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Does a diesel 700r4 shift differently than a gasser?

Can Can

Can Can

Pusher Man
Joined
Feb 17, 2000
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Cochrane, Alberta, Canada
I've been meaning to ask this question for a long time and tonight while I was adjusting my TV cable I made a promise to myself to come straight home afterwards and ask you guys.

Off the line, halfway before the tranny drops into 2nd, it almost feels like there's a half shift in there somewhere. Kinda like:

1st
Half shift
2nd
3rd

I assume this has something to do with the torque converter unlocking, but why would my tranny be setup like this? Does the diesel 700r4 stay locked in 1st until a certain RPM is reached? FWIW, the convertor locks and unlocks as it should in all the other gears......

I'm scratching my head over this one. And don't tell me that my tranny is on the way out. It's been shifting like this for over 70K miles.
 
the lockup should only engage in 4th. the 1-2 accumulator spring may be a little on the soft side, or the seal on the piston is worn.

to fix this you need to drop the pan and pull the filter, valve body. the 1-2 accumulator is under the valve body and the 3-4 is above the valve body (with tranny still in the vehicle) it may be agood time to replace the seals and springs in the accumulators. its more work than it is parts but the tranny will thank you for it /forums/images/graemlins/waytogo.gif
 
Ah man.........that kind of tranny work is out of my scope(I think). Like I said, the truck has run this way for over 7 years with no trouble. If it was gonna blow you'd figure that it would have done it by now.....

BTW, now that I think of it, my converter on my 84 305 locks up in D for sure(so does my diesel). Are you saying that it shouldn't do that? /forums/images/graemlins/confused.gif
 
/forums/images/graemlins/doah.gif i meant 3rd and 4th /forums/images/graemlins/doah.gif some trannys do it in second, depends on the electronics the trans has

if you wanna stop by my place id be happy to do it for ya /forums/images/graemlins/whistling.gif /forums/images/graemlins/whistling.gif
 
[ QUOTE ]
if you wanna stop by my place id be happy to do it for ya

[/ QUOTE ]

Well, it's been a while since I've gone on a road trip...... /forums/images/graemlins/rotfl.gif /forums/images/graemlins/waytogo.gif
 
I have been meaning to ask this question for almost 10K miles. Mine does the exact same thing. I had a relative tell me it felt normal but he had never heard of the diesel shifting different before, and he is a trans mechanic. Then again this was the first diesel K5 he had ever seen.
 
Sounds to me, from your description, that it's just the TC locking when it goes into second. In other words:

1st
2nd
TC locks
3rd
4th

When the TC locks, it will feel like it shifts again. So it's probably just shifting from 1st into second, and then the TC locks.

Whether it's supposed to lock in 2nd gear or not is a good question. There are about 47 different ways the 700R4 was wired internally for different applications from the factory. Some lock in 2nd, 3rd,and 4th, others only lock in 3rd and 4th, and yet some only lock in 4th.

Casey
 
Thanks for the info, Casey.

Why were there so many different setups in these trannies? Was it because of economy or power concerns? I would think that a tranny designed to be behind a 6.2 would benefit from having more lockups because, at the time, it was used primarily for towing or rugged service....
 
I haven't the foggiest idea! /forums/images/graemlins/confused.gif /forums/images/graemlins/dunno.gif

It probably has to do with GM trying to figure out which combo gave the best performance, and which gave the best fuel economy. I do know that I don't like to drive vehicles with a TC that locks up in the upper three gears. You lose the torque mulitplying effect of the TC when it's locked up. It's also harder to accelerate a heavy truck like we have with a locked up TC. That's why I prefer to have only 4th gear lock up. That's the way my TC is wired, to only lock in 4th. One of these days I'm going to add a switch so that I can manually lock it up in 2nd and 3rd when I'm towing. For regular driving around town, the unlocked TC is much smoother.

If you have access, read these excellent articles about a performance 700R4 for a diesel:

http://www.thedieselpage.com/reviews/lee700r4-1.htm

http://www.thedieselpage.com/reviews/lee700r4-2.htm

Casey
 
[ QUOTE ]
I would think that a tranny designed to be behind a 6.2 would benefit from having more lockups because, at the time, it was used primarily for towing or rugged service....

[/ QUOTE ]

Actually, I would disagree with you here. The 6.2L was designed for one reason and one reason only: economy. It's main duty was in a 1/2 ton pickup with tall gears and the 700R4 tranny. Here, the lockup TC was utilized to keep the rpms down and the engine/drivetrain locked together to maximize fuel economy. A shortbed 2WD 1/2 pickup with 2.73 gears and a 700R4 would probably net somewhere around 25 mpg or more.

Yes, the 6.2L could be had in 1 tons and motorhomes, but it was never designed for serious heavy duty use. Now, it can be modified to do better at those jobs, and I think it does a fine job, but we must remember when GM introduced the 6.2L and why. 1982. Right after the fuel crisis. Nobody else offered a diesel in a 1/2 ton truck.

Not until GM offered the Duramax did they really get serious about heavy duty diesels designed for towing and rugged use. You can only get a 6.6L Duramax in the HD 3/4 tons and up, unlike the 6.5L diesel, available in 1/2 tons until just a few years ago.

I think it's a shame, really, that you can't buy new light duty vehicles with diesels. The 6.2L did and continues to do an excellent job at powering a full-size vehicle for very little fuel.

Casey
 
Correct me if I'm wrong, but it isn't so much "wired" as it is "plumbed", right? If I'm not mistaken it isn't terribly different to have it changed. I may be way off base though, so that's why I ask.

The reason I ask is that when I put a 700 in my '72 I bought it from a guy who worked in a 4x4 shop and had it in his pickup. He used the truck mainly to tow his sleds from Edmonton to Vailmont. From what I understood when I bought the transmission it's a series of valves or check-balls that determine if it will lock up in a given gear. Changing those allows the trans to "decide" if the torque convertor is able to lock up. If it is plumbed to lock up in a given gear then only the locking and unlocking is controlled electrically.

My trans will only lockup in OD and it is manually switched. With the switch on it automatically locks up as soon as it shifts into OD. Flipping the switch will lock/unlock. The PO set it up this way so the convertor wouldn't lock/unlock while towing in OD (he had the gearing for it). For me it works great - I never touch it unless I'm cruising around 65 mph (low rpms) and see a long hill coming up that I want a few extra rpms on to prevent it from lugging.

I got the impression that it isn't too hard to change the valving for the lockup (I could be way off here though) - that might get you the shifting characteristics you like.

In theory with the right valving the trans would be able to lock up in every gear. I've always wondered what it might be like to be able to lock the convertor in first for compression braking on downhill descents. I've never done more than wonder though because it would be a serious PITA on the street, remembering to unlock it as you came off the freeway.

The PO actually had two 700R4s with the AA output shaft and two NP205s. He would just keep one on his bench at work to tear down and rebuild in his spare time and swap them back and forth at will.
 
Actually, from what I understand, it's both 'wired' and 'plumbed'! /forums/images/graemlins/smile.gif

When I installed the 700 into my '81, I had to determine how many pressure switches were present. Basically, I have one hot wire going to the tranny, and it's hooked to the TC clutch solenoid. The other wire coming from the solenoid goes to a pressure switch. When it's in fourth gear, pressure activates this switch, grounding the wire, completing the circuit, and engaging the TCC. There were also a couple of other switches, one for third gear, and one for second. I didn't use those.

Seems like there are several different valve bodies used on the 700, all with different amounts of plumbing and switches.

So, in a sense, we're both right! /forums/images/graemlins/thumb.gif

You can see how confusing it can get when you start talking about a 700 behind a diesel that will lock up in the three upper gears, and also realize that there is a TPS on the injection pump on the stock vehicles which also controls the lockup function. That's a lot of stuff going on!

So, like on my old '86 Suburban, the TPS would unlock the TC whenever the throttle went to idle, and then would lock it at mid-throttle, and then unlock it again at full-throttle. That's a lot of stuff to have to keep straight when adjusting/troubleshooting a 700 in a stock situation.

There was no TPS used on the gasser models, at least to my knowledge. They did utilize a low-vacuum switch, so that it would unlock the TC when the throttle was wide open.

So, when you start thinking about all of the different setups, you can see a lot of it had to do with wiring, and lot of it had to do with plumbing.

Casey
 
Thanks, that makes sense to me. I have a simple two wire setup - one hot wire to the trans and then a ground wire to a toggle switch in the cab. The lockup will only engage in OD, so I'm guessing that there is either only one pressure switch (for fourth) or the rest are unused.

It seems to me that if you used the right combination of pressure switches in the trans and electrical toggle switches to interupt the ground circuit you could have a 700R4 in which you could select whether it locks up in any one of the gears.

Now you have me thinking .. is there a provision for a 1st gear pressure switch? I'm imagining a bank of toggle switches which individually break the ground circuit from each pressure switch. That way you could still cruise around town with only one or two of the switches on and then when you wanted to make that long downhill descent you could flip on the first or second gear toggle switch, allowing the convertor to lock the drivetrain, take advantage of compression braking and possibly keep the trans a little cooler by eliminating the heat build up for torque convertor slippage.

Not something that I'm likely to do, even if it is possible, but it makes me go hmmmmm .... Then again, transmissions are not my forte and if I'm off base, please edumacate me.
 
You seem to have it just right! Yep, you can set it up to have switches for each individual gear, or just one master switch for all gears.

There is no provision for a lockup switch in first gear though.

Casey
 
[ QUOTE ]
You seem to have it just right! Yep, you can set it up to have switches for each individual gear, or just one master switch for all gears.

There is no provision for a lockup switch in first gear though.

Casey

[/ QUOTE ]

Too bad, that would have been the perfect solution.
Still second in lockup is still better than none.
I think I will try to do that on my burb.
I need to get it running first to see if it had lockup in 3rd or not.
 
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