700R-4 Overdrive Downshifting

[4pilgrim4]

I own an 83 Jimmy "trailering special" with a 6.2 liter diesel turbo'd with a Banks sidewinder, axle ratio 3.73 and normally run 235-75 x 15 rubber. I also tow a 16ft Scamp fiberglass camper trailer weighing around 2500 lb.

Several years ago the automatic went out near Los Angeles. I dropped the pan beside the road cleaned out all the debris including torrington bearing parts I could, put in a new filter and refilled it with ATF. It limped, but I got into Pomona and had it rebuilt at a great little shop that specializes in rock climbers. All the hard parts including the case were toast, but the clutches still looked like new.

The fellow did some upgrades (not exactly sure what) and said I could tow in overdrive. All is fine and it has gone about 50K without any problem. However, whenever I give it more than about 1/4 to 1/2 throttle it downshifts especially from OD down to 3rd. This doesn't allow the diesel to develop much power as I would like.

My question is -- Was this done to make the 700R-4 withstand the power and torque of a diesel? If so, is there another solution -- possibly by using upgraded hard parts that would allow full or nearly full throttle operation?

The reason I ask, is I am in a financial position where I could dispose of my older GM trucks and upgrade to a new or newer vehicle. Or, more economically I could upgrade the ones I have eliminating the nuisance factors such as the overly sensitive downshifting 700R-4.

Ideally, all that is required is a minor adjustment, however I am not interested in going this route if the result would be decreased reliability. If it would take a full blown overhaul with gold plated parts, that would factor into my decision, but I think I would still be money ahead and then some.

Any good words of advice?

[tobiahr]

it sounds like they "upgraded" the governor and valvebody springs with the kit for a gasoline trans. the problem with most shift kits is that they are for gasoline engines that create more horsepower at a higher rpm and allow the engine to downshift under a lighter load. the diesel engines prefer to use the lower end torque and don't have much horsepower in stock form so they shift up at lower rpm and don't downshift as soon. i would suggest getting a shift kit specific to a diesel trans and see if it helps. you might also just need to adjust the tv cable.

[JURKFASE]

I have a 91 burban with the 6.2 banks sidewinder turbo diesel. Just got it about a month ago and have put about 600-700 miles on it. Running 1 1/4 axles with 4.56 gears and 37" tires, but mine downshifts from O/D to 3rd constantly when I am running 55 or higher on the highway. PO says the 700R4 was rebuilt about 6K miles ago. I have read on numerous forums now, that downshifting is common with the 6.2/700R4 setup. Hasn't bothered me too much, however I guess I am not towing like you are...

[swissarmychainsaw]

I had some issues with my 700R4 (gas 305); but specifically I'll agree with a comment above that says this:
You need to have the proper springs installed for your application (desired shift point).
It sounds like it's set up for the gas motor, which *should* downshift at 1/2 throttle.

http://www.transmissioncenter.net/subject.htm

Says:
#6. Do not attempt to adjust the shift timing with the throttle valve cable. The shift timing is controlled by the governor springs and weights. For higher shift points at wide-open throttle install lighter springs. For lower shift points at wide-open throttle install stronger springs. After the cable is adjusted the approximate shift points should be 15 to 18, 25 to 30 and 40 to 50 MPH at very light throttle depending on tire size and axle ratio. Light throttle shift points are measured in MPH and full throttle shift points are measured at what RPM. The full throttle 1-2 shift will usually be at a higher RPM than the 2-3 shift. The governor will work with only 1 spring or 2 springs. You can remove one spring to raise the full throttle shift points.

[4pilgrim4]

When the weather warms up I'll climb under the Jimmy and try just one spring on the governor.

Thanks for the tip.

[special-K]

I'm putting my '85 Jimmy 6.2/700r4 on the road this spring,so don't know how it acts yet. I am driving my first 700r4 truck now,'89 K1500 w/350,and sure feel it shifts funny. It wants to go down to 3rd on much of any grade when I'm trying to stay at 70 mph and this is pretty much empty. I don't know if it's just how these trannies are or what. I know I don't want my Jimmy to run that way,so I'm watching this thread to see how it goes. I have driven a '91 3/4t 4wd Suburban 6.2 4L80E with 35" tires and felt it shifted perfect. Just like a TH400 + o/d. This 700r4 wants to go into o/d at 40 mph with a light peddle and then it just lugs. But,on the hiway it wants to downshift. Going through the gears I never know what gear I'm in. It's just all wishy-washy mush to me. I hate it. The truck's just a loaner but I hate it. I may end up putting the 4L80E in the Jimmy,which I was going to try to just drive w/o playing with a lot...for once

[68Timber]

The R4's in mid-80's Camaro's wouldn't downshift until 3/4 throttle. At least not the TPI cars. That shouldn't be too hard to rectify.

[Jonboy]

Originally Posted by BRUISER

The R4's in mid-80's Camaro's wouldn't downshift until 3/4 throttle. At least not the TPI cars. That shouldn't be too hard to rectify.

The "performance" cars like the Corvette and F bodies use a differennt shift servo. Scroll down the page a little bit to the servos on this page: http://www.4l60-e.com/ $19.00 for the GM servo is a lot cheaper than my cost here at a dealership, FWIW.

[4pilgrim4]

Originally Posted by BRUISER

The R4's in mid-80's Camaro's wouldn't downshift until 3/4 throttle. At least not the TPI cars. That shouldn't be too hard to rectify.

You are probably right, Bruiser, but while there are some good publications on rebuilding a 700R4, I haven't found much that explains how to "tune" them.

What it looks like to this simple old guy is the spool valve that is operated by the TV cable needs to have a slightly different spacing between spool flanges for diesel applications. What, how much and where to get are questions I don't answers for.

Another approach would be to change the geometry of the TV cable hookup at the injector pump such that almost full TV cable pull-out would occur in the first 1/4 travel of the accelerator pedal. I'm sure I could figure this out. Again, I'll want to wait for better weather before I start experimenting.

Also, I can't believe it doesn't already have the Corvette servo or equivalent already installed as the shifts are much firmer and quicker than original.

In fact, I'm happy with it except for that one issue of down shifting prematurely. I can certainly live with it as-is. It is just a minor annoyance. If it helps prolong the life of the transmission, I'd gladly accept the annoyance.

[68Timber]

Originally Posted by Jonboy

The "performance" cars like the Corvette and F bodies use a differennt shift servo. Scroll down the page a little bit to the servos on this page: http://www.4l60-e.com/ $19.00 for the GM servo is a lot cheaper than my cost here at a dealership, FWIW.

Originally Posted by 4pilgrim4

You are probably right, Bruiser, but while there are some good publications on rebuilding a 700R4, I haven't found much that explains how to "tune" them.

What it looks like to this simple old guy is the spool valve that is operated by the TV cable needs to have a slightly different spacing between spool flanges for diesel applications. What, how much and where to get are questions I don't answers for.

Another approach would be to change the geometry of the TV cable hookup at the injector pump such that almost full TV cable pull-out would occur in the first 1/4 travel of the accelerator pedal. I'm sure I could figure this out. Again, I'll want to wait for better weather before I start experimenting.

Also, I can't believe it doesn't already have the Corvette servo or equivalent already installed as the shifts are much firmer and quicker than original.

In fact, I'm happy with it except for that one issue of down shifting prematurely. I can certainly live with it as-is. It is just a minor annoyance. If it helps prolong the life of the transmission, I'd gladly accept the annoyance.

Wouldn't it hold it in OD up to 3/4 throttle just using the servo's Jonboy linked?

[thirdstreettito]

No offense, but you gasoline guys dont have helpful input, considering a gasser/700R4 acts COMPLETELY different from a 6.2/700R4.

Our Suburban(my sig) downshift's from OD to 3rd at 1/3-1/2 throttle on a 20* or steeper hill when under 75mph. What the original poster describes is PERFECTLY normal for a 6.2/700R4. I'm not sure what the guy did to be able to say you can tow in OD, I've searched and searched and never seen a way you can do it.

My advice is, DO NOT TOW IN OD. By towing in OD the trans downshifts and then upshifts too often, it is called "searching" because the trans is searching for the right gear. Hence why they always say to not tow with a 700R4 in OD. You can tow in OD if you have a lock-up kit. What the lockup kit does is, it alloys you to drive(or tow) in OD without the trans "searching". It will only downshift from OD to 3rd with full throttle, it will literally save you thousands in new transmissions.

Lock-up kit - http://www.summitracing.com/parts/TRG-700-LU

[68Timber]

Bradley, I've got a good handle on what he's trying to accomplish and why. That lock-up kit accomplishes the same thing as making it shift like a mid to late 80's F-body or C4.

[4pilgrim4]

The problem is that I don't know if the 700-R4 presently installed is a "gasser" or a "oiler" type. When I had it in a shop in Pasadena the report was that it was so far gone that even the case was 'toast'.

So the shop sourced an entire replacement unit to rebuild. I believe it was from a early 90's application. From thirdstreettito's post, apparently the rebuilder set it up as a diesel application, if it wasn't already set up that way.

His shop specialized in 'rock climber' applications and the manager inquired into what axle ratio I had, as well as what size and weight trailer I was pulling and he was aware the engine was a diesel. Also the up-shift points are very low, which is great for a diesel.

Additionally he retained the torque convertor switch function I have installed on the dashboard that allows me to lock the torque convertor in manually from second gear up. It is just this situation in OD where the shift-down point is too sensitive to throttle position, occurring at approx 1/3 throttle.

The comment that it is questioned how I can tow anything in OD -- my trailer is a 16ft fiberglass Scamp weighing about 2500 lbs wet. With anything much larger, towing in OD would really be marginal.

Perhaps the best solution is to leave the transmission as it is and change the axles to 3.08 : 1 and forget about towing in OD. It might just make for much better transmission reliability over the long run and also allow the Jimmy to get better fuel economy solo. It also might gain in fuel economy towing in 3rd since that is direct drive with no planetary gears soaking up energy.

Whaduyathink?

[BigBlocksRule]

I've never seen any 700R4 build that recommended towing in OD, that's one of the weak points of this tranny.
I tow a good bit, my 4L80E doesn't care.

[4pilgrim4]

Originally Posted by BigBlocksRule

I've never seen any 700R4 build that recommended towing in OD, that's one of the weak points of this tranny.
I tow a good bit, my 4L80E doesn't care.

That is the reason I'm inquiring -- i.e., I've been warned about this from several other sources. Also the failure that got me to the shop in Pomona was due to a hard-part failure from towing in O.D.

However, the rebuilder specifically told me towing in O.D. was OK with the size trailer and axle ration (3.73) I had with the later model transmission he had rebuilt. Actually, it would be quite disagreeable to tow in 3rd with the engine RPM's required for 55 - 60 MPH travel. (know, because it wouldn't run in 4th from about 100 mi north of the Bakersfield exit down to Pomona.)

So I suspect he set it up so the engine would never produce anywhere full torque or power in O.D. I guess I would call the strategy successful since I have towed approx 15K miles that way. But I have often wondered when the next episode might occur. There really isn't much warning.

I think I'll keep my eye out for a 4L80 and transfer case. Might just save the day sometime.

Thanks for the responses. Much appreciated.

[thirdstreettito]

4pilgrim4

I found out how the guy upgraded your overdrive! The common Corvette servo upgrade. Or the Sonnax Super Hold Servo, super freakin strong.

http://www.transmissioncenter.net/pe...tive.htm#Parts