2006 Volvo S60 2.5 AWD, Endless search of problem

[mrbrian200]

That's worn enough to start binding in my opinion.

Worn.JPG (31.36 KiB) Viewed 999 times

When you zoom in more closely, the profile of a couple of the teeth on the bottom of the large center gear is worn down to a "V" shape as opposed to the others that have retained their original profile more like a pencil with flat sides with the "V" shape at the tip. The throttle plate gear wears down by just a little bit and it's less obvious. The teeth on the small gear turn/mesh in at such a high angle it doesn't take much until the end of the tooth on the small cog jams against the side of the tooth on the throttle plate gear instead of cleanly meshing/turning together. If that small gear were say 20-25% larger this issue would be much less of a problem as the 'angle of attack' decreases. But that's not how Bosch designed the thing so all the people driving around millions and millions of various makes/models that uses Bosch engine management and this type of throttle body are kind of stuck with it.

This is Bosch engine management. Thoughtfully designed systems.. then key components that make it all work end up being made sort of cheap and breaky. Sort of their MO.

[EngineeringBloke]

I do think I finally see the issue.

Although the cogs wear so that their outline changes, that is not the failure. While the cogs are indeed worn and their shape changed in outline as seen from the front in the pictures, I do not think that is enough to cause slippage, they still appear to engage but there is an issue shown in the pictures. Take a close look at the few cogs that are dirty. Those marks are where the slippage happens.

The main problem will be seen from the side - looking at the edge of the disk - the ends of the cogs are worn in cross-section. You can even see dark marks on the cogs where they have slipped past each other. The gears are only dirty at those points.

New:
-----||-----
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Worn:
----\..\-----
-----\..\----

I think this happens because the middle gear disk rises up from the gear back plate, then the bottom corner of the middle disk cog wears against the upper corner of the motor gear resulting in excess wear at those points. Similarly the lower corner of the middle gear's small gear wears against the top of the throttle plate large gear cogs.

1. Can you confirm that the gear cogs are worn in this way?
2. What holds the middle gear down? Is there a spring on the gear cover? The other two gears appear to be held to the pins they rotate on - this is needed for the motor to drive the first gear on the right, and the throttle plate on the left.

[Unownreality]

looking at the cogs more in depth, there is more of a sharper V on the cogs that were used. There is a handful (4 or 5) cogs that have a more flat shap at the top. Also playing with the gears, there is a little bit of play where you can turn the center gear without contacting the other 2.

There is a little plastic divet that holds the center rod and gear in

[mrbrian200]

EngineeringBloke, it's not that the teeth are slipping past each other. This is pretty difficult to put into words..I'll try.. the small cog is such a small diameter that when the individual teeth are rotating into position to drive the larger gear, at a point just as they're beginning to engage, the teeth of the larger and smaller gear are some where around a 45-70 degree angle to each other. When the cogs wear down enough, the additional play in the system results with this this angle becoming even greater, closer to 90 degrees and the whole thing locks up/binds at the point where those teeth are rotating into position and just beginning to engage. If the smaller gear were say 20% larger diameter this probably wouldn't happen and you wouldn't see problems until the individual cogs are worn down enough to begin slipping past/failing to grab each other. I hope that helps.

[EngineeringBloke]

Mrbrian, I do see your point. If the cogs wear thin, then they can bind end of cog jamming on end of cog on the other gear due to the large amount of space between the cogs as a result of the loss of material.

The consequence of your issue is a jamming of the gears. However, I do believe I see a second issue. Unknowreality mentions

Also playing with the gears, there is a little bit of play where you can turn the center gear without contacting the other 2

In your picture where you highlighted wear, I think the wear includes loss of depth to the cog. And I think the marks on that cog show the cog on the other gear slipping past it. I sketched what I think I am seeing:

worn cog.png (10.71 KiB) Viewed 957 times

[mrbrian200]

Vertical misalignment wasn't happening on mine. The vertical wear of the individual cogs was even. For this to start happening either the top surface of 'large diameter side' of this gear or the inside of the black plastic cover would have to be worn down noticeably by at least a mm or more. Those surfaces were all fine on mine/not visibly worn. Unknownreality's pics doesn't show any noticable wear on the top side of this gear. His plastic cover isn't in any of his pictures. My gut says it's probably ok like mine was, though.

I suspect the blemish is just a spot of errant grease or engine oil.

The double fluted shape of these cogs have them just barely missing each other as they rotate into position and start to grab. As the cogs wear one gear starts aligning a little too far ahead or behind the other similar to what starts to happen between pulleys driven by a stretched timing belt. Instead of cleanly coming together you get this. Green is where it should be. When they wear the angular rotational relationship between these two isn't maintained and they 'crash'. Demonstrated in '2D' looking down from the top.

[EngineeringBloke]

Thanks, mrbrian, I do understand how the cogs can fail to mesh, and did not think you would have missed the wear I thought I could see in Unownreality's pictures on your own TB.

I look forward to inspecting my own TB, but with no garage and the poor weather, I have not got to it yet.

[Unownreality]

I think with mine, the center cog is just too worn between teeth. There is at least .5mm of play at thr idle position before the teeth grab traction with each other. The main cog is metal, not sure why the put platic for the other two. Seems like a design flaw to me.

[mrbrian200]

I'm guessing during accelerated wear testing Bosch saw more immediate problems with the gear at the stepper motor, so thay made it of metal. The ones down in that give us problems as the vehicles age were probably deemed 'good enough' to outlast a typical new vehicle warranty. Or their accelerated wear testing didn't consider/test for the effects of engine oils/additives/cleaners that might be getting pushed into the mechanism under turbo pressure. I think I mentioned in this or another thread my sister used to own a VW Jetta that used this same basic design throttle body from Bosch. It was a naturally aspirated engine. I looked under the cover of that TB. There was no sign of oils or wear whatsoever at 180k miles and not one bit of play in the gears it was bascially still like brand new in there.

EngineeringBloke:
You can kida-sorta feel this binding by working the throttle plate slowly by hand, but your're fighting both the main butterfly spring and turning the stepper motor under heavy gear reduction. It's easier to feel by taking the cover off and turning the large gear with your finger. The mechanism should feel silky smooth as you rotate the gear to open the throttle plate. If you feel any spots where the mechanism seems to resist motion a little, or feels 'notchy', that's this wear which is responsible. Also, turn the large gear until the throttle plate is over 1/3 open. But instead of releasing your finger from the gear to let the mechanism spring back to idle, slowly bring it back to idle with your finger. Observe where the throttle plate want's to settle- this is when worn cogs start binding and causing the throttle plate to sit around 12% open (too far). Then manually rotate the large gear it the other direction bringing the idle plate to fully closed. When you release from closed the secondary spring loaded idle stop mechanism will open the plate back to that ~7% open position, which is where it should normally be at 'rest' but doesn't when the gears become worn.