Should You Change the Filter in Your Automatic Transmission?
If you’re considering having your automatic transmission serviced, congratulations! You’re already ahead of the curve as many owners of a vehicle with an automatic transmission don’t even realise their gearbox needs servicing! This isn’t helped by the likes of BMW, with their claims of “lifetime oil! do not change”. But if you are looking to service your automatic transmission, you may find yourself hitting another stumbling block.
Should you change your automatic transmission filter?
This may seem counter intuitive. Why wouldn’t you change your filter? It seems obvious, right? You’re changing the oil in your transmission, you should change the filter that has been soaking in your old oil for the past thirty thousand miles!
Well, not exactly. But before we get into that, let’s briefly look at why an automatic transmission needs a filter at all.
Why Service an Automatic Transmission?
It's not particularly easy, so let's understand why you should do it at all.
Automatic transmissions (for the most part) rely heavily on hydraulics. To put in simplistic terms, this means pressurising a fluid and using it to move things. In the case of an automatic transmission, your oil is that fluid. It is pressurised (often by pump directly driven by the engine) and directed around the transmission as needed to engage or disengage components. Automatic transmission fluid is not merely a lubricant, though it does serve that purpose as well. This is why an automatic transmission with no oil at all will suffer little to no damage, whereas an automatic that is short on oil will burn itself out. If there’s no oil, nothing can happen inside the transmission, whereas a box short on oil may have enough pressure to partially apply components, damaging them.
The filter is necessary because the way in which the gearbox moves the oil around involves a series of delicate valves and springs and balls, and small pieces of contaminants—such as debris from worn clutches—can easily cause these delicate components to stick, causing a lot of damage. Debris like this will get into your oil, it’s an unavoidable fact of wear and tear, but the filter is there to stop that debris getting somewhere it could do harm.
So again, why would you not change it?
How to Service an Automatic Transmission Filter
Why Might you NOT Change an Automatic Transmission Filter
Despite the importance of the filter, there are still reasons to leave it alone.
There are a few reasons why you might opt not to have the filter changed when getting your gearbox serviced, and they have varying levels of legitimacy in terms of being a good excuse or not. Let’s go through them.
Your mechanic gets your car in the air, drops the oil, and discovers it’s as fresh looking as the day it was put in! Of course, any professional would recommend having the oil changed anyway, but if it is clean, clear, and doesn’t smell burned, you’d be forgiven for just topping it back up and walking away. If the oil is clean, there’s no reason to believe the filter won’t be.
Generally speaking, if you’re getting an oil and filter service for your transmission, most of the expense will be taken up by the labour of your mechanic and the cost of oil. That might not be the case if you’re taking this task on yourself, however. The material cost of a filter might still be relatively small, but changing it does add a significant amount of labour over changing only the oil.
There are also some transmissions, such as the ZF6HP series of gearboxes, where the filter is built into a larger component of the transmission. In these cases, it is not possible to change the filter without changing the whole component, which will often increase the cost of the filter dramatically. In the case of the aforementioned 6HP transmissions, the filter is built into the sump, and replacing the whole thing can easily end up costing ten times more than a regular transmission filter.
As mentioned above, not changing the filter makes the job that little bit easier on any transmission. Even transmissions where the filter is a simple screw in/out affair—such as the DSG DQ250—not changing the filter is less work that changing it. Where it starts to make a real difference, however, is in transmissions where the filter can only be changed by stripping down the gearbox itself.
There are many examples of this, such as the AG4 found in Volkswagen Transporters, or the 01J in Audi A6s. In these cases, it is necessary to remove and strip down the transmission in order to remove the filter. The amount of stripping down required varies from transmission to transmission. Some would only require you to remove a part of the casing, whereas others need the entire box stripping down. Either way, it’s a considerable amount of labour, and quite an expense if you’re paying a professional to do it. In this situation, many people will opt to leave the filter alone.
There is no Filter!
It’s not particularly common (for now) but some transmissions don’t actually have a filter to change, such as the DSG DQ200. Transmissions like this are designed in such a way that a filter is not necessary.
So What's the Risk
How bad could not changing a filter really be?
So, we’ve looked at why you might decide not to change your filter, now let’s look at how risky it is do so.
We know that your automatic transmission needs clean oil to control components in the transmission, and we know that debris can contaminate your fluid even in a perfectly healthy transmission through wear and tear. We also know that a partial lack of oil can do more damage to your transmission than a total lack.
If the oil dropped from your transmission is clean and clear, you’re probably fine leaving your filter alone. If the oil is clear but dark, it really would be better if you can have the filter changed, but it’s probably not the end of the world if you don’t. If your oil is not clear (as in you can see through it, whatever colour it is), that lack of opacity is caused by debris in the oil, and changing the filter is definitely recommended. Where it starts to get really serious is if the oil is black and opaque. In this case, changing the filter is essential. Unfortunately, so is changing many other components as transmission fluid in this state means the transmission has already gone bad.
The problem is, short of some catastrophic internal failure, filters don’t clog up all at once. This means that there are no outward symptoms that you’ll be able to detect before the transmission goes down, because once the filter is sufficiently blocked to cause issues, there are already bigger issues in the box itself. Also, the effect of a clogging filter is exponential. As the filter becomes blocked, the flow of oil to clutches is restricted, causing them to wear quicker, contributing to the debris clogging the filter and… you get the idea.
And that brings us to the end. Hopefully you now find yourself well enough informed to decide whether a new filter should be part of your next automatic transmission service.
Do I Need to Change my Automatic Transmission Filter?
© 2017 John Bullock