We don’t think much about servicing an automatic transmission. Fill it until the dipstick shows full, and off we go. Even if the dipstick read low, it didn’t seem to matter, at least back in the old days. You could even find technical information recommending running the transmission low on fluid. If you’ve ever installed a TransGo® Shift Kit® for a THM 400, you’ll recall the instructions using the pan rail as the fluid fill line. The idea was to loosen the pan bolts, allowing fluid to flow over the pan rail. Once the fluid came to a trickle, you’d tighten the pan bolts and recheck the fluid. The level on the dipstick was now the new fluid full line. You’d mark the dipstick with the new level. Now, drop the pan, clean the pan rail, install a new gasket, and you’re all set.
This procedure was to fix a problem with fluid blowing out the vent. We’re going back over 50 years, but you get an idea of how fluid issues gave technicians fits even back then.
This singular example of using the pan rail as the full line was just that, a singular example for a specific problem — on only one transmission type, the THM 400. Some technicians saw this as the new norm and began using it on practically everything they worked on. Before you knew it, many problems and failures crept up, like falling out of gear at a stop or driving hard around a corner. I used to get these on the ATRA Hotline, and I might react with, “It sounds like it’s low on fluid.” They often responded, “No, I checked it twice.” Only after a deeper dive would I learn that they used the pan rail as the full line.
Then there were problems like delayed engagement when cold or no movement after being parked on a hill overnight. You get the idea. And we’re not even getting into issues caused by swapping 4WD and 2WD pans, filters, and dipsticks. It turns out that filling an automatic transmission is bigger than we thought. So let’s first talk about the word “full.”
Every transmission has a fill level with the suction port of the filter about two to three inches below the surface of the fluid line. Figure one shows an 8L90 pan. Notice that the top of the standpipe (the full line) is above the pan rail. The filter intake is about two inches below the pan rail. This puts us around that two to three-inch mark. Of course, the fluid level will vary based on the location of the filter intake, along with other factors like fluid temperature, but this gives you an idea of how it works. If your transmission has a dipstick know that sometimes fluid will splash on the stick and read full when it’s not. If you have any of the low-fluid conditions mentioned above, treat it like it’s low. Add a pint of fluid and retest. A couple of tests you can make for a stick misread is to put the transmission into gear (To stop the gear train) or turn the engine off and check the stick immediately. In any event, don’t presume it’s full just because the stick says so. Many manufacturers have eliminated the dipstick, so it’s become less of an issue. We’ll cover that later.
So now you have the transmission full. But what’s it full of? It used to be that you could get away with using the wrong fluid. Or at least, you might not notice any significant shifting problems. That all changed about 30 years ago. Problems like harsh shifts, flare shifts, and chatters became more common as the years passed. The ATRA Hotline had a lot of shifting problems when the Ford TorqShift came out, and shops filled them with Mercon fluid instead of the recommended SP1 fluid. Honda transmissions are another. Good luck getting one to work right with any ol’ fluid you have lying around the shop.
Today, manufacturers have a variety of fluids, depending on the transmission type and year. Why so many, you ask? Several reasons. Before all these variations, when you had Ford type F or Dexron II as the bulk oil you’d keep in your shop, an automatic transmission might last only 100,000 miles. It was commonplace when I was rebuilding to have a transmission fail at 60,000 miles (or even less). Heat wasn’t much of an issue, and fuel economy wasn’t much of a concern.
The demand for better fuel economy took a front seat as time passed, so the number of gears increased. In some cases, the expectation of longevity took transmission failure to 150,000, 200,000, and beyond. ATF now had to provide better lubrication, a higher heat tolerance, less parasitic drag for released friction components, and better compatibility for hi-tech friction plates. Compatibility with the friction plates is where you’re most likely to notice a problem with fluid type. This is where you wind up with chatters and harsh shifts, along with other unwanted shifting problems. Many of which will either bring your customer back with a complaint. Or worse, you’ll replace a good transmission because it doesn’t work right.
Car manufacturers today have a sophisticated procedure for servicing (or filling) their transmissions. Here, we’ll use a Chrysler 845RFE.
Figure 2 shows the drain plug. If you’re servicing the unit, you’ll replace the filter and pan as an assembly. Now it’s time to fill the unit.
Figure 3 shows that they give you the part number for their fluid and a temperature spec. They also put it in a place you can’t miss, right below the fill plug. This procedure is from Chrysler, and ATRA published it in the 2018 seminar. They’re using it as a fluid check, so it already had fluid in this instance. The only modification we’ll make is to add the initial fluid after step one.
- Raise and support the vehicle on a level hoist.
NOTE: If this is a service or a first-time fill, you’ll want to fill the transmission and continue with Chrysler’s procedure. - Start the engine. The engine must continue to run for the entire test.
- AWD vehicles only: If necessary, remove the front propeller shaft (Refer to 03 – Differential and Driveline/Propeller Shaft/Removal
- AWD vehicles only: Using a scan tool, enable Emissions Rolls Test Mode in the PCM.
- Using a scan tool or the vehicle information center, verify that the transmission fluid temperature is below 30°C(86° F).
- Disable traction control (ESC).
- Remove the fill plug from the right rear of the transmission case.
- Add transmission fluid until it trickles from the fill opening.
- Install the fill plug.
- Lower the vehicle for access to the inside of the vehicle, leaving the tires at least 8 inches off the ground.
- 1With the brakes applied, place the transmission in Reverse and hold for 5 seconds.
- Place the transmission in Drive and hold for 5 seconds.
- Release the brakes, slowly accelerate to 2nd gear and hold for 5 seconds.
- Apply the brakes and place the transmission in Neutral.
- Raise the engine speed to 2000 RPM for 5 seconds.
- Return the engine to idle and place the transmission in Park.
- Remove the fill plug to allow excess fluid to drain from the fill hole, or add fluid as necessary.
- Install the fill plug.
- AWD vehicles only: If removed, install the front propeller shaft.
- AWD vehicles only: Using a scan tool, disable Emissions Rolls Test Mode in the PCM.
- Using a scan tool, clear any DTCs.
NOTE: A full transmission will have fluid at the fill hole with the transmission between 30°C (86°F) and 50°C (122°F). Do not overfill.
Car manufacturers take these measures to ensure the transmission is filled correctly. Don’t shortcut it. If you’re unsure of the procedure, contact ATRA for the procedure and other service tips to help satisfy your customer and prevent an unnecessary comeback. It’s easy as one, two, three.
