Technically Speaking
- Subject: Noise from transmission in Park or Neutral
- Unit: ZF 6HP
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
ATSG’s main office is in Miami-Dade County, Fla. Several years ago the county passed an ordinance requiring 16 hours of continuing-education courses for certified mechanics each year. To facilitate this requirement, ATSG has been conducting several evening classes for the shops in Miami. During this time, Luis Zabala from WiWi’s Transmissions attended these classes and brought in a ZF 6HP spacer plate from a BMW for me to look at (Figure 1).
He described how the transmission was rebuilt but soon came back with a noise complaint. When he inspected it he noticed that upon initial startup he could hear no noise, but after 20 seconds or so the noise occurred while the vehicle was idling in either Park or Neutral. When he placed it into gear the noise went away, but as soon as the vehicle began to move the noise returned. As the transmission shifted the noise lessened, and by the time he reached third or fourth gear the noise was no longer present. He also noticed that every once in a while the vehicle would try to stall when coming to a stop.
Luis described the noise as similar to that of a bad planetary. But when he removed and disassembled the transmission, there were no signs of planetary failure. Since he could see nothing that would produce this noise, he separated the valve-body halves for inspection. Immediately he saw how the two relief valves had violently hammered the spacer plate by the deep impressions they left on the plate (figures 1 to 3).
Now that he had discovered the source of the noise, there still remained the question of what was causing the checkvalves to regulate so violently. In answering this question, he then checked each valve for proper operation. The TCC regulator valve was stuck in a partially stroked position. After freeing up the valve and thoroughly cleaning the valve body and cooling system, the shop reinstalled the valve body with a new plate. Once they started the vehicle and topped off the fluid level they waited for the noise. After several minutes at an idle they detected no noise, and after a complete road test there still was no noise. Problem solved.
Thanks, Luis – a nice tip that could prevent another shop from pulling the unit first.