Well, thanks to member Fuddy, I received in the mail today a non-functioning Throttle Actuator for a little science experiment.
So, here we go:
Front Side of Actuator
Back side. The 5 T-20 Torx screw are removed next
Inside the actuator. The motor contacts are in yellow, more on these later. The Second Reduction Spur gear (black ) is a half gear, not fully round and stops on a metal stop in the pink circle.
Operation: There is a DC motor in the housing which drives the brass Input Pinion Gear (yellow circle). The First Reduction gear (pink circle) is made of what appears to by white nylon and multiplies the torque from the drive motor. The Second Reduction gear (green circle) is made of what appears to be black nylon and drives the output arm which connects to the throttle bodies. It is spring loaded to return to idle when the motor is deenergized.
Motor Contacts: The DC motor connects to the control board (mounted to the inside of the cover) which only has two contacts, a positive and negative lead.
First Reduction Gear removed. Here you cans see the lubrication used and there isn't much of it. This isn't necessarily a problem, in fact probably prevents dirt buildup. Nylon gears are used frequently as they require little lubrication.
First Reduction Gear
First Reduction Pinion Detail. Here you can see the flaw in the actuator. Given the mileage that some of these have failed at, this component must be incredibly underdesigned. You can see the wear on the pinion half of the gear on at least three teeth. The only reason I can think they didn't use metal for this is the metal motor pinion meshing on a metal first reduction spur would produce conductive dust which would be bad for the unsealed circuit board inside.
Actuator Arm removal. I was surprised to find this is a 1/2" nut and not metric.
Once the retaining nut is removed, you can see the two flats on the output shaft which key to the actuator arm.
With the arm removed, here's the parts layout
The last two pieces to remove are somewhat difficult. This is a press fit spacer which has about .001" interference fit to the shaft. I used a small bearing puller to remove it.
Complete arm parts layout. Here it all is in order for reassembly.
Second Reduction Gear and shaft
Second reduction spur good teeth. This is what the gear tooth profile should look like.
Second reduction spur gear worn teeth. This is not
what they should look like.
Output shaft needle bearings
Complete Parts layout
Circuit board details. I haven't stared at the board much yet, but here are the ICs which control the output power to the motor. The yellow circles denote the polarity of the black connector next to it. The pink circles highlight the output ICs for that terminal.
So, from this teardown, two things occur to me.
1. First off, the actuator has no internal means of knowing its own position. The motor control board has no feedback without the hall sensors which are located on the opposite end of the throttle body assembly. This means the DME monitors the position and constantly has to send a control signal to the actuator to move/maintain the position. Also, there is bound to be some slop that develops over time in the mechanical linkages from the actuator arm all the way downstream to the throttle body which is monitored via hall sensor. I'm wondering if this is part of what causes the mild lurching sensation when the gears are aligned with the bad teeth meshing. Why the hell BMW didn't use a optical rotary encoder directly on the output shaft from the second reduction gear, so that it would operate more like a traditional servo, I have no idea.
2. Material choice. I'm not a materials guy, but I used to mess around with RC helos quite a bit, and the metal pinions and nylon spurs used on those little 2000 RPM machines at a few horsepower are far more resilient than these apparently are.
So, why are these so expensive to replace? Because there is no viable alternative for replacement/repair. So, I'm going to give these to a local machine shop and see what they can come up with.
Of course, all of this assumes that there is nothing wrong in the electronics side of the house. Once I read a bit about the control signals from the DME, I'll see if I can find a way to test it.