700R4/4L60 GM transmissions

I have an '89 Suburban with the 700R4 and 3:73 rear gears. Not 4 W/D. About half of my miles are towing an 8500 lb trailer. First tranny lasted less than 60,000. Shop that rebuilt it suggested a device(can't recall the name, but will check if you want) that keeps the TC out of lockup unless the trans is in OD. If manually run in 3rd, the TC will never lock. Along with that they suggested a shift kit and an oil pan available from Summit Racing. It is cast aluminum with fins on the bottom, a drain plug, and holds two extra quarts of oil and a large, external oil cooler. I change tranny oil every 30,000, not the filter, just drain and refill. Never dark or smelly.

I am happily motoring towards 200,000 on that tranny and have had NO trouble. Fuel mileage might suffer just a little when towing without the lockup TC, but I can live with it. About 9 MPG @ 65 MPH highway. Not towing highway mileage between 16-17.

Yes, the TC is now slipping instead of being locked up, but again, in my opinion, this probably generates less heat than the TC constantly shifting in and out. You are also carrying more engine rpm which increases pump speed and therefore flow through the cooler and to other internal trans parts.

It has worked and worked well for me. Your mileage may vary.

The 4L60 is not a stacked shift where gear changes are additive, one on top of the other, like the old 400s or newer 4L80 transmissions are; but rather, the 700 uses a "timed" shift.

What this means is that for every gear change, it requires one friction set to 'release' at the same time another is 'engaging'. There is always going to be a certain amount of "slipping" during a 4L60's shift. It is the nature of the beast. Put a heavy trailer on behind that truck, and this shift 'burn' is magnified.

What these shift kits do, is to monkey around with the engineering of the springs, weights, pressures, etc. to "shorten" this 'shift/burn time'. End result is always a "stiffer" shift.

Works better when towing; trouble is, when you unhook the trailer, that "stiff shift" itself is then magnified resulting in a somewhat 'harsh' shift.

Since automatic transmissions were never, ever designed to tow, we can't have our cake and eat it too; we have to find a somewhat happy medium and learn to live with it.

Allan

Why don't you just put a tranny heat gauge in your truck. Keep 'er under 270 degrees and leave the truck like it is. It sounds to me like your truck is operating just as it should.

Let the darned TC shift; that is what it was designed to do.

Allan

Where is your gauge hooked up?

It should be reading right at the output of the cooler line connector at the tranny. This oil is coming directly from the torque converter and also, it is in the torque converter where the heat is generated because of all the 'slippin'/sliddin' reduction going on in that converter.

The reason I ask is because if you have it hooked into the return cooler line, you are reading oil that has already been "cooled" out front and this would be kind of a false indicator.

At 250 degrees, like you say, things are starting to happen like the oil turning brown color and wanting to puke, but you are still okay.

Allan

Couldn't really figure out why you were gettin' brown oil at only 250 degrees. This usually starts showing up at around 280 degrees.

That pressure port is just one step away from the coolest part of the transmission, the sump.

Tempreatures over at the cooler line out will be showing higher than that. A 'cool' transmission running down the road will always show around 200-225 coming off that converter (depending on the conditions, of course).

A "stock" truck will always have tranny temps running at least at the 195 degree mark because this is the temp the truck's thermostat is set at and is as cold as that radiator's tranny cooler can get it down too. Follow me?

Allan

Doggone! Sounds like an ordeal goin' on down there in Kansas today. Ha!

No Sir, your tranny guy is wrong. The bottom line is pressure out. Transmissioin coolers work exactly backwards of radiators in that the flow is from bottom to top.

I'd sure leave in the radiator cooler; the more cooling, the better, I'd think.

Allan

I lived off those trannys for 20 years; rebuilding 700s was how I made my living.

You are doing everything right. Forget the lockup kit and put on that external cooler you speak of. Not the pretty flashy one from NAPA, but the big, black, thick, square guy from GM.

Also, I'd use steel brake line and compression fittings to pipe it in. Hoses and clamps just aren't meant for heavy service, especially in the wintertime. That thick oil will blow a hose off and you loose the transmission.

I've seen those converters get so hot that they have turned blue and you could smell that hot iron/oil 1/2 a mile away. That scrawny little radiator cooler just cannot get the heat away from 'em, even tho the dealer told you that it had the "tow package".

Allan

In heavy load conditions, cooling the transmission fluid first is also a good idea because it reduces the heat load on the engine cooling system.

I would certainly hook the auxiliary transmission cooler in ahead of the radiator.

Synthetic transmission fluid is an excellent idea for towing. I would also run synthetic lubes in the differentials if doing lots of heavy towing.

John

The 3:73s are the best "all around" ratio for that year of truck in a 4x4 version, but that is one heck of a job swapping the ratios; I've done it and that front differential is nasty if you haven't been schooled on 'em. Also, you would have to change the prom because otherwise, the computer thinks you still have the 3:42s. I'd just leave it alone.

GM finally got that tranny right in the '91 and '92s before going electronic. You've got a tough tranny.

I like to run the cooler oil thru the radiator first, then the auxirally, then back to the dump. The cooler, the better when it gets back home.

Allan

Yes Sir, you're right; on a '92, there is no shifting on command from the ECM and the shifting is controlled via that �other� throttle cable that controls the TV pressure against governor pressure and the various shift valves� spring pressures.

However, all that VSS is, is a little electrical A/C �generator� that picks up the tooth count on the output shaft. Changing this item will do absolutely nothing for the correctness of your speedometer or the torque converter�s locking/unlocking strategy, as all VSS units would throw the same identical count for any given reluctor ring. You have to change the PROM inside the ECM to get that ECM to �decipher� and �understand� that thrown count correctly for any changed differential ratio.

Yes, setting up the backlash and the bearing preload is just plain �nasty� in that type of front differential. I�d personally trade trucks before swapping out the ratios.

On that particular transmission you have in your truck, it�s only enemy is heat. At 305 degrees, the bonding on the clutch discs begins to melt and she comes down. Keep it cool and let it shift as it was designed to do (to provide you with the proper acceleration) and it will last a long, long time.

Allan

The computer senses engine load by accelerator position and manifold vacuum, sensed by the MAP sensor.

So you can have a perfectly good transmission, and have a heavy load, and maybe a engine with some miles on it, and when the computer decides the engine is lugging too hard, the computer will unlock the torque convertor. (Numerically LOW axle ratios will make the engine lug more, and the computer will unlock the convertor sooner.)

If you have access to a scan tool, you can see the computer's opinion on convertor lockup at any given time.

If it unlocking is intermittant, and not load-related, check and adjust, or replace the brake switch.

The aftermarket kits simply supply a steady current source to the lockup solenoid, regardless of the engine computer's opinion of whether the convertor should be locked or not.

• B&M

When you say Lock up, do you mean the lock up torque converter? Or are you trying to drive in overdrive and the transmission is shifting in and out of overdrive?

Never tow with a 700R4, 4L60, or 4L80 when in overdrive. Always tow in 3rd gear.

If you're towing in 3rd and the torque converter clutch is unlocking, then that could indicate a problem with the transmission or torque converter. As I understand it, the torque converter clutch locks up at a pre-determined vehicle speed regardless of load. If that's true, then I don't know why it would lock and unlock while driving at a steady speed.

Like I said, i'm not an expert, but I hope this helps. Maybe some of the others know more than I do.

OK-AL

Good luck,

OK-AL

• B&M

Shift kits are just like that electronic ignition kit you bought. Stay away from 'em 'cause you are not going to like the result.

The only positive shift kit attribute I have ever seen was the little positive-connection fix on the TV valve to keep it from sticking. That idea at least made some sense. I used to buy those kits, use that one feature and then throw the rest of the kit away.

Other than that, they are nothing more than snake oil marketed at the bubble-chewing hot-rod crowd.

Just my view,

Allan