///Teardown!\\\ S85 Throttle Actuator - BMW M5 Forum and M6 Forums

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Old 20th November 2012, 02:26   #1
jcolley
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///Teardown!\\\ S85 Throttle Actuator

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
///Teardown!\\ S85 Throttle Actuator-1-front-side.jpg

Back side. The 5 T-20 Torx screw are removed next
///Teardown!\\ S85 Throttle Actuator-2-back-side.jpg

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.
///Teardown!\\ S85 Throttle Actuator-4-cover-off-first.jpg

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.
///Teardown!\\ S85 Throttle Actuator-4-cover-off-second.jpg

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.
///Teardown!\\ S85 Throttle Actuator-5-contact-closeup.jpg

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.
///Teardown!\\ S85 Throttle Actuator-6-first-reduction-removed.jpg

First Reduction Gear
///Teardown!\\ S85 Throttle Actuator-7-first-reduction-pinion.jpg

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.
///Teardown!\\ S85 Throttle Actuator-8-first-reduction-pinion-detail.jpg

Actuator Arm removal. I was surprised to find this is a 1/2" nut and not metric.
///Teardown!\\ S85 Throttle Actuator-9-removing-actuator-arm.jpg

Once the retaining nut is removed, you can see the two flats on the output shaft which key to the actuator arm.
///Teardown!\\ S85 Throttle Actuator-10-actuator-arm-removed.jpg

With the arm removed, here's the parts layout
///Teardown!\\ S85 Throttle Actuator-11-actuator-arm-components.jpg

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.
///Teardown!\\ S85 Throttle Actuator-12-actuator-arm-press-fit-spacer.jpg

Complete arm parts layout. Here it all is in order for reassembly.
///Teardown!\\ S85 Throttle Actuator-13-all-external-components-layed-out.jpg

Second Reduction Gear and shaft
///Teardown!\\ S85 Throttle Actuator-14-second-reduction-spur-shaft.jpg

Second reduction spur good teeth. This is what the gear tooth profile should look like.
///Teardown!\\ S85 Throttle Actuator-15-second-reduction-spur-good-teeth.jpg

Second reduction spur gear worn teeth. This is not what they should look like.
///Teardown!\\ S85 Throttle Actuator-16-second-reduction-spur-bad-teeth.jpg

Output shaft needle bearings
///Teardown!\\ S85 Throttle Actuator-17-housing-detail.jpg
///Teardown!\\ S85 Throttle Actuator-18-outer-needle-bearings.jpg

Complete Parts layout
///Teardown!\\ S85 Throttle Actuator-19-full-tear-down.jpg

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.

Enjoy!

Jim
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///Teardown!\\ S85 Throttle Actuator-20-motor-output-terminals.jpg  

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Old 20th November 2012, 09:19   #2
Fuddy
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awesome
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Old 20th November 2012, 12:20   #3
potassium
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Would expect metal components on something this critical and frequently failing - I had mine replaced last year but would be willing to get mine upgraded with a better build version.
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Old 20th November 2012, 13:30   #4
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Old 20th November 2012, 13:36   #5
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Thanks for sharing. Do you think lack of lubrication is the main contributor?
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Old 20th November 2012, 14:31   #6
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this is a great write up, thanks.

Some questions:
- what symptoms was this defective throttle actuator exhibiting for Fuddy? Or was it not working at all?
- do you think it failed due to the mechanical issues you highlighted (insufficient lube, bent/misaligned teeth) or something electronic on the circuit board?
- how does the actuator arm actually connect to the throttle body?
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Old 20th November 2012, 14:41   #7
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You, sir, are a welcomed resource in my book! Kudos!
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Old 20th November 2012, 14:45   #8
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Quote:
Originally Posted by jcolley View Post
well, thanks to member fuddy, i received in the mail today a non-functioning throttle actuator for a little science experiment....

... 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.

Enjoy!

Jim

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Old 20th November 2012, 14:50   #9
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Quote:
Originally Posted by douglas_ngo View Post
Thanks for sharing. Do you think lack of lubrication is the main contributor?
I honestly don't know. Materials aren't my specialty, but this link: Gear Design
shows that one of the advantages of nylon and delrin gearing is no lubrication required.

It seems that it's more of a design flaw to me. The actuator is not designed for continous rotation, so when the throttle bodies are in their most often used range of travel, only 3-4 teeth at most are in contact. This causes wear on those teeth specifically. As the trottle actuator rocks back and forth for the very slight variations in thottle, these 3-4 teeth are just working back and forth against each other and this is the resultant wear.

How does this end up affecting our cars? Here's my theory so far, which is based on absolutely nothing. When I got the actuator from Fuddy, I noticed the actuator arm could be moved by hand. About 1 time in 5 as I moved it all the way from lock to lock, if releasing it slowly, the spring tension would move the arm back to its normal (throttle closed) position, but it would stick about 20-30 degress from fully closed. As I moved it from lock to lock, I could feel the notchiness in its travel, so I had a pretty good idea what I would find when I opened it up.

If you are having any doubt whether your actuators need to be replaced, this would be the easiest way to check. Pop the ball joint of the arm and move it by hand.

I think that when the car (at least for mine) is around 3000RPM, it is right in the range of the bad teeth. As the DME senses a slight bit more throttle is needed, it signals the actuator to open the throttle bodies a bit more. I believe the actuator motor is not a brushless motor, but a standard DC motor (haven't confirmed this yet). The control circuitry applies a little more current to the DC motor causing it to start to move. But here's the catch, the teeth are somewhat bound due to wear and the actuator circuitry has no way to know if it's moving yet. The position feedback sensor is on the end of the throttle body assembly! So, more current is applied to the motor until the gears rotate slightly, the output arm moves the throttle linkage shaft, the linkage shaft moves another ball joint which rotates the throttle body furthest from the actuator and it it turn moves the hall sensor which provides the feedback signal to the actuator. Now the DME also sees that it moved the throttle bodies a tad too far, eases up the signal to the actuator and the whole cycle repeats in the opposite direction. Evenutally, the hunting goes away and it settle somewhere. What does this constant hunting do to those 3-4 teeth? You guessed it, extra wear.

The Siemens-VDO website shows simliar actuators, but most seem to have the position indications built in. I emailed them inquiring about replacement parts, but honestly don't expect to get anywhere with it.

Last edited by jcolley; 20th November 2012 at 16:26. Reason: edited for technical accuracy
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Old 20th November 2012, 15:01   #10
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Quote:
Originally Posted by petesamprs View Post
- what symptoms was this defective throttle actuator exhibiting for Fuddy? Or was it not working at all?
I'm not sure exactly, maybe he can answer. Mine are giving me tiny power surges around 3000 RPM, so I wanted to take one apart.

Quote:
Originally Posted by petesamprs View Post
- do you think it failed due to the mechanical issues you highlighted (insufficient lube, bent/misaligned teeth) or something electronic on the circuit board?
I haven't had a chance to test the elex yet, but doubt I'll find anything there.

Quote:
Originally Posted by petesamprs View Post
- how does the actuator arm actually connect to the throttle body?
RealOEM.com * BMW E60 M5 Throttle Housing Assy

and

RealOEM.com * BMW E60 M5 Throttle body/acceleration

will help show this. There are two actuators, one for each throttle bank. Notice part number 4 in the first link is the hall sensor for position feedback.
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