Don’t be Afraid, Little One

The beginning of my first day at school involved me standing alone and crying pitifully, feeling overwhelmed and scared. Then, another little girl, Sarie Joubert, came over, took my hand and led me away, comforting me and saying it’s all going to be OK.

So now I’m sometimes in the same situation again, feeling overwhelmed while trying to figure out the symptoms on two 2000 Audi A6 project cars with the 4.2 liter engine and the ZF 5 HP-24A transmission. The transmission on each car slips in first gear, there’s a jarring thud, and then the car goes into limp-home mode.

One at least one of these two cars, the car was fine as it approached a particular traffic intersection, and when it left the intersection, it was misbehaving, and it’s been doing that ever since. The instant nature of the onset is helpful to know. My best guess is that an electrical component failed, but that’s sort of like saying that the needle I want is in that haystack over there.

Unless the transmission control computer is well-matched to the car, the car’s electronics won’t talk to it. I have plugged in about nine separate transmission control computers, spanning seven different variants, and that didn’t fix the problem.

The plug on the transmission control computer is a massive 88-pin thing. Hoping to be able to analyze the signals, I tried to find some information as to what each pin means, but when I found it, the totality of it was overwhelmingly complex.

The ZF 4 HP-22 transmissions that I know and love have hydraulically controlled operation, and the only electrics on the transmission itself is when it announces that it’s in reverse, so that the car’s two back-up lights can turn on.

For the ZF 5 HP-24A, the electrics seem to be all over the place. I once made the mistake of crawling under the car to study the various places where electrical wiring attaches, and that was overwhelming too.

Not just is the transmission control computer already intimidating by being a computer — but it actually talks to the car’s other computers (ECU, ABS, etc.) across a serial bus network. And then there are wires going directly to the transmission too, and to all sorts of sensors.

The way I understand how an automatic transmission (not the torque converter, just the transmission itself) basically works, is:

  1. Mechanically, it uses shafts, gears and clutches to transmit power from the input shaft to the output shaft using, at any one point in time, a particular gear ratio that is one of a small, finite set of gear ratios specific to that transmission.
  2. Hydraulically, it uses a high-pressure pump and various channels to guide the fluid.. These channels are centralized in a part called the valve body, where the fluids either open or close valves, and that in turn controls the mechanical components. Various fluid pressure conditions cause various valves to open or close under the desired conditions.

So, as a made-up example, to engage first gear, clutches A, B and D should be activated and then clutches C and E should not be, and to enact this, we want valves 1, 3 and 4 to be open and valves 2, 5 and 6 to be closed. The various conditions and consequences can be diagrammed using a grid that some call a “truth table.”

In electrically controlled transmissions, the valves are opened or closed under electrical control, using (for each valve) a solenoid, which I understand to be the sort of electromechanical device that makes the passenger door’s knob pop up when I pull up the knob on the driver door.

In relatively simple electrically controlled transmissions, a particular solenoid can be either energized or not, and so the valve controlled by that solenoid can be either open or closed.

So, to stay with my made-up example, to engage first gear, you’d energize the solenoids for valves 1, 3 and 4. With that premise, it’s not that complicated. I would measure whether or not any particular solenoid wire has battery voltage when it should, and if it does, great — and if does not, then hey, there’s an electrical problem upstream of the solenoid. And if the solenoid gets power but doesn’t react, it’s a bad solenoid. It’s as Simple as that. It’s sort of like being pregnant or not. There’s not much of a gray area.

By contrast, on the ZF 5 HP-24A transmission, a solenoid can be on, off … or somewhere in between, and there’s not just the one mid-way point, either. So it’s a lot, a lot, a lot more complicated.

Or is it? I finally found a truth table for this transmission, which shows how the neither-on-nor-off conditions are not that common, and besides they don’t affect first gear, anyway.  For first gear (which is where I’m having the problem) the situation is a clean, simple combination of some solenoids being completely off and others being completely on.

I also found a wiring diagram, and I finally took a deep breath and started analyzing it. It turns out to be almost comically simple. The wires from the transmission control computer to the transmission are simple voltage-energized lines, not a serial bus or any sort of weird encoding.  There are maybe eight or so wires. One wire goes to the transmission fluid temperature sending unit, which I understand to be a simple temperature-sensitive rheostat. As for the other wires, each one goes, quite simply, to one solenoid. Nice and simple, really.

Then, I read that the solenoids on this type of transmission are not deep inside the transmission, but just below the valve body — and the valve body isn’t all that hard to remove, with the pan off.

So it’s possible to test each solenoid and to have some reasonable clue as to whether or not it’s reacting to being energized or de-energized.

Given their relative accessibility, it might even be possible to replace a solenoid without having to remove and replace the entire massive 350-pound transmission. I’m hopeful.

As to the cable, a.k.a. the bundle of eight or so wires from the transmission control computer to the transmission, it is not that complicated either. If there’s continuity on each wire, end to end, it’s good and if not, it’s bad.

As to the electrical components sprinkled around the transmission, they’re not that complicated either. One is for the position of the control cable, and the rest are for speed sensors. Nice and simple, too.

Suddenly, I feel much better. It’s all going to be OK.






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