Bucko74

Newbie Here,

I've been bothered for some time about the operation of the downshift and the kickdown of my 84 Auto.

I'm no mechanic, but I expected the box to behave a certain way and it doesn't.

Can anyone simply explain the mechanism for downshift in the auto (other than kickdown switch interaction or using the gear selector). I'm fairly sure after reading other posts that my kickdown is not functioning properly, but to establish this (with out getting the tools out), I need to understand how the GB should behave without it to rule out issues with the box first.

This bowden cable I hear about obviously must govern part of this system. does the tension of this establish at what revs gearshift occur, both up and down. Or is it only the upshift. And how does the position of the throttle impact?

I'm sure a lot of you are thinking.....maybe I need to sit "GEARBOXES 101", so sorry if this is fairly common knowledge....I just dont know.

cheers and thanks in advance.

WallyP

The kickdown switch under the pedal operates a solenoid on the transmission. When energized, the kickdown makes the transmission more enthusiastic about shifting down and less enthusiastic about shifting up.

The Bowden cable is the primary adjustment. Too tight, and it is irritating because it downshifts too easily and upshifts too late in normal driving. Too loose, and it is irritating because it doesn't want to downshift and upshifts too early in spirited driving.

You can install a switch in parallel with the throttle pedal switch and have more fun in spirited driving. Search is your friend - there is a lot of discussion of Bowden adjustment and kickdown bypass in past threads.

Get the car in a very quiet location, preferably next to a wall or curb. Roll the passenger window down. Turn the ignition switch on and listen as you press the throttle pedal to the floor. There should be a quiet "click" as you reach the floor. If not, check the carpet for interference, then start on the electrical checking.

martinss

Why do people persist in calling the cable going from the throttle quadrant to the transmission THE Bowden cable? In fact all of the cables there (four in the case of an automatic) are Bowden cables, and my bike uses Bowden cables to actuate the brakes and gears. See: http://en.wikipedia.org/wiki/Bowden_cable

Here are some examples: https://www.google.ca/search?q=bowde...urce=univ&sa=X

Search works better if the right terms are used... Sorry to rant on a pet peeve

ammonman

As I understand the A28 gearbox, shift points are controlled by the relationship of two hydraulic pressures in the gearbox system: Control Pressure (modulated up via the "Bowden Cable" connected to the throttle when the throttle is opened) and Governor Pressure(determined by the centrifugal governor driven by the pinion shaft, thereby giving a "road speed" hydraulic signal to the gearbox.) these two pressures control the behavior of a set of Command valves in the valve body. Each shift point is controlled by a separate Command valve but they all operate basically the same. If Control Pressure (as set by throttle position) plus spring pressure in the Command Valve (biasing the valve toward the "lower gear" position) is higher than governor pressure (heavy throttle and low road speed) the Command valve stays in the "lower gear" position. As road speed rises, Governor Pressure rises until it's higher than Control Pressure plus the spring pressure in the Command Valve and the affected Command Valve shifts to the "higher gear" position. When the Kickdown solenoid is activated, Control Pressure is bumped up slightly, raising it above Governor Pressure and causing the Command Valve to move to "lower gear" position. My assumption is the sequencing of the Command Valves is controlled by the relative pressures of the "biasing spring" (my term) inside each Command Valve spool that ensures the Command Valve is in the "lower gear" position in the absence of any Governor Pressure.

Clear as mud right?

Mike

jpitman2

With a RHD car, its easy to open the drivers door, ignition on, not running, floor the accelerator, listen for the solenoid click. Kickdown bypass works really well in my car, although its a 3 speed.
jp 83 Euro S AT 55k

Hilton

How many bowden cables does your transmission have?

Bucko74

Thanks Wally. I definitely read some stuff about in series switches hookup to the tbar shifter for easy access to the kickdown switch. Sounds like something I'd be interested in trying down the track. Cheers

Bucko74

Actually Mike, that makes a lot of sense, and is what I really was trying to understand. It also makes sense understanding why low oil levels in the box impact on efficient shifting. I think I'll test my new understanding, but I'm clear that my kick-down switch is faulty. Many thanks all for the input.

John Speake

Have a read of your car's Drivers' Manual. There is a good description and diagrams of how the box should work.