Automatic transmission hiccup - have any idea why?

I drive daily my pickup truck in the city. It’s a Toyota light truck, popularly known as the Hilux here is Asia.

It’s a 2012 4x4 with Auto transmission, mechanical transfer case with factory LSD model variant.

This happens sometimes, and getting more frequent, btw:

Moving in Drive in city normal traffic @ about 1k to 1.2k rpm and following another vehicle in front that slows down near to halt, then resume. Of course I do the same with the fellow in front of me.

Here is my “problem”: On the road, when the fellow in front lightly brakes to a near stop, naturally I lightly brake too and my truck goes down in speed to near stop (to keep distance with the guy in front) but when I step-OFF the brake to resume speed, I can feel and hear a moderate thud in the truck as if there is still some “momentum” inside the transmission just as I let go the brake pedal (as if the brake held back the momentum of the truck).

Btw, two months ago I change the transmission fluid with the recommended T-IV full-synthetic, though this has been happening before, albeit infrequently.

It does not happen each time but it’s annoying as it gets to happen more frequently.

Any idea guys?

Edit: The mileage is about 88k kilometers, or 55k miles.

When you changed the transmission fluid was it just a change or a flush and did you change the filter as well ? Also what color was the transmission fluid when you changed it.

did you also change the transfer case fluids

does it also happen when you use either 2wd or 4wd?

The color of the old transmission oil was still clear red, hardly used-looking. It has no transmission filter.

The shop that did the fluid change used a machine that was attached to the transmission. The machine is called AT fluid exchanger (they call it a dialysis machine). In the front of the vehicle there is a radiator transmission cooler that has two hoses, removed and one was used for outlet of old oil, while the other one for inlet of new oil.

The procedure is done while the the engine is running and the trans in neutral. There are two bar graphs in the machine, one for the old oil and the other for the new oil, showing the % decrease in the old oil and at the same time the % increase of the new oil.

When the new oil is about 90% IN (as shown in the bar graph), the transmission is engaged in different gears while the parking brake is of course engaged, thereby flushing out almost 98% of the old oil, including the oil inside the torque converter, etc. thereby consuming up to 11 liters of total ATF capacity. The procedure lasts about 7 to 10 min IIRC.

Since the transmission has no drain plug, just sucking the old oil from the dip stick will only get about 6 liters of the old oil.

Btw, this problem was already present before the oil change, though not as frequent.

changing the transmission oil would not cause a problem but using the wrong oil will. I would look to see what oil they used. Transmission oils are completely incompatible with each other.

The transfer case fluid is still factory. The front drive is only engaged on very short runs for lubrication purposes.

I notice it in 98% of my driving, which is in 2wd. I guess it’s also most probably present in 4wd.

How many miles do you have on this TRuck/transmission? Is that the only time you feel the thud, when slowing down to a near stop, but not stop fully stopping if I read your description correctly?

I believe in checking the cheapest and easiest fixes first. It could be play in the Universal joint in the drive shaft. Have you ever had those replaced? There are connection points that wear out over time and cause “play” that could be felt during specific times of acceleration like you describe. Also could be your rear differential at the connection point between the rear axle and drive shaft.

Both of those are easier to diagnose and cheaper to repair then the transmission, and could be a good starting point for diagnosing a potential problem.

If you climb under the truck and grab the drive shaft and try to “rock it” front to back or rotate it clockwise / counter clockwise you’ll know if one of the U-Joint is bad by the “play” in it.

These rear end and axle problems are more likely to be present if you have higher mileage and if those parts have ever been replaced before.

Good luck!

I used the factory-recommended ATF, as embossed in the dip stick: use Type T-IV ( the shop was aware that I should use this specific type )

Oh, sorry, I edited my op……it’s 55k mileage.

Yes, not fully stopping. The thud happens when I take off my foot from the brake pedal, as it there is still some forward momentum left in the trans that comes off when the brakes are off.

On the other hand, I just talked with a close cousin of mine about this problem.

He also has several Toyotas models, from light trucks, to Land Cruisers and Sequioa, and also some Mazda, Ford, Isuzu light trucks.

He said ONLY his Toyota Hilux light trucks also exibit this problem.

Since it was still clear and didn’t look like it was used then it seems it wasn’t old fluid or unusual wear.

I know most cars don’t have a drain plug for transmission fluid normally I would remove the entire pan and change the filter while I’m at it then check the bolts on the transmission valve body that hold it against the transmission since overtime it can loosen its always a good idea to check if its tightened properly. But from the sounds of the issue since the transmission fluid is clean it so theres no unusual wear it could be slippage in the gears.

You may want to try something like this:

While your linked product would be good for its intended purpose, I’m afraid my problem is not slippage but seemingly an “abundance” of bite, so to speak.

Everything is normal, except as described in the op.

I understand however, there is absolutely nothing the owner of a vehicle can do to fix their own transmission. The only option is to take it to a repair shop. They will no doubt make it worse.

Do you know your transmission code? It seems that it have 4 gears, right?

Nobody need to try this.

My 2102 Acura MDX needed a fluid change and I remember reading somewhere specifically saying not to hook it up to any type of flushing machine .

Yes, 4 speed. Don’t know the code.

If you change your ATF without a flushing machine, you only remove 20% of the fluid. All the flushing machine does is get 95% of it out.

Toyota code is A340F, Aisin code is AW40.
This gearbox have been used by many truck manufactures for long time. Quite cheap and robust.
Its manufacture has been started long time before T-IV transmission fluid standart have been introduced (first they used other fluid inside).
However, common suggest that works for any transmission with T-IV fluid - never mix it with any other type of fluid.
How much fluid you needed for change? Did you check fluid level on horizontal surface with heat-up car (I guess it is always heat-up in your region lol).
I don’t completely understand what does this mean (I can feel and hear a moderate thud in the truck as if there is still some “momentum” inside the transmission just as I let go the brake pedal).
It could be close to the feelings of person who used to drive modern many-gears electronic ruled gearboxes, and after tried old 3-4 gear transmission without electronics (when gear change signal was a pressure difference in hydraulic lines).
Automatic transmissions used in trucks always have some extra “idle” torque. Small car needs small torque to start without throttle in jam, while truck driver usually wants to be able to go little uphill without throttle.
How long have you checked out brake system? There might be a problem with brake piston return, or whole caliper might stay not parallel to the brake disc (easy recognizable by wedge-shaped brake pads).