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Discussion Starter · #1 ·
As you know, Lucas CU14 uses a speed sensor in the speedo cluster to figure out when it needs to activate the AIC, in addition to the TPS.

Does anyone know what signals the sensor produces?

I think my issue related to stalling when coming to a stop could be related to it. I came to that conclusion when my speedo cable broke, and it didn't affect driveability, which it should. So either the sensor isn't working, or the signals are not getting to the ECU. I need to test for that.

James
 

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ECU uses the AIC valve to dampen the on/off throttle loading of the engine - giving the smooth deceleration of the engine at speed.
If no road speed signal is sent to the ECU, it would calculate that car is standing still and not have any reference to the car speed or engine loading.
LH 2.4 and 2.4.2 have the same feature.
LH 2.2 use the "dashpot" damper for the deceleration smoothing.
 

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I think the sensor is in the speedo James. The thought at one time it used the ABS sensor but yes, it is in the speedo. It produces a voltage of between 0.5-11 volt. I am really not sure it has any bearing or input upon your idling or controlling the AIC. After all the engine should idle correctly at any speed if you are in neutral. Well at least mine does. The speed sensor is there primarily to allow the ECU to shut off fuel on the overrun when you have the throttle closed. Perhaps with no speed input, there is no fuel cut-off as you close throttle to stop, so it may be over-fueling as the revs reduce. Don't know at what revs the fuel cut-off disables. Maybe 2k like the Bosch LH. Just an idea you understand. Perhaps a measure of the O2 voltage may give you some clues.

PS There should be an error code for road-speed sensor if it's non-functional.
 

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........try driving without the AIC and then you will understand it is more than just idle control.
But if history is any guide, most of what I suggest here is poo poo'd......
 

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Discussion Starter · #5 ·
I think the sensor is in the speedo James. The thought at one time it used the ABS sensor but yes, it is in the speedo. It produces a voltage of between 0.5-11 volt. I am really not sure it has any bearing or input upon your idling or controlling the AIC. After all the engine should idle correctly at any speed if you are in neutral. Well at least mine does. The speed sensor is there primarily to allow the ECU to shut off fuel on the overrun when you have the throttle closed. Perhaps with no speed input, there is no fuel cut-off as you close throttle to stop, so it may be over-fueling as the revs reduce. Don't know at what revs the fuel cut-off disables. Maybe 2k like the Bosch LH. Just an idea you understand. Perhaps a measure of the O2 voltage may give you some clues.

PS There should be an error code for road-speed sensor if it's non-functional.
Paul, the Lucas CU14 service manual clearly states that the AIC will only function when the TPS senses idle and the speed is below 5mph. It also states,

As well as idling control, the signal from the road speed sensor is also taken into account by the ECU when determining when to trigger the shift-up signal (certain variants only), and for control of fuel shut-off during deceleration (engine over-run). This latter function is necessary to ensure that fuel shut-off does not take place at low speeds.


The last statement seems to indicate that the speed sensor is needed to ensure fuel does NOT shut-off at low speeds. However, your statement is the opposite - that the sensor is to make sure it doesn't overfuel at low speeds.

And yes, I ought to get an error code with no speed sensor signal. I don't have one. I have also swapped instrument cluster before and it didn't make a difference to the behaviour. Perhaps the ECU is faulty?
 

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The last statement seems to indicate that the speed sensor is needed to ensure fuel does NOT shut-off at low speeds. However, your statement is the opposite - that the sensor is to make sure it doesn't overfuel at low speeds.
I did not mean to imply that. I was suggesting that as you were slowing down and above 5mph (I didn't remember that figure) the ECU would still be fueling with a closed throttle, thinking you were stationary, and cause a rich fuel situation. It was a dubious, unfounded idea anyway. Perhaps your ECU is faulty. Surprised that there is no CEL.

PS Wouldn't harm to check the O2 voltage anyway, just to eliminate it from the equation.
 

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Just been looking at the early Range Rover fuel system which has an ECU similar to the CU14. It says,
"The ECU in turn detects vehicle movement from the road speed input and ensures that idle speed control mode is disengaged. Should the speed transducer fail in service the ECU idle speed control would become erratic".
Seems likely that the CU14 could behave the same way.

I did experience stalling when coming to a stop a year or so ago, particularly from speed. I never got to the bottom of it, and my speedo was working. I substituted nearly every component, including ECU, to no avail. However, a short time later my fuel pump failed. When I replaced the pump the stalling never recurred. You need a particular type of twisted logic to follow that argument through, but hey, it hasn't happened again.
 
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