Throttle Linkage Joint
The rotating connection point where the cable end is attached to the throttle lever; play and binding here spoil speed repeatability directly.
The throttle linkage joint is the connection point where the end of the cable is attached to the throttle lever and which allows rotation. Play here swallows part of the command; binding here makes the actuator spend force for nothing. A large part of the repeatability problems in speed control is solved at this single point, without touching the electronics at all.
The two opposite faults to look for
Two opposite faults are looked for in the joint. The first is play: wear between the pin and the hole makes the lever fail to follow at once when the actuator changes direction. This turns directly into hysteresis — a different speed is read at the same command value depending on whether it is approached from above or from below. The second is binding: rust, drying out or a fastener connected at the wrong angle makes the joint stiff; the actuator draws more current, and on the GS740-3P this current can come close to the limit threshold.
| Symptom | Probable fault | First check |
|---|---|---|
| The lever moves late on a change of direction | Play | Wear of the pin and the hole |
| A different speed every time at the same setting | Play | The joint and the cable end connection |
| The servo strains and goes into current protection | Binding | The movement of the joint by hand |
| The lever does not return fully to idle | Binding or a weak spring | Release the lever and watch the return |
The right way to check
The way to check is simple. Take the cable off and move the throttle lever with your finger from idle to fully open and back. The lever must move without catching, easily with one hand, and must return to idle with its spring when released. A mechanism that does not pass this test will not be put right by any electronic adjustment. During the test rock the joint with your hand as well, not only the lever: if there is play you can feel in the hole of the pin, that is where the play is.
There is a practical way of measuring the play on its own too. Push the lever up against one direction and let go, then start pushing it slowly the other way. The amount of free movement you feel in the joint before the lever itself starts to move is the command the system loses on a change of direction. If this movement has become visible to the eye, a pin or bush change is needed; increasing the tightening torque does not close the play, it only binds the joint and this time creates a friction problem.
Connection angle and linearity
When the connection is made, the angle at which the cable comes to the lever should not change much through the working range. A connection at a large angle changes the effective lever arm as the lever turns and moves the relationship between the potentiometer and the speed away from linearity. The practical result is this: in one region of the potentiometer a small turn changes the speed a great deal, while in another region it changes it hardly at all. This behaviour is most often taken for an electronic fault; its source, though, is mechanical.
When the calibration is repeated
If the pin, the bush or the cable end in the joint has been changed, the relationship between the range of movement of the lever and the stroke range of the actuator has changed. In that case the calibration has to be repeated; repeating the calibration after a cable change works on the same logic. In the same way, a cable that has been in use for a long time stretching over time also shifts the range.
What to do in practice
If you have a repeatability problem in the speed, do not break the order: verify the play and the freedom of the joint by hand first, then the cable route, and the electronic settings last of all. Both the complaints a different speed at the same position and the servo turns but the throttle does not follow start at this point. Do not leave the joint dry at maintenance; rust causes trouble faster than play does.
The context in which this term is used: Electronic speed and throttle control on a diesel engine