Sounds like you found a dealership where someone is willing to take the time to explain things.
A "drain and fill" means you remove a drain plug or cover, let whatever fluid falls out fall out, put the plug or cover back on, and refill it. That is what we did for decades to automatic transmissions. In addition, while the bottom cover was off, we replaced the filter. In fact, that was the only way to get to the filter. Draining this way only removed about half of the transmission fluid. Unlike engine oil, the additives in transmission fluid last a long time, so the fluid doesn't have to be replaced so often. It's important to understand that some people run in for this fluid and filter change when a problem shows up with the transmission, but by then it's too late. There ARE some shifting problems that are caused by a plugged filter, but you have to ask what plugged the filter? That debris that is collecting in it came from the clutch plates, seals, bushings, and thrust washers, so that means mechanical wear and / or damage has already occurred. New fluid and filter isn't going to fix existing wear or damage. Usually the problem gets worse or there's a total failure, then the mechanic gets blamed unfairly.
With a drain and fill, the fluid in the torque converter, oil passages, clutch packs, and servos does not drain out so it doesn't get replaced. The typical transmission holds up to nine quarts of fluid but it only takes four or five quarts to complete a drain and fill.
A flush uses a machine to pump in new fluid which pumps out the old fluid. A strong detergent is added first, the car is driven a few miles to circulate it, then it has to be completely removed. A lot of new fluid goes in and right back out to be discarded.
Old transmission fluid starts to turn to varnish. That reduces its lubricating properties, and if it adheres to the bores valves ride in, it can cause some valves to stick. The most common symptoms are a delayed up-shift or a failure to down-shift to first gear when you come to a stop. The detergent will remove that varnish, but that can create other problems. Most of the valves have fluid pressure from various sources on both ends, and when one pressure overcomes the other, the valve moves. When it moves, it opens or closes various ports for other fluid to flow. Those ports can leak and fail to seal properly when the varnish is removed. You might have mushy shifts, harsh shifts, engine "runaway", meaning it feels like it went into neutral for a second when it was up-shifting, and things like that.
A lot of people run in when they find the transmission fluid looks and smells burned. By that time it's way too late for a flush. It's the slipping clutch plates that create the excessive heat and burn the fluid. Replacing the fluid doesn't fix the burned clutch plates. The slippage that caused that is going to continue. A transmission rebuild is in the near future.
Engine coolant is alcohol and will always be alcohol. We replace it because the water pump lubricant and corrosion inhibitor additives wear out in about two years, and acids normally build up in it over time. A drain and fill only replaces about half of what is in the system, but it replenishes the additives. It doesn't really matter if you drain and fill a gallon of coolant or flush and refill two gallons. Think of placing two ice cubes on the sidewalk. If one totally melts in ten minutes, two will totally melt in ten minutes. You'll just have twice as much water. In the cooling system, the additives wear out in about two years. Twice as much new additives will still wear out in two years.
Some cars have a rather miserable coolant that turns to mud after a few years. With those, a flush is a better idea. The advantage is the flush is done with water, not an expensive fluid that is going to be discarded. The disadvantage is it is very difficult to drain all the water out of the engine after flushing. That makes it hard to know exactly how much new antifreeze to add. There are ways to overcome that, but the last thing you want to do is buy that pre-mixed, half-water antifreeze.
Brake fluid is never flushed. There used to be pressurized "bleeder balls" that we used when a brake job involved rebuilding calipers and wheel cylinders, but today there are way too many variations in master cylinder designs, so it's impossible to have the right adapters to cover all the applications. Very few professionals even use a helper to bleed brake hydraulic systems. Most of us gravity-bleed when necessary. That is also how we replace brake fluid. First you must understand the only reason we SAY to replace brake fluid is because it absorbs moisture out of the air. That lowers its boiling point from over 400 degrees to 212 degrees. With extensive city driving, or coming down out of a mountain, the brake fluid can easily get over 212 degrees. The moisture in it will vaporize and turn into air bubbles. Air can be compressed leading to a low and mushy brake pedal. That is one form of brake fade.
The other reason to get that moisture out is it leads to corroded metal parts. The fluid will turn dark from being hot. That is normal and is not a cause for concern on its own. Very few of us change our own brake fluid. It gets ignored because it causes such little trouble. When we do, we just loosen the cover on the reservoir to prevent vacuum from keeping the fluid from flowing, then open the bleeder screws. To speed the process you can use a turkey baster to remove most of the fluid from the reservoir first, then fill it with clean new fluid and start the draining. As the old fluid flows from the bleeder screws, it's important to prevent the reservoir from running empty. Doing so will allow air to get in and make more work later.
There's two things to be aware of during this draining. First, never fill the reservoir to the top unless that's where the level was when you started. As the disc brake pads wear, they get thinner, and the pistons have to work their way out of the calipers to take up the space. As they do that, brake fluid fills in behind the pistons. When the brake fluid is low in the reservoir, there is either a leak that must be addressed, or the disc brake pads are worn and in need of inspection and / or replacement. To install new, thicker brake pads, the pistons have to be pushed back into the calipers, and that pushes the brake fluid back up into the reservoir. If someone filled the reservoir previously, the brake fluid is going to overflow, make a major mess, and it eats paint.
The most important issue with brake fluid is it must not have even one drop of petroleum product contamination. The slightest hint of engine oil, power steering fluid, transmission fluid, axle grease, or penetrating oil will destroy every rubber part in the hydraulic system that contacts the fluid. All of those parts must be replaced and the steel lines must be flushed with brake parts cleaner and dried. That is a very expensive repair. This is why mechanics do not use that turkey baster I mentioned. They can't be sure where it was used last, and it could have come in contact with some oil.
Power steering fluid doesn't typically get replaced. Years ago it lasted the life of the vehicle. Today, with rack and pinion steering gears in most cars, those develop leaks eventually and have to be replaced. The power steering fluid runs out during that service so it will be replaced then.
Monday, June 29th, 2015 AT 5:54 PM
