Hi: I am still negotiating with a local car lot on maybe buying this 97 white XK8 they have and was wondering about throttle control. I do not like the motorized butterflies in my X type and noticed a post on here about the cable on their XK8. So what is the definitive answer on exactly how these big cats get their leaping orders? Thank you. --Tom

 

AJ26 engines (so the 97 model) are semi drive by wire, where idle is controlled by the ECU, and normal operation via cable. From the AJ27 engines, its full drive by wire.

 

Avos: The Factory Description of the AJ26 Throttle indicates that "Drive By Wire" operation is in effect under normal operating conditions:

Operating Modes:

Normal
1. Throttle cable inputs driver demand
2. Input shaft turns mechanical guard
3. Sensors supply ECM with inputs of driver demand
4. ECM operates dc motor
5. Sensor supplies ECM with inputs of throttle valve position
6. Gap maintained between throttle valve and mechanical guard

Mechanical Guard (Fail Safe Mode)
1. Throttle cable inputs driver demand
2. Input shaft turns mechanical guard
3. Mechanical guard turns throttle valve
4. dc motor drive disengaged from throttle valve

 

Good info.

Whats odd about it is that the TPS only works within a very small range, arround idle, so the ECU has no idea how wide the throttle is acutally open. It wil get info from the PPS of course, but contrary to the AJ27 models it is missing info about the exact postition of the throttle. Also the ECU can control the motor int the closing direction.

This is why I said its semi drive by wire.

 

THank you guys for this. I studied the picture but can't figure out where the DC motor is? Would it be toward the right or rear of this drawing? Is it sort of like a power assist function? And what does acronym PPS mean?

 

PPS = Pedal Positioning Sensor

The motor is connected to the smaller wheel at the right side. And it can only control the throttle toward closing position, it can't open it more than mechanically opened.

 

Exactly. Andre, I have studied the operation of the AJ26 Throttle and the AJ27 Throttle and they are completely different. One seems "Upside Down and Backwards" compared to the other.

This is my Analysis:
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AJ26:

Springs on the Throttle Motor and the Throttle Shaft that, un-restrained, try to OPEN the Throttle.

With the Pedal released, the Throttle is kept closed by the Mechanical Guard Tang that has Stronger Springs than the ones on the Throttle Shaft. (the Guard Tang is on top of the Throttle Shaft Tang).

With the Pedal not released, the ECM can close the Throttle via the Throttle Motor.

With the Pedal Released The Pedal and Guard Tang override the Throttle Motor and force the Throttle Shaft to Idle.

One effect of this system is the inability of the ECM to ease the closing of the Throttle (to aid in reducing emissions).

Also, when the Pedal is rapidly released, I'm sure that the two tangs come together before the ECM-Throttle Motor can close the Throttle.

The two tangs come together occasionally and are never very far apart.

With the ignition off, the Pedal should be able to open the Throttle to the Full open (90deg) position.

Cruise Control Servo lifts or lowers Guard to adjust Throttle Shaft position. This is a Mechanical Coupling, the Throttle Motor is not used.
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AJ27:

The Pedal and Throttle tangs are normally 60 degrees apart. The Pedal Tang is BELOW the Throttle Tang.

The only time the tangs come together (to couple the Pedal Shaft directly to the Throttle Shaft) is:

1. If the ignition is on and the Throttle Motor has Failed and the Pedal Shaft has rotated 60 degrees.

2. If the ignition is off and the Pedal Shaft has rotated 60 degrees.

With a failed Throttle Motor, the Pedal can only open the Throttle 30 degrees. (Fail Safe Mode).

With the ignition off, the Pedal can only open the Throttle to 30 degrees.

To open the Throttle 90 degrees, turn the ignition on but don't start the engine. Then press the pedal fully down. The Throttle Motor (assuming it has not failed) will open the Throttle to 90 degrees.

Cruise Control is a function of the ECM via the Throttle Motor.
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