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Long gone are the days of transmission dipsticks in the engine bay. On most newer vehicles you can’t simply open the hood and check the level or condition of the fluid. Why would the manufacturer remove the driver’s ability to access and check the fluid level? We have asked that question of several engineers, and the answer is the manufacturer wants qualified technicians working on these vehicles, i.e., dealership technicians.

Why is the fluid being guarded so closely now? In the old days, if you got aggressive filling the transmission and put a little extra fluid in, it would likely puke out of the vent and not much more. So, if you happened to add ½ qt. too much, there was not much concern. Now we have units like 6T40, 6T70, and other transmissions that will overheat if overfilled by that same 1/2 qt. It is because of the damage that comes from under or overfilling the transmission and using the wrong fluid type, that the manufacturers have restricted access. Over the last 20 years, fluid type and level have become more and more important. In later articles, we will discuss clutch learn and adaptive strategy and you will see the importance of correct fluid level as it relates to drivability. With that being said, it is very important to set the fluid level correctly. For this article, we will focus on the different styles of fill procedures and how to access them. With many different transmission types available there are almost as many ways to correctly fill the transmission.

For the reasons above, many manufacturers have ditched the stick in place of a level check plug, or a short stick that is accessed from under the vehicle to be used by professional automotive technicians.

I believe it comes down to money like everything else. This process keeps the customer coming back to the service and parts department to spend money. The perfect example is DaimlerChrysler’s service tool (see photo below) to check the fluid level. It could be purchased from the dealer parts counter for about $50. Now, if they wanted only professionals working on their transmission, they would have restricted the sale of this tool to professionals only, however, they made it available to anyone. Hmmm???

I’ll try to explain as many fill procedures as possible, but this article would go on forever, if I covered everything in exact detail. You should consult the service manual on your specific application.

Before we get into the different fill processes please remember:

  1. When reading a dipstick, the lowest reading you see is the correct reading. If you see one side of the stick higher than the other the lowest one is the actual level.
  1. When filling a transmission through the dipstick tube you may get a false reading on the dipstick from air pockets or residual fluid on the sides of the tube. You will want to allow time for the fluid to settle in the pan before checking the fluid level for the final time.
  1. If your readings are erratic or it is difficult to read the fluid on the stick, you may have build-up on the stick. You can remove the residue with brake clean and wipe it with a clean cloth.

Under the hood:

  1. Traditionally you check the transmission dipstick (usually Red or Yellow in color) located in the engine bay with the transmission temperature, between 150-200 degrees, while the transmission is in park or neutral, and the engine running at idle!
  2. Your transmission dipstick may have the words “must check in neutral” etched into the steel near the safe mark indicator. Why does it need to be neutral you ask? Well, on the transmission type in question fluid doesn’t circulate to the cooler circuit in the park position. This means the fluid level will be higher in park than in neutral and give you a false high reading.
    Tip. If you are filling this transmission type you want to make sure the engine is running with the transmission in neutral or the fluid will overfill the pan and leak from the transmission vent. This makes a heck of a mess on your shop floor.
  3. For most of the Honda transmissions and some CVT-type transmissions the dipstick is under the hood, but the fluid must be checked with the engine off.
  4. For applications like Daimler Chrysler that use a service tool (dipstick) as mentioned above there will be an accompanying chart in the service manual indicating what the level should be on the stick at a certain temperature (see the example below). You would check this like any other dipstick; pull the stick, wipe it clean, reinsert in the tube, wait for a second or two, and remove it. Make sure the level matches the criteria on the chart and add or subtract fluid as needed.

Under the vehicle:

  1. Transmissions with a standpipe located at the bottom of the transmission pan such as the 5R55 (W/S/N) allow fluid to flow out of the plug if the fluid level is above the height of the pipe (see figure below). This would require the engine to run at idle, and the shifter in park or neutral with the fluid at a specified temperature. Because there is no “range” It is important to be very accurate with the temperature it is checked at. Fluid expands and contracts with temperature so you cannot remove the plug once the temperature falls out of that specified temperature range. The specified Temperature will vary depending on what type of transmission we are checking, it may be cold, hot, or any temperature in between. When we talk about temperature, we are referring to the fluid temperature in the transmission pan. The best way is to use a scan tool to monitor the temperature sensor electrically. A temperature gun pointed at the transmission pan can be used, but just like the technician using an impact gun to tighten wheel lug nuts, this is not very precise and could get you in trouble if not done correctly.
  2. Transmissions with check plugs use the same flow/level indicators as above but the fluid will flow from the side of the case instead of the bottom. The plug may look like what you would find on a 4WD transfer case or rear differential.
    Note. You will want to check the service manual to verify set-up conditions, i.e., engine running or not.
  3. The Ford 6L80 transmission and others are a bit of a hybrid of these processes, it does not have a traditional dipstick for checking the transmission fluid level. Instead, it has a small plastic stick that must be accessed under a fill plug located on the side of the case just above the transmission pan.
    Here are the steps to check the transmission fluid level on the Ford F-150:
    1. Park the truck on a level surface and engage the parking brake.
    2. Raise the vehicle to access the fill plug.
    3. Break the plug free but do not remove it.
    4. With the transmission in "Park" let the engine idle for a few minutes to warm up the transmission fluid.
      Tip. If additional fluid temperature is needed you may place the shifter in “2” with your foot planted firmly on the brake and the area clear of anyone or thing that could contact the vehicle, apply throttle until the wheels just start to turn. Hold steady throttle for about 15 seconds and release the throttle and wait 30 seconds and repeat the process until the desired temperature is reached.
    5. On the underside of the vehicle, look for the fill plug on the side of the transmission just above the pan. It will be a small, round plug with a hex head.
    6. Using a socket or wrench, remove the fill plug.
    7. Remove the short plastic dipstick under the steel plug and check the transmission fluid level.
    8. If you need to add more fluid, this will require additional tools to pump fluid up and into the open hole that the dipstick came from.
    9. Once the fluid level is correct, reinstall the dipstick and the fill plug, then tighten it securely with a socket or wrench.
    10. Lower the vehicle and turn off the engine.

Additional fill locations:

  1. If you are working on a transmission that has an internal differential such as a 42RLE, or Subaru 4EAT, 5EAT, and others you will need to find and access the dedicated fill plug. In these types of units, the differential fluid is isolated from the main transmission fluid. If left empty severe damage will occur in a short amount of driving.

Note. If the wrong drain plug is removed you may unknowingly drain the differential fluid. When this happens the transmission usually makes it down the road until the differential gears seize and make a spectacle exit from the transmission case. Quick lube facilities sent us this type of work a lot.

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