Transmission, Automatic

Kickdown Relay Tutorial

Kickdown Relay Tutorial

>I recently tried doing the "kickdown switch modification" as documented on
>many sites. Result was no discernable change in shifting behavior.
>After checking my connections and ensuring the toggle switch is really
>switching the circuit (probing continuity across the foot switch connector),
>I'm fairly sure there is nothing wrong with my wiring job. That leaves me to
>suspect faults in how this circuit is sending control signals to the AT. I
>think I'm in for some under-car exploration...
>Has anyone BTDT? I'm guessing there may be some kind of relay involved.
>Anyone know how/where the kickdown circuit connects to the AT?
>TIA dw

As usual, it would have been nice to know the specific year model, but ....

The automatic transmission is primarily controlled by internal hydraulic pressures. This includes the speed at which upshifts and downshifts occur, and the firmness of the shifts.

The kickdown solenoid is only one of several devices that adjust the control pressure to change the shifts. When the kickdown switch is made (closed), power flows from fuse #10 (on the '87) thru the switch, then into the 30 terminal of the kickdown relay (XV on the '87), thru the normally closed contacts of the relay, and out the 87a terminal to the kickdown solenoid mounted inside the transmission. When the solenoid is energized, it reduces control pressure, raising the shift speed close to the maximum speed.

The kickdown relay doesn't do what most people think that it does. The power flow from the switch to the solenoid is thru the NORMALLY CLOSED contacts of the relay. This means that the power normally flows thru the relay. In order to control the shift points more precisely (since the speed is very close to engine redline), the kickdown relay receives a signal from the tachometer when engine speed hits 5800 +/-60 RPM (USA and Japan - it's 100 RPM higher for the ROW). This signal open the contacts in the kickdown relay, breaking the power to the kickdown solenoid. This has the same effect as instantaneously lifting your foot from the throttle switch, causing an instant shift. As soon as the engine speed goes back below 5800, the relay closes the contacts, lowering the control pressure to hold the next shift.

So, the kickdown switch (and the added parallel kickdown switch) supplies power to the kickdown solenoid only when the engine speed is below 5800 RPM. Above that speed, the kickdown relay opens, breaking the circuit.

You can check by pulling relay XV (on the '87) and jumpering terminals 30 and 87a in the socket, then driving the car. If there is now a difference in the shift speeds, relay XV or the contacts for it are bad.

I would suggest that you NOT try the full throttle shift points with the relay jumpered.

Wally Plumley
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