Automatic Transmissions 101

[Craig99SE2]

As I browse this forum and see discussion on the weaknesses of our 4T45E transmission and the "fixes" offered by some companies, I try to sway people away from these "quick fix" products because of how they throw-off, if you will, the delicate balance within a transmission. Usually, in the thread it takes too much to explain why, so allow me to do so here. Having rebuilt and modified a 700R4 GM RWD transmission, I have better understood an auto trans and how it works.

1. Torque Converter

The torque converter "couples" the engine flywheel to the input shaft of the transmission via a fluid coupling. The flywheel turns the housing which has fins (Pump) welded internally. These fins force the fluid to rotate which turns the Turbine which is connected to the transmission input shaft. The fluid is then directed to the center of the housing where it is gathered by the Stator and directed back into the transmission. If it were not, the currents would flow back into the pump thus loading down the engine. Some converters are "lockup" which means when the pump and turbine are moving at the same speed, a clutch plate "locks" them together thus increasing gas mileage by allowing a direct connection from engine to converter.

Stall speed is the RPM at which the converter "holds" I.e., if you accelerate hard from a stop, the TC will spin the Turbine up to stall speed. Most cars are factory-equipped with a low stall so they apply power slow and smooth. When modifications are made to the engine, you want a higher stall to apply the power to the transmission quicker and more effieciently. The general rule of thumb is to choose a stall speed 500-700RPM below where your engine makes peak torque. It is HIGHLY recommended to consult with a professional because driveability and vehicle weight must also be taken into account.

2. Planetary Gears

This is the "heart and soul" of the gearbox! These gearsets determine the ratio between engine and driveshaft. By applying power to different parts of the gearset, you can control this ratio. If power is applied to the inner gear, it is transferred to the outer which in turn spins slower. By applying to the outer, you force the inner to spin faster, etc. The transmission applies power to the diffrent parts via sun housings driven by clutch plates and bands.

This is where the transmission needs to be strengthened to handle more power. These work EXACTLY like a clutch in a manual transmission. When a sun housing needs to be engaged, the computer sends a signal to the valve bodt to open a solenoid. The solenoid applies fluid to a piston. The piston pushes these spinning clutch plates toghether. When the friction material on the faces start to grab, the housing slowly starts spinning. The wear of high performance is on the friction material. When performance of the engine is increased, so must the friction material to handle it. Ultimately, once the housing is spinning, it is coupled to the gearset which turns the drieveshaft.

3. Hydraulic System

This is what makes the clutches and bands "grab". A hydraulic pump is on the tranmission input shaft. As soon as the TC spins the trans, the pump starts pumping fluid. The fluid sits in a reservoir under pressure waiting for the valve body to direct it to the appropriate piston. When the PCM tells the transmission to shift, it opens a solenoid which sends fluid to the proper piston. In the same fluid path, there is also an Accumulator. This is a shock absorber in that it absorbs the extra hydraulic energy of the fluid that the piston cannot.

One of the BIGGEST misconceptions is that you can increase transmission line pressure and handle more HP. WRONG! By increasing the line pressure, you are only applying a fraction more to the piston because the accumulator has the same spring rate and therefore just absorbs quicker. In order to apply more pressure to the piston, you must increase the accumulator spring rate. Just like a shock absorber! A harder shock transfers more energy to the chassis. So, if a transmission is PROPERLY modified to increase line pressure by increasing fluid pressure AND accumulator springs, this causes the piston to move faster which in turn engages the clutch plates much quicker. This reduces the slippage and therefore wear on the friction material.

I hope I have shed some light on this mystery drivetrain component and helped a few people understand how they work. If anyone has anymore specific questions, feel free to post them here or PM me direct.

MODS ... Can we make this a Sticky?

[fast_ga]

Cool, reminds of an ASE test i just took! Or the GM training i have to go through at work

[Craig99SE2]

I've been doing quite a bit of research the past few weeks and sometimes I think people jump the gun on pointing to the transmission as a fault when something goes awry. Remember that we have a PCM-controlled transmission that gets input from the MAF (Mass Air Flow), MAP (Manifold Absolute Pressure), ECT (Engine Coolant Temperature), TPS (Throttle Position Sensor), among other sensors on the transmission itself. Before pointing a finger at the transmission, it might be a good idea to have the PCM scanned for trouble codes to rule out a sensor malfunction.

ONE OF THE BIGGEST CAUSES of transmission mis-diagnosis is after porting the MAF sensor. Some people have noticed "soft" shifts and the transmission "hunting" for gears. The reason is that by porting the MAF, you have nullified the calibration of the unit and it now provides more airflow for a signal to the PCM that originally meant less airflow. Since the MAF input controls the line pressure of the transmission fluid by sensing load, this will cause soft shifts. Other things the MAF controls is TCC clutch pressure and lockup.

Here is an article that gives a little more detail on what controls what: http://www.jeepnuts.com/12sGASE/what_controls_trans.pdf

[fast_ga]

I agree with that! Out of all the trans problems that come through the shop i work for, 85% of them are diagnosed as a electronic problem and fixed. But there are times where we have to put in a new trans or do a rebuild, just not that often.

[Craig99SE2]

If you search this board, you'll see a lot of contradicting information about running synthetic ATF in our cars. In order to dispell the myths and explain the contradictory info, I did a little research and this is what I found:

I called Red Line again. Yes, I realize they are "biased", but I put him on the spot and he was very diplomatic and backed everything up.

He said GM releases the Dexron III spec that must be met in the areas of viscosity, coefficient of friction and thermal breakdown. They do this to prevent things such as slippage, varnishing, and to keep the trans spinning without restriction.

Red Line D4 FULLY meets Dexron III requirements and even exceeds them by showing a 30% coefficient of friction improvement in cold and hot temps beyond conventional ATF. In the normal zone however, they are identical fluids as far as the Dexron III spec is concerned.

The "base" oil has absolutely nothing to do with the friction modifiers. The base oil only relates to thermal breakdown and it's effects on viscosity which in turn determine how the friction modifier bonds with the base oil.

So, from a purely fluid versus fluid standpoint, they are identical except that synthetic is more stable across the temperature band (just like Pat's Amsoil chart shows below).


Now, for people that had slippage. He said that this is common regardless of what fluid is put in after a complete system flush. If a trans has been run hard, the clutch material burns off (just like a brake pad) and the resulting particles remain in suspension in the fluid. As the clutch wears, these particles give a grit just like sandpaper that help the packs grab one another. When the system is flushed, the grit is removed. Now, the damaged packs will slip because the grit is gone! It has nothing to do with synthetic versus petro-based. He said it happens all the time on higher mileage cars with standard petro -> petro flushes.

Spaz's situation stumped him. He said he understood slipping post-flushing especially since the car has seen a supercharger and nitrous. But flushing synthetic out and petro back in should have yielded the same results unless a friction modifier was introduced (like B&M Trick Shift). This is just fluid with "built-in" grit to provide better plate adhesion. Considering SPAZ said "A secret non-synthetic blend", that is my guess!

His recommendation is this: Check the fluid and magnet. If the fluid is brown with no trace of pink/red or if there is metal on the magnet ... DO NOT FLUSH PERIOD! A pan drop change with either synthetic or petro will be the safe bet. Either that, or flush and add a friction modifier. For a car that has good fluid, a full flush with synthetic will enhance the transmission and provide better longevity and performance because the trans is less prone to slip and get hot when cold or hot (vs. conventional fluid).

I apologize for writing a book, but I think that makes sense according to all the conflicting information floating around. I am still waiting to hear back from a GM Powertrain Engineer, but like the Red Line guy said: They only test transmissions, we test oils!

I also confirmed this information with a GM Powertrain Release Engineer who actually was on the N-Body Team. He summed it up by saying as long as you use a Dexron III-approved oil, it doesn't matter what brand you put in.

After hearing all this positive response, I flushed and filled my own car with Red Line D4 Synthetic ATF. The car runs great! The shifts are more positive in the colder temps before the fluid reaches operating temps. It also seems to shift faster now as well. I also noticed trans fluid temps are 5-10 degrees cooler than before. So far, all of Red Line's claims have been backed-up! If you have been wanting to switch and are scared by the horror stories, use your own judgemnent on the condition of your trans or when in doubt, take it to a trans shop and get a fluid analysis done ... that is still a helluva lot cheaper than a rebuild!

Now about the running higher line pressure to allow more horse power... THIS IS A MUST what you are doing is basically increasing the slip point of the clutches (tourqe required to cause slipage)
YES changing the acumulator spring is a good idea as well but keep in mind this only effects how quickly the clutch pack sees maximum pressure (max pressure is related to line pressure available)
The sooner the acumulator piston reaches its full travel the sooner full pressure is aplied (more thump when it does shift)

NOTE: Too much of either can be a bad thing as well!

[mjhurley1]

Great post Craig. Very informative.

[Draug]

Greetings,

I just purchased a 99 Grand Am GT with 140,000kms (~90K miles). Part of my standard practice when buying used vehicles includes going through and changing each fluid. I generally use the best fluid (oil, coolant, whatever) that you can buy.

Regarding the tranny fluid, my question is whether it would actually be worth the added expense to use synthetic fluid when there is already this many miles on the car.

Secondly, I am usure whether to flush the tranny, or just change the fluid and filter. Even with cooling systems, I have always been hesitant to flush, just because I wonder how detrimental it can be to dislodge bits of junk that are currently stuck somewhere that they are not doing any damage. Again, with the high amount of miles on this car, I am leary to flush the tranny for fear of pumping around large amounts of debris. I will check the condition of the fluid, as suggested earlier in this post. If (as I expect) it is good and clean, would you recommend I go ahead with the flush, or just the change?

Thanks in advance,
Draug.

[Craig99SE2]

I would definitely NOT suggest a flush at 90K miles. I would take it to Jiffy Lube and get an analysis done of the fluid and if the metal particle count is low, go ahead and switch to synthetic with a pan/filter drop. If the particle count is high, use standard fluid and just do a pan drop.

[99alero]

nice job! I learned Alot!!!

[Hot_Stud18]

I heard alot about driving hard in this thread, so my question is how hard is to hard from a stoplight? 4,5,6 RPM's? Also how come my transmission shifts hard for like 5-10min. after the thermostat hits operating temp?

[ryGT1]

I was about to go and get my tranny flushed (force feed) today! You made me think twice. What do you think Craig? I have a '99 with almost 60k on it, I just assumed it would be normal to get that done. The tranny does whine, and it feels like it shifts a little slower than it used to (but I bet thats from my stupid self messing with the MAF).

Think I should get it done?

Very informative I learned a lot. I was thinking of doing a fluid and filter change, but now I am going to get my fluid analzyed first. How much should I be looking to pay for something like this? It shouldn't be too much right?

Thanks a lot,
-G-

Originally posted by Hot_Stud18
I heard alot about driving hard in this thread, so my question is how hard is to hard from a stoplight? 4,5,6 RPM's? Also how come my transmission shifts hard for like 5-10min. after the thermostat hits operating temp?

The real answer to that is finding out what power and torque your transmission is rated for. At least on the GM 700R4/4L60E it's not too far above 300 hp. When I did heads and cam on my car I blew the tranny after 3 runs at the track on factory street tires.

In general your tranny should last for a while if your power levels are at stock. Stoplight-to-stoplight pounding won't be too bad. It's the WOT shifts that you get at the strip that will really do damage.

For moderate bolt-ons, a shift kit should help. For more serious stuff, start hunting for a GOOD tranny shop that knows what they're doing.

[AznGA]

How bad is it for auto tranny when you hit the gas, let it go, and hit it again to make it sound like a manual? I have done that a few times with new exhaust.

[BTarb24]

Not sure, but thats what i had to do with mine to make it actually shift when it was supposed to. The problem actually ended up being a clogged catalytic converter, its in the process of being repaired under a standard 80,000 mile pontiac warranty now. guess ill find out if i did any tranny damage soon eh?

[AznGA]

Well if you hit the right spot and have the auto tranny shift when you let off gas, but if you don't the tranny will kinda jerk a little bit you know.

[BTarb24]

i was always at 3-4k RPMs so i didnt have to worry about that much . awlays went smoothly.

[AznGA]

Well when I accerate slow, it shifts fine, but when i floor it, tranny takes longer to shift. I guess that's the way auto tranny is.

[BTarb24]

i think thats just you confusing the heck out of the computer. probably sitting there wondering why you floored it then suddenly let go. then it finally realizes, oh..... he thinks he has a stick, ok ill play along