ZF 5 HP-24A Inputs and Outputs

2015-12-19 14.31.53

The concept of this computer-controlled 5-speed Quattro Tiptronic automatic transmission as a whole is pretty darn overwhelming, but now that my tech & I have dismantled one, we feel much less intimidated. This article covers what we have figured out so far.

I’m basically a software development geek, so I tend to think in terms of input, process and output.

Inputs to the transmission as a whole:

  • Analog direct current from the transmission control module to the solenoids
  • Transmission fluid from the radiator
  • Rotating force from the engine
  • Shift lever
  • Analog direct current to the speed sensors
  • Analog direct current to the temperature sensor
  • Counter-force at the physical mounting points to hold the transmission in position

Outputs from the transmission as a whole:

  • Current-draw feedback to the transmission control module from the solenoids
  • Transmission fluid to the radiator
  • Rotating force to the rear drive shaft
  • Rotating force to the front drive shafts
  • Analog direct current from the speed sensors
  • Analog direct current from the temperature sensor
  • Force on mounting points that hold the transmission in position

As to the processes … it’s complicated. So let’s worry about them later, and worry about how the inputs relate to the outputs.

Analog direct current to and from the solenoids

The transmission control module is horribly complex, and it chats with the rest of the car using a complex serial bus network protocol, plus analog direct current, all via a massive 88-pin plug. Fortunately, the way it chats with the solenoids inside the transmission is relatively simple: analog direct current.

The wiring bundle from the transmission control module ends up at a round plug with more than half a dozen, but less than a dozen, electrical contacts. From there, a wiring bundle runs inside the transmission, along the valve body, until it reaches the solenoids. These are all arranged in a row. The wiring branches off to each of the solenoids. Two or three wires run to each solenoid.  The wires are different colors. One of the colors is purple. The purple wire is probably a ground wire, common to all the solenoids, since each of the solenoids has at least one purple wire running to it.

Some of the solenoids are simple on/off solenoids whereas others can be completely on, completely off, or somewhere in between. This is probably to enable the complex and subtle timing of the transmission being able to shift while the car is accelerating, yet without flat-lining the accelerating curve. This would also explain why, if the transmission control module goes haywire such as when it gets wet, it sends unintentionally destructive commands to the transmission.

The transmission has no intelligence built-in. When the transmission control module tells a solenoid to energize, then it’ll energize, regardless of the consequences.

Presumably, the transmission control module can sense when a solenoid isn’t firing, due to the current draw characteristics.

Transmission fluid to and from the radiator

Two black steel lines run connect the transmission and the radiator. One line is for fluid flow from the radiator; one is to the radiator. As to which is which, the clever ZF folks have stamped the directions on the casing at the appropriate position.

Rotating force from the engine and to the drive shafts

The torque converter is bolted to the engine, and its outside casing spins at the speed of the engine and with whatever torque the engine puts into the movement.

This rotational movement and torque comes out of the transmission via the two front drive-shafts and the rear driveshaft.

Shift lever

Regardless of how the shift lever chats with the rest of the car, the shift lever is mechanically attached to the transmission and as it moves, mechanical things inside the transmission move accordingly.

Analog direct current to and from the speed sensors

The transmission has a speed sensor by the torque converter, another one approximately central to the transmission, another one attached to the driver side front drive shaft, and another one in or by the tail housing.

Each sensor independently generates pulses along a simple to-and-from wiring circuit that runs from the transmission to the transmission control module and back. The speed of the pulses show the speed of the rotation, probably due to a Hall effect switch at each speed sensor. This data is presumably useful to the transmission control module such as to calculate whether or not excessive slippage is occurring.

Analog direct current to and from the temperature sensor

The transmission has a transmission fluid temperature sensor inside it. The sensor is probably a heat-sensitive rheostat, meaning its electrical resistance varies with its temperature. Analog voltage goes in one side, and there’s a voltage drop out the other side. The size of the voltage drop depends on the electrical resistance of the sensor, which in turn depends on the temperature. A simple to-and-from wiring circuit runs from the sensor to the transmission control module and back.

In-balance forces at the physical mounting points

This transmission is a heavy beast. I’d guesstimate it at 350 pounds or so. Keeping it in position as the Audi goes bouncing around on a road, stopping, accelerating, leaning, pitching … it takes some impressive counter-force to keep the transmission in position, not least when maybe 300 HP is being pumped through it.

The main mounting surface is from the housing to the engine. With the exception of the tail shaft, the entire housing is one piece of cast aluminum. Perhaps the tail-shaft varies depending on the variation of front-to-rear differential that’s used.

There is, by implication of the one-piece casting, no separate bell-housing, and the Audi 4.2 V8 is the only engine that the ZF 5 HP-24A bolts up to, with some massive bolts.

Other 4WD vehicles such as the BMW X5 also use this basic transmission: the ZF 5 HP-24 but not the ZF 5 HP-24A. The “A” variant has Quattro 4WD technology built-in; the “non-A” variant is only RWD, and an external mechanism (perhaps a transfer case) enables the 4WD functionality on the BMW X5. Because the “non-A” variant is used with various BMW engines, and also Jaguar engines, it presumably has a removable bell-housing.

Attached to each side of the transmission is a transmission mount, that in turn attaches to the body of the car.


The various processes inside the transmission enable the desirable outputs using the available inputs.



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