Why does VVT have to be "unloaded" before removing the timing belt ?

Post by **oragex** » 08 Aug 2020, 06:09

Technical question here, in the official Volvo procedure for timing belt replacement it says, After aligning the timing marks
"Turn the crankshaft a further 1/4 turn (90 degrees) clockwise and then back again until the markings correspond."

I understand this will "unload" the VVT hubs. What is the reason for this ? Is it only so the VVT hub(s) don't spring back when the belt is removed ? So they stay without moving and the timing marks remain aligned ? So is it only to help not make a mistake and miss the timing marks when placing the new belt ? Or is it for a more technical reason ?

http://www.ondemand5.com/mric/common/as ... ileid=6198

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Post by **jonesg** » 08 Aug 2020, 15:47

Maybe to remove any lash or play in the cam system, a bit of slop here and there add up, its called ganging.
Helps to put the correct tension on the timing belt too.
It establishes an accurate end point for reference.

I transferred the timing marks to the rear of the cam sprockets, make sure to paint both extremes of the cam movement from rear of sprocket to the block if you're removing the cam gears, if doing cam seals etc.

Post by **abscate** » 08 Aug 2020, 18:44

The Timing involves setting the cams , not the cam hubs, in definite relationship to the crankshaft.

Because the cam hub can move relative to the cam on VVT cars, the designer has to choose which,way to load the hub on the cam to designate the relationship

The design could have been done either way, clocked clockwise or reverse.

The VVT hub is. Marvel of engineering, adjusting cam position at about 300 Hz frequency

Post by **jimmy57** » 08 Aug 2020, 20:09

The reason is simple: on the the 99-2001, the exhaust VVT has a spring. If you turn it CW to the marks then when you remove belt the exhaust gear moves and you have to fight it back to the correct place while fitting the new belt and getting everything in place. If you go past and then CCW back to marks then the exhaust cam gets CW relative to gear and the spring is unloaded with the marks aligned. Remove the belt in that position and the cam gear doesn't move and there is no fight with the belt. Effective 2002 the exhaust hubs lost the spring and there is no need to do the CCW shuffle.

Post by **oragex** » 09 Aug 2020, 05:14

Thank you all. On my 2003 non turbo, when I removed the belt, the non VVT exhaust cam rotated by itself 1-2 teeth clockwise. I then put the new belt first on the intake cam, then on the exhaust cam - and the exhaust cam moved back by itself and the marks got aligned. So I understand as long as the timing marks are properly aligned, there is no functional issue with the timing advance

Post by **abscate** » 09 Aug 2020, 09:28

As long as the cam didn’t move, yes

The cam tolerance for position is only about one tooth. More than that, the VVT will give a cam flank angle error code.

I went deep into this in the p80 forum on my 1999 in April

Post by **oragex** » 09 Aug 2020, 10:19

jonesg wrote: 08 Aug 2020, 15:47 Maybe to remove any lash or play in the cam system, a bit of slop here and there add up, its called ganging.
Helps to put the correct tension on the timing belt too.
It establishes an accurate end point for reference.

I transferred the timing marks to the rear of the cam sprockets, make sure to paint both extremes of the cam movement from rear of sprocket to the block if you're removing the cam gears, if doing cam seals etc.

Great idea to paint the cams on the engine side, even doing just the belt, the front markings with the upper cover alignment are not easy to check, especially the exhaust side

jmartin919 · Post by **jmartin919** » 04 Apr 2023, 07:35

I have a 2000 S70 GLT SE (2.4 turbo) with the exhaust VVT. When doing the timing belt I didn't have the cams locked and the VVVT cam moved - it wasn't just movement due to the spring action. Trying to figure out where the cam should be repositioned to. I didn't break any of the bolts on the VVT hub or timing gear. When the Volvo procedure is done using the cam locks it looks like the VVT cam has to be turned a little ccw to get the tool in place and then the VVT setup is done and the belt is installed - do I have that right?. So my question is when the setup of the VVT is complete (before the locking tool is removed) where should the timing mark on the VVT cam be? From the instructions it seems like it should either be on the mark or one tooth before, not sure which.