Automatic transmission fluid turns chocolate brown or mud-like

What could make ATF to look mud-like or chocolate brown?

Automatic transmissions are very delicate part of automobiles. They ought to be handled with care and owner or users ought to have a good level of maintenance culture.

One of the practices that kill automatic transmissions is the use of incompatible automatic transmission fluid in the transmission of a hybrid car.

Automatic transmission fluids transform in different ways depending on the cause, ranging from age, overuse, water intrusion into the transmission, transmission overheat, clutch wear, incorrect fluid and so on. ..

The one we shall discuss here is quite uncommon and that is automatic transmission fluid looking like chocolate or mud.

We are going to be using a case study; a 2010 model Toyota camry hybrid.

Before we assessed this vehicle, the code that the vehicle was diagnosed to have was P0A94 DC/DC converter performance.

Below were the repairs that were attempted on the car by a previous mechanic:

  • The traction battery (high voltage battery) was replaced
  • The Inverter/Converter assembly was replaced.

Unfortunately, non of the above two actions fixed the problem.

When the car was towed to us for diagnosis and fix, we checked the Inverter/Converter assembly, the wiring and connectors and the high voltage battery but all were good

We then decided to check the fluid in the automatic transmission and behold, it looked exactly like a chocolate or watery mud.

We began to investigate the root cause of that and after interrogating owner, we discovered that a dexron III ATF was put in the hybrid car's eCVT (electronic Continuous Variable Transmission)! That sounds amazing because the fluid of that transmission is a special type because of the electric coils of the motors/generators in the transmission. This oil is special in the sense that it does not create electrical short circuit or electrical conduction path due to its own conduction.

Another fault we discovered during troubleshooting is ineffective cooling of the engine and transmission, though the owner didn't notice this. The cooling fan was no longer firm on the shaft of the electric motor  hence the fan itself could not spin as fast as the electric motor. This worsened the situation of the transmission that already had a bad Automatic transmission fluid in it. This is because the transmission and the transmission fluid overheated.

Unfortunately  that transmission had to be replaced to fix the problem because the problem was not detected for a long time by previous mechanics.

I forgot to say that the car could actually start and drive but after a very short distance, it was shutting down with P0A94 DC/DC converter performance code. It was shutting down because the high voltage battery was not charging.

Another thing that could cause this color change is water getting into the transmission. This is similar to coolant mixing with engine oil.

It is a must to use the recommended transmission fluid in a hybrid car hence, the transmission would be destroyed after a short time.