Hello, need some help,
How to test headlight stepper motor for funtcion. 2007 XJ8L.

Arne

 

Hi Arne,

What is your situation there?
Do you have one headlight not adjusting itself, or do neither appear to be working?
If one side is working OK, then you might just want to quickly swap the headlight over to the other side cabling to check if it now responds to commands.
If neither headlight is leveling then you might have a lack of PWM controlling signal from the air suspension module reaching the headlights - wire continuity check.

Delving further into the headlight assembly itself- Stepper motors have multiple windings (coils) that are phase driven to create the stepping force and rotation of the motor.
The more windings the finer the rotational increments.
The headlight stepper motor has 2 windings and is driven via an integrated driver module within the headlight assembly.

To physically test the stepper motor itself, firstly you would manually turn the motor to ensure it was fee and not seized.
Next you would disconnect teh stepper motor from the driver module and measure the continuity (resistance of the two separate windings) to ensure both were present and of identical resistance. I would hazard a guess that a 12 volt stepper motor that doesn't need to generate a lot of force to probably have winding resistance measurements ranging somewhere between 5-20 ohms (depends on torque the motor is designed to generate).

The headlight stepper driver module is controlled by receiving a pulse width modulated (PWM) drive signal that is applied to both headlight assemblies simultaneously. There is just one command signal applied to both headlight assemblies equally.
The duty cycle of the PWM waveform (the ratio of "on time" versus "off time" of the square wave) is interpreted by the headlight stepper driver module to activate the stepper motor in the forward or reverse direction to set the headlight level.
The PWM signal is most likely a logic level waveform, so 0-5v peak, as that signal comes from the air suspension module to the headlight assemblies.
I have no information as to the frequency of the PWM signal being sent, nor the practical PWM ratio that the controller responds to, but if you are able to put a scope on the PWM control to the headlight and measure the amplitude (peak voltage of the PWM to determine if 0-12v or 0-5 volt signal) and "period" of the waveform, you can easily calculate the frequency. It is likely to be a relatively low frequency signal, somewhere between 150 and 500Hz for such an operation.
As for the duty cycle, again the driver module is probably expecting a duty cycle ratio range of perhaps 3% to 50% for intended full range of stepper movement.

If you really want to independently bench test the headlight, you might use a variable frequency variable duty cycle PWM generator, which are dirt cheap on eBay.
You would just power that with a 5 volt source if the output PWM waveform needs to be 0-5v or 12 volts if the PWM signal needs to be 0-12v, as the generator supply voltage denotes the drive signal amplitude.
You will also need to powering the headlight driver supply with 12 volts. The earth (negative) of both your 12v supply and the PWM generator supply (if it needs to be a 5 volt supply)will need to be linked together otherwise the drive PWM signal will be floating and not be recognized by the driver module.
Set the frequency of the generator to initially perhaps 150Hz and a radio of 1%, then gradually increase the ratio (not frequency) upwards towards 50% to see if the headlight assembly responds. If it doesn't, then you might try increasing the PWM frequency up to 350Hz and repeat.
The headlight stepper drive module may not be particularly worried about the exact frequency, but ratio of the duty cycle will be what it is more sensitive to.
You can take some of that guess work out by first measuring the PWM output from the air suspension module with a scope.

Here is an example of the type of PWM generator you could use for such testing (using it in PWM mode not just the Pulse mode).
https://www.ebay.com/itm/354792671110

 

Are you sure you even have leveling headlights? From a post in another thread on self-leveling headlights: "Self-levelling headlamp units were only fitted up to chassis number G39154." Basically, the leveling of the car through the air suspension is what leveled the headlights after that.

Of course, coilover replacement puts paid to that...