Although the Prius that I currently own is my 4th, it is the first one that I have had enough time to try and understand exactly how it works. The members of this forum have been very helpful, and I am very thankful for the support. I am changing the headlight assembly on my 2007 Toyota Prius. In the process of doing so I have run up against several questions that I am unable to find an answer to. It appears as though the difference between high and low beam is controlled by a little shudder. That's easy enough to understand. However, I am scratching my head trying to understand how the self-leveling mechanism works. I know there is a sensor on the rear axle and a little servo motor on the headlight assembly, but I don't understand how it works. The servo motor does not appear to be indexed so how does it know or communicate with the sensor on the rear axle and then make the necessary adjustments. Will anyone venture an explanation or perhaps point me to a place where I can find an explanation.
There's a little motor in the bottom outside corner of the back of the headlight case and the leveling sensor is mounted to the rear axle control arm. That's all I've got, haven't studied it more than that.
The missing link between the two is the headlight-leveling ECU, which takes in the sensor reading and computes what to do with the headlight motors. I'm not sure where that ECU is located. A good guess would be in the ECU village behind the glove box. The wiring diagram will show the location. Toyota Service Information and Where To Find It | PriusChat
There's also a micro adjustment on the box to make incremental adjustments to the system overall I haven't read the section in the manual for it but I've seen the adjustment on the outside of the box I'm sure there's some discussion of it for overall use in the manual and what have you. I just change them to the LED conversions and they get so much brighter and so much clearer don't seem to need much adjustment or anything but in a northern car the system could go south if you will no pun intended pretty quick The rod and everything connecting to the little box in the sensor in the back and all that is pretty fragile it'll rust very quickly.
I actually just replaced the headlights on my old Camry, and the basic idea should be similar. Here’s how I think it works: Basically, there’s a "digital scale" (the rear axle sensor) on the back of the car that keeps an eye on whether the rear end sags—like when you load heavy stuff or brake hard. Whenever it notices a change in the rear height, it sends a signal to the "tiny motor" (that servo motor you mentioned) in the headlight. That motor basically turns a seesaw-like mechanism. By rotating, it tilts the headlight beam up or down. For example, if the rear sinks under weight, the motor angles the lights lower to avoid blinding oncoming drivers. As for how it knows how much to turn? I’m guessing it’s like tuning an old radio dial—the motor kinda "feels" its way by turning until it hits a preset resistance point (which probably matches different load conditions). After swapping in a new headlight assembly, you might need to let the system "relearn" its settings, like how you’d recalibrate a phone screen after a factory reset. My tip: After installation, try parking on a slope facing downhill, shift into Drive, and see if the lights adjust automatically. If the lights flicker or act crazy, check if the sensor linkage is jammed or if the motor plug’s loose. For more details, you’d probably need the repair manual. But Toyota’s electronic systems are usually pretty robust—if you’re using OEM parts and plugged everything in right, it should sort itself out.
From the description in the manual, the leveling ecu looks at the height sensor signal. If that signal is steady (and headlights are on and vehicle speed & accel are in certain ranges), then the ecu sends commands to the leveling actuators. The actuators have constant power and ground. The command signal varies from about 1.0 to 12.6V (don't know if it's analog or digital PWM). So it sounds like these are some type of smart actuator. Any position feedback would be internal to the actuator - not to the ecu. X voltage from the height sensor equals Y % of actuator travel. You are supposed to "set the height of the car" with a specific weight - 10 liters of fuel, one 150lb driver, etc. Then adjust headlight beam aim when the leveling system is "where it's supposed to be". Posted via the PriusChat mobile app.
Most motors of this type are stepper motors which are designed to move a fixed amount per step. Many types of motors can determine the positive stops as end points, either by limit switches or by increased current and then stop. The control can send steps from a known position or it can reset to a known position by driving the maximum steps to an end point and then reverse to reach the needed number of steps. It is essentially how printers and plotters work.
How does this work? Automatic headlight beam level control system https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/New%20Features%20Manual/04priusn/be/light0.pdf
I had composed a rather lengthy post, detailing what I had learned thus far before opening this thread. Although what I had determined was spot on, the subject is very well documented by the link provided by MAX2. Thanks! Where did you get this document? Is it a source of information that I can access? The only thing I would add is that when the servo motor is R&Red it is not indexed. I burnt a few brain cells over thinking it all. The worm drive must have enough movement that it does not required an exact installation. mr_guy_man I always appreciate you insightful comments. The only thing I would add to, "X voltage from the height sensor equals Y % of actuator travel." is that there is an ECU between these two parts. It is a single function ECU located behind the glove box on the right side. There seems to be very little that can be done on this vehicle without involving an ECU.
A simple strategy for a stepper motor that doesn't require you to index anything would be like what the gen 3 ECM does to drive the EGR valve. It knows that valve has 111 steps from full open to full closed. If it isn't sure where the valve is, it can just send 111 steps in the close direction. When it hits closed it's not going any farther, so wherever it was before, it's closed now, and the ECM can count steps to open it as far as wanted.
This is a collection of various manuals for cars (diesel, petrol, hybrid). Some enthusiast collected them from different sources on the Internet for his site. As you can see, the names are in Chinese. share.qclt.com - / The structure of the site is in the form of a tree diagram with folders that contain scans of some pages in various formats. Unfortunately, there is no single, understandable structure even for one car, and it is difficult to find the necessary document due to the large volume of folders and files.
