch still locked and only unlock it once it`s in 3rd?

With a stock setup, what locks the clutch? Would it just be locked the moment the transmission shifts into 4th?

As far as I know shifting with the converter locked shouldn't hurt anything but I don't know exactly how your transmission is set up. I'm guessing that whoever put that label there must have had a reason. I have driven trucks that were set up so that the clutch locks right after it shifts into second and stays locked after that and never had a problem because of it. Shifting a manual with the clutch out isn't the same. Completely different transmissions. I almost never use the clutch when I shift my manual anyway (saves the clutch and doesn't hurt the transmission if it is done properly).

Also, I would think that unlocking the clutch during shifts would put a lot of wear on it.

As far as I know shifting with the converter locked shouldn't hurt anything but I don't know exactly how your transmission is set up. I'm guessing that whoever put that label there must have had a reason. I have driven trucks that were set up so that the clutch locks right after it shifts into second and stays locked after that and never had a problem because of it. Shifting a manual with the clutch out isn't the same. Completely different transmissions. I almost never use the clutch when I shift my manual anyway (saves the clutch and doesn't hurt the transmission if it is done properly).

Also, I would think that unlocking the clutch during shifts would put a lot of wear on it.

There have been automatic transmissions in the past with no torque converter at all. Chrysler had one called the "Clutch-flite." Basically just an automatic trans hooked to a conventional clutch and flywheel. You had to push the clutch pedal in to stop (while in gear), but otherwise shifted without it.

I suspect though, with the 700R4, it needs NOT to be engaged at low RPM-high torque conditions or it will get trashed. It's not a very rugged lock-up converter. It's basically made for cruising - not hard pulling. GM did make a special HD converter with six lugs - but it's rare and I don't know if it's any beefier inside.

In regard to shifting manuals without using the clutch - it shortens the life of modern day cone-clutch synchronizers unless you do it absolutely perfectly. But yeah, if done right with matching gear speeds/RPMs so the synchros don't have to do the speed-matching work, it does little wear and tear. But, use the clutch correctly - and that also works fine - probably better.

Yes, the 700R4 lockup clutch is kind of a weak mechanism, but it shouldn't hurt anything engaging in second gear. There is no way that I know of to get it to lock in any lower gear so there is no danger of stalling the engine or anything like that as long as the TV cable and governor are setup properly. I've driven plenty of trucks that locked the converter as soon as the transmission shifted to second and they worked great and seemed to last just fine. I do agree that it is a light duty mechanism and does need to be treated carefully, but the whole transmission is a light duty transmission. It isn't made for heavy duty work. Towing with that setup probably wouldn't be a good idea.

I learned to shift without the clutch with a non synchro HD truck. You had to get the shifts right on or it would grind. You could shift it with the clutch - but you would have to double clutch it. After that I drove quite a few HD trucks that were the same way and got pretty used to it. Ever since I've only used the clutch about half the time. In the city I use the clutch through all the gears but if I'm out on the highway headed for the next town I don't bother. True, if you don't know what you are doing you shift without the clutch and just let the shifter ride on the synchros until it drops in and that will wear them out (not to mention give you a bit of a rough ride), but if you do it right it shouldn't hurt it any. Even when you shift with the clutch you are still putting some wear on the synchros.

I've driven plenty of trucks that locked the converter as soon as the transmission shifted to second and they worked great and seemed to last just fine.

I can't say the same for my Ford 7.3 turbo with the E40D trans. Several times I've made the lockup converter clutch slip and squauk while towing - usually when gaining speed climbing a hill and it tries to engage. I now turn off the OD when towing.

I learned to shift without the clutch with a non synchro HD truck. You had to get the shifts right on or it would grind.

Yeah, that's how I learned when I first starting driving big trucks in the 1960s. First Brockway I drove - I couldn't shift without grinding no matter what I did and often, I'd get stuck in neutral and could not get back into any gear without stopping. Then my boss would get in the truck and shift the damn thing without even using the clutch - with no grinding at all. I eventually learned, though - ridicule and embarrassment can be a good teaching aid. We had several old 50s era gas-engine Brockway trucks - along with some same-vintage diesel Macks. But, those transmissions were sliding-gear which is a totally different technology than constant mesh as used today in auto transmissions. Only exception I can think of is 1st gear only in the GM HD four speed where 1st is sliding gear but all else is constant-mesh. When you screw up a shift with an older sliding gear trans, all you do is peen the ends of the gear teeth a bit. But, screwing up a shift on a constant mesh trans can screw things up much faster since there's a lot less metal involved for engagement.

My guess is that the transmission will overheat if not locked up in 4th gear. But the label doesn`t say it must not shift between 4th and 3rd gear when locked :) So I`ll still try to prevent that but I`ll stop worrying about it too much.

What holds the converter clutch together in a 700R4? If it`s oil pressure, is there a minimum rpm required?


Yesterday, I was at the junkyard: As expected, oil lines from gassers do not fit, and Diesels will be awfully hard to find here. I got a flywheel cover from a gasser --- even that doesn`t fit because the opening for the starter isn`t wide enough. But I can make it fit :)

My guess is that the transmission will overheat if not locked up in 4th gear.

Yesterday, I was at the junkyard: As expected, oil lines from gassers do not fit, and Diesels will be awfully hard to find here. I got a flywheel cover from a gasser --- even that doesn`t fit because the opening for the starter isn`t wide enough. But I can make it fit :)

I've heard many arguments about unlocked converters causing trans. overheating. I don't believe it. I drove two GMs for years with the converter wireharnesses unhooked - my 86 S15 Jimmy, and my 86 K5 diesel Blazer.
If lack of lockup causes dangerous heat - how did the TH350s, TH400s, and Powerglides survive? My 87 Suburban has 530,000 miles on the non-lockup TH400.

In regard to my gas-engine 86 Jimmy. The lockup went bad and made an awful noise. So, with nothing to lose - I unplugged the converter wire-harness and drove it another 80,000 miles before selling it. The trans still worked fine - other than having no lockup. Since the engine was only a 2.8, it ran much better on the highway without the lockup and no longer shifted in and out of OD at every slight grade.

I later did the same with my 86 diesel Blazer since it lacked power to stay in OD on a hilly highway. Unplugged, it runs much better. I later added a shutoff switch and plugged the harness back in. So now, if I'm on a flat highway, I turn the lockup back on.

In regard to small parts for 6.2 trucks - if you really get stuck, I can probably help. I've got a two fields full of junked 6.2 diesel trucks, Suburbans, and Blazers - many with 700R4s.

If lack of lockup causes dangerous heat - how did the TH350s, TH400s, and Powerglides survive?


My guess is that they were designed to work without a lockup, and that they had sufficient cooling :)

The only cooling I have for the transmission now is this small cooler on the passenger side:

[/URL][URL="http://dieselplace.com/forum/attachment.php?attachmentid=43301&d=1186441855"]http://dieselplace.com/forum/attachment.php?attachmentid=43301&d=1186441855 (http://dieselplace.com/forum/attachment.php?attachmentid=43301&d=1186441855)
(Image has been resized. Click it for full size.)

(Image has been resized. Click it for full size.)

I`m afraid that this may not be enough for a truck with a GVWR of 8600 pounds. And it`s going to be 98F here this weekend.


In regard to small parts for 6.2 trucks - if you really get stuck, I can probably help. I've got a two fields full of junked 6.2 diesel trucks, Suburbans, and Blazers - many with 700R4s.Hehe, I was sending you a PM while you were typing this :)

My guess is that they were designed to work without a lockup, and that they had sufficient cooling :)


The radiator is the same for a TH400 or a 700R4 trans, so the stock cooling capacity is also the same.

The radiator is the same for a TH400 or a 700R4 trans, so the stock cooling capacity is also the same.

Ok, but does the aftermarket cooler on my truck have sufficient cooling capacity? :)