The fluid in your car’s transmission should be replaced every 30,000 miles. Over time, the assembly’s friction creates an enormous amount of internal heat. That causes the fluid to oxidize and lose its ability to lubricate the moving parts. Eventually, the lack of lubrication will cause the operational temperature to rise further, leading to varnish, cracked seals, and a loss in pressure. If your transmission fails, you can expect a hefty repair bill.

Below, I’ll quickly take you through the steps involved with changing your car’s transmission fluid and filter (it’s recommended that you do both at the same time). You can pay a mechanic to do the job, but you’ll save time and money by doing it yourself.

Draining The Fluid

If your car has an automatic transmission, it will also have a service pan that contains both the fluid and the filter. On the pan’s underside, there will normally a plug. Remove it and drain the fluid into a catch container. If the pan does not have a plug, you can drain the fluid when you remove the entire pan.

Removing The Transmission Pan

After completely draining the fluid, you’ll need to remove the pan. Look for bolts around the pan’s perimeter that are securing it in position, and remove them. Before removing the last two bolts, place your hand on the pan’s underside to hold it. Then, remove the remaining bolts.

Cleaning The Pan

After you have removed the pan, place it on a flat surface. Clean out the remaining fluid and look for small residual metal particles. There will usually be some, but an excessive amount can eventually cause problems within your tranny. Clean the pan as thoroughly as possible. Then, place a new gasket (included inside transmission service kits available at auto supply stores) along the rim of the pan.

Replace The Transmission Filter

In most cases, the filter will be housed inside the pan. Occasionally, you’ll find it secured with bolts to the front of the tranny. Remove it and replace the corresponding seal. It’s critical that the seal is in good condition in order to prevent fluid leaks. Replace the filter and reinstall the pan. As with the filter seal, make sure the gasket along the rim of the pan will prevent leaks.

Replenish The Fluid

At this point, your work under your vehicle is complete. Lift the hood and replenish the fluid level in the appropriate reservoir. Then, turn your car on and allow the engine to idle for two or three minutes. Check the fluid level again and add more, if necessary.

That’s it. You’ve successfully serviced your vehicle’s transmission. It’s a good idea to check the fluid level after a few days. Also, check the ground directly underneath the assembly to make sure that a badly positioned seal or gasket isn’t causing leaks.

As a general rule of thumb, transmission fluid should be changed every 30,000 miles. Unlike modern cars, where the trend is to do away with service access ports altogether, these Mercedes transmissions allow us to drain not just the pan but also the torque converter. In other words, we can remove almost the entire fluid charge without resorting to such gimmicks as power flushes.

Whereas engine oil should be changed hot, transmission fluid is best changed when merely warm to protect the mechanic from a very real risk of scalding. The steps are as follows:

1. Drive the front of the car on to ramps or support on jack stands.
2. Make sure that the torque converter drain plug is accessible. Unless you have been very lucky, you will usually need to blip the engine on the starter to get the plug to come into view through the access hole. It can be “walked” the rest of the way with a large, flat-head screwdriver. It may also be necessary to remove the cross-member between the two front subframes (”dogbones”). The six retaining bolts have 17mm hex heads are are torqued to 45Nm.
3. Loosen the transmission pan drain plug with a 5mm hex socket, then allow the pan to drain into a suitable container. If possible, collect all the used fluid in one container and then measure it; this will help us dispense the right quantity when we refill.
4. When the pan has stopped draining, loosen the plug on the torque converter (same size as on the pan) and allow the torque converter to drain out. There is much more fluid here than in the pan.
5. When the two main flows have ceased, loosen the six bolts retaining the pan (13mm heads) and carefully remove the pan. Don’t spill the remaining fluid all over yourself! Resist the temptation to wipe the innards with a shop towel: we don’t want any pieces of lint in here.
6. Remove and replace the transmission filter, retained by phillips-head screws.
7. Thoroughly clean out the pan with a lint-free cloth and replace the rubber gasket, ensuring a proper fit all the way around. Reinstall the pan, torquing the bolts gradually and evenly up to 10Nm. (The manual says 8Nm; a little more is fine. But please don’t go overboard: these pans can be warped and they’re not cheap.)
8. Replace both drain plugs, using new copper crush seals. Factory spec. is 14Nm, or about 10lbs/ft. Tight with a 3/8 drive works fine. If you removed the cross-member, replace it, being careful not to cross-thread the bolts.
9. (Optional) Crack open the trans cooler lines at the radiator (17mm wrench) and drain the small amount of fluid from this area. Replace the lines if their condition is dubious; they are not expensive. Retighten all connections.
10. Using a fine-mesh filter, add four quarts of fresh Dexron-Mercon transmission fluid (ATF) through the dipstick tube. If you can afford it, use synthetic fluid: it helps keep temperatures down by reducing internal friction, and heat is the number one enemy of automatic transmissions.
11. Start the engine and slowly add another three quarts.
12. Back the car off the ramps, run it through all the gears, pausing for a couple of seconds between each gear, then check the fluid level. You do not want to be over the minimum mark at this stage. Transmission fluid expands greatly when hot; the reference marks on the dipstick are calibrated for hot fluid. So it is o.k. to be half-an-inch or so below that mark while the fluid is cold.
13. Drive the car for about 20 minutes (yes, at least that much) to warm the fluid thoroughly. Transmission fluid takes longer to warm up than the engine oil or coolant and requires the friction of actual driving to get up to temperature. Check the fluid again on level ground. Make sure you don’t leave any lint on the dipstick before replacing it in the tube. Any level between the marks is fine, but under no circumstances do we want to exceed the maximum level. If you need to add fluid, do so in very small increments and check again.

If you used synthetic fluid, you may notice that the shifts have become noticeably firmer. Compensate, if needed, by adjusting the modulator valve on the driver’s side of the transmission. Turn the little key counter-clockwise until the shift quality is to your liking. Bear in mind that too much slip is bad for the internal clutches.

Many of you add aftermarket coolers to your cars and this is always a good idea. Here are some hints.

1-Always mount the cooler in a place that has good air flow and if mounting in front of the radiator always make sure the fin’s in the cooler are parallel with the fin’s in the radiator to avoid restricting air flow to the radiator. If you must place the cooler in an area with little air flow a FAN is a must.

2-When Installing coolers make sure lines are clear of anything that can rub them and always use line that’s approved for transmission/hydraulic fluid. Fuel line will not work and will break down in transmission fluid.

3-By passing the radiator is the way I prefer a cooler be set up. Generally the transmission will run cooler this way and additionally this may take some of the load off the radiators job of cooling the engine.

4-When hooking up multiple coolers several things should be noted. The cooler itself is a restriction of flow and this restriction while not a problem for the cooling or lube to the trans can cause excessive pressure inside the converter. This can force the converter against the flex plate more so than normal and cause wear to the engine thrust bearing. A stopped up or severely restricted cooler or cooler lines can do the same. So when installing more than one cooler its advised to put them in parallel with each other, This means one line from the trans is split with a Y and one line run to each cooler then out of each and into a Y back to a single line running to the trans. This will assure adequate cooling while reducing the wear to the engine thrust bearing. Always use line that’s at least as large as the inlets in the cooler, Avoid extreme twist or turns in them particularly with rubber lines.

5-Type and brands of coolers; I really have no preference here as far as brand. From my experience and expensive brand name or a generic unit appear to do the same job. I have found the stacked plate (Flat tube) Types when measured inlet to outlet temp drop tend to be more efficient in the same dimensional sizes with a greater temperature drop. I did a test once with two coolers in parallel one round tube and one flat with a laser thermometer to determine this.

6-This is not specifically about a cooler but about cooling. Always be sure that the metal lines coming from the transmission under car do not contact the headers or exhaust. I once had a car where the transmission temps would sky rocket when the car was being run hard and couldn’t understand why. I after some looking around found that the cooler lines in and out were pinned against the headers. This would super heat the fluid when the headers got hot. Moved them away and the problem was solved, So always keep trans cooler lines factory or otherwise as far away from the exhaust as possible.

7-Trans temps- This question gets ask a lot. What’s TOO HOT? Well ideally you want you average temp to stay under 190 degrees however I would not get to concerned till the temps get at and or over 230 degrees. Heat can damage a trans but with the exception of extreme heat 250 or higher this damage is a long term killer by breaking down fluid and causing rubber seals to harden and break.

8-Never use your cooler lines to remove fluid from your transmission by starting the engine or in and attempt to flush by catching fluid in a pan while adding fluid to the dipstick. When this is done no lube is being sent to the planets and bushings and they can go dry and melt down very quickly destroying or significantly shortening the life of your transmission.

9-It is advisable to fully and properly flush a cooler especially when a converter failure is involved. Where a transmission internal failure is unlikely to put debris in the cooler since all fluid coming from transmission must travel through the trans filter. The fluid coming from the Pump or converter does not and any debris from these will certainly end up in the cooler. Its always a good idea to power flush a cooler or replace after a failure but is particularly important in the case of this type failure. Anything that’s in your cooler or lines will end up in your Valve Body, Bushings and Planets. A cooler cannot be properly cleaned simply by blowing air through it. A can of cooler flush which most transmission shops and or transmission parts suppliers should have followed by a blow out with air is the correct way to do it.

10-When installing rubber cooler lines to metal lines it’s a good idea to put a small flair at the end of the metal line. Not too big because you will tear the line on the inside during install. Then place a clamp just ahead of the flair. Also roughening up the outer surface of the metal line with very coarse sand paper in area where clamp will ride will help assure a good seal and reduce the possibility of blow off. Never over tighten clamps to the point of the rubber oozing through or being cut this will cause a failure too.

11-Temp sending units- The best place for these is in the pan since that will give the best average temperature which is what’s important, using the pressure port is ok however you will show temps 10 to 20 degrees higher average than you would in the pan. I do not suggest placing temp senders in either the inlet or outlet from the cooler as they will increase cooler restriction and increase the odds of a leak.

A car requires different fluids to run. Transmission fluid is one among the vital fluids that cars those that use both manual and automatic transmission require to run optimally. Transmission fluid is a slick lubricant that facilitates the smooth movement of parts when driving. The fluid also acts as a coolant and its viscous nature helps transmit engine power to the transmission. Moreover, it is vital for gear lubrication, torque conversion, brake friction and valve operation. Transmission fluid should be changed after a given mileage usually as stated by the manufacturer. However, if one buys purchases a car and they are not very certain of its history, checking the transmission fluid is the only way they can ensure that the it will run smoothly. Leaks may also necessitate the user to inspect transmission fluid levels. Although changing this is best left to professional mechanics, checking the levels is easy and can be done from one’s home garage.

Most cars are equipped with dip sticks specifically used to check the oil and transmission fluid levels. The transmission dipstick is shorter than that used to check the engine oil level. The person checking the levels should first wipe the dipstick and then deep it into the engine compartment where the transmission fluid is held. After the dip and the subsequent confirmation of the level, one should wipe the dip stick on a white cloth specifically to check the color of the fluid. Ideally, it has acquired a darkish tone either indicates issues with the transmission system, misuse of the vehicle or fluid that has run its course. Unlike engine oil, the difference between low and high levels is usually a pint only. Thus, it is vitally important that the person taking the transmission levels take clear readings from the dipstick, which should guide them on whether to add, change or retain the current levels.

In manual cars, transmission fluid contamination is a significant issue and car owners have to regularly check for this. The constant use of gears, bearings and synchronizers during power transmission results in the eventual wear and tear of these metallic parts. The released metal particles subsequently end up in the transmission fluid. Therefore this affects the ability of the fluid to effectively lubricate and if a replacement of the contaminated transmission fluid is not carried out, the contaminants eventually shortens the lifespan of the transmission system as it hinders the proper lubrication of these parts.

The automatic transmission fluid however is prone to degradation, which occurs when it breaks down due to exposure to the heat generated during transmission. At such, the fluid looses some of its vital lubricating ability and eventually accumulates contaminants. Therefore, if it is not drained and changed out, it then shortens the performances of the system. Constant inspection and change is advisable when the need arises. This not only minimizes the trips to the mechanic, but also reduces the amount of funds used resolving transmission system problems.