How to: Automatic transmission service

Jan. 1, 2020
Routine fluid maintenance on the transmission not only helps protect the health of the transmission, but it also allows periodic inspections that might uncover developing problems before they become serious. It also might restore the smooth shifting

Just like the engine, the tranny needs routine maintenance to ensure a long and happy life.

underhood transmissions transmission fluid transmission repair repair shop training technician training A/C training automotive aftermarket Your customers know the importance of having their engine's oil changed on a routine basis. Fathers have been pounding it into teen drivers' heads for decades. But few customers understand the importance of maintaining the other fluids on their cars.
Routine fluid maintenance on the transmission not only helps protect the health of the transmission, but it also allows periodic inspections that might uncover developing problems before they become serious. It also might restore the smooth shifting performance your customers knew when they first bought their car and have lost over time.
The main objection from consumers is the cost of a professional fluid service. While not difficult or even time consuming, the material cost involved is significantly more than a routine engine oil change. But what is the cost of a major transmission repair in comparison?

At the Counter

I've always been leery of customers who walk in requesting services that aren't often requested. If they're asking for a tune-up, I know there is likely a drivability issue they think will be fixed with new spark plugs. If a customer walks in requesting a transmission service, I have to wonder if he's experiencing any transmission related problems.
So, the first step is to test drive the car, preferably with the customer, to see if the transmission is operating normally or if there is a fault to diagnose. I also scan for codes that may or may not have turned on the Malfunction Indicator Lamp (MIL). The MIL is a warning for any fault that will impact emissions, and some transmission faults won't. The only message you might see in the Engine Control Module (ECM) is a code P0700, which just lets you know the Transmission Control Module (TCM) has found a fault that requires you to access the TCM directly. If there is a performance issue, that is handled as a separate service.

If all checks out, and the customer is indeed asking just for the sake of maintenance, I move on to the next step.

Visual Inspection

The visual inspection starts under the hood with a quick check of the transmission fluid level and condition. Be sure to read up on the proper service procedure. Some models are checked with the engine running and the transmission in Park, while others are checked in Neutral. A few are read with the engine off, and many late model makes are not even fitted with a conventional dipstick, requiring special tooling and procedures to properly measure fluid level. Generally, all models are checked with the transmission warmed up to operating temperature with some requiring a specific temperature range for accurate fluid measurement.
Fluid levels that are too low or too high both can have an impact on transmission performance. In both cases, the fluid can aerate in the pan, sending tiny bubbles throughout the hydraulic circuits. In-service fluid color often is a brownish red instead of the bright red of most new fluids. Carefully look for any metallic particles in the fluid, and smell it to see if it has a burnt odor. Both can indicate internal problems that bear closer inspection and a notation on the repair order.

As the transmission fluid ages and breaks down, it can leave a varnish-like deposit on the internals of the tranny. These deposits can interfere with the action of valve body and clutch components, restrict fluid flow through internal filters and screens and even lead to transmission failure. The use of a cleaning agent, if approved by the manufacturer, often can help dissolve these deposits prior to changing the fluid and restore transmission performance.

With the car raised, inspect for signs of fluid loss. Common leakage areas are aging transmission pan gaskets and tail shaft seals on rear wheel drive vehicles. Fluid leaking from the engine/transmission mating area could indicate a leak in the front seal or the pump itself. Leaks also are common around any point an electrical connection or sensor passes through the casing. Note any leaks on the repair order and advise the customer that additional repairs might be needed.

If all is well to this point, there still is one last step before proceeding with the service. Check the service information system for any applicable technical service bulletins (TSBs) and any unique procedures or cautions that might affect your work. Check the maintenance schedule for the recommended service interval. Some models use "filled for life" transmissions that, according to the manufacturer anyway, do not require periodic maintenance.

Changing the Filter

If the vehicle has a serviceable filter, the transmission oil pan usually will need to be removed to access it. (Some models use a spin-on filter). Some transmission fluid exchange machines allow you to remove the fluid from the pan by sucking it out through the dipstick tube. But if you don't have that luxury, then you'll have to drain it yourself. You know this can get a little messy if you aren't careful. To minimize the mess, remove all the bolts except a few on one side of the pan. Just loosen those and allow the pan to tilt slightly during removal. Not all of the fluid will be out of the pan, but most will and, more importantly, most of the fluid will be in your drain pan and not on you.
With the pan removed, inspect for signs of clutch debris or metallic particles. If it looks like the bottom of a gold miner's pan after hitting the "mother load," stop where you are and speak with your customer. Some is normal and alone, is not a cause for alarm. Clean the pan thoroughly, including any magnets used to attract and hold normal deposits. Don't forget to put the magnet(s) back!

Next, remove the filter. Be sure to look for small bolts that might be holding the filter in place before going after it with your pry bar! Make sure that any seals that go with the filter came out with it. Then clean the gasket surfaces on both the casing and the pan.

Very few OEMs use cork gaskets on their transmission pans anymore, yet I still see some packed with some aftermarket filter kits. Other OEMs use reusable gaskets that don't require replacement while others use silicone sealant in place of the gasket the trans originally came with. Check that service information to be sure you reseal the pan correctly. When reinstalling the pan, snug all the bolts down by hand and then use a torque wrench to finish the installation. Follow any torque pattern outlined by the manufacturer to insure a good seal when you're done.

Use your machine to reinstall fluid you originally removed, or refill the pan with clean fluid. If you're just doing a drain and fill, you're done. But is that the best way to service your customer's transmission?

Still More Fluid

Only about a third of the total fluid fill is in the transmission oil pan. The rest is in the torque converter, cooler, lines and internal passages. The best way to service a transmission is to perform a fluid exchange, using a machine designed for the purpose. These machines allow you to connect in series with the normal fluid flow of the transmission and replace all the old fluid with new.
While the car is in the air, locate the transmission cooler lines and follow them toward the cooler. The cooler can be an independent heat exchanger, but more typically it is built in to the radiator. Look for the line connection between the cooler and the line. This is usually the easiest place to attach the fluid exchange machine's lines. Follow the machine manufacturer's instructions for connecting to the car, using any appropriate adaptors as required. Fill the machine with the fluid specified by the car manufacturer in your service information. I like to add a few extra quarts just to make sure I get all the old fluid out with enough left over to ensure a correct fill in the vehicle's tranny when I'm done.
Warm the transmission to operating temperature by letting the car run. Follow all your normal safety procedures when dealing with a running automobile! Personally, I like to keep the car lifted just off of the ground. With the wheels in the air, I'm pretty sure it isn't going anywhere.
Follow the instructions for beginning the exchange process and turn the machine on. Most machines have some sort of sight glass that will show you when the service is completed, or go into a bypass mode once all the new fluid has been depleted. When the exchange is done, shut down the car and wait a few minutes for any remaining pressure in the lines to bleed down before disconnecting the equipment. Put the car back together, verify the fluid level, and check for any leaks. Test drive the car before returning it to the customer.

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