Centre Diff / Drop Gear Problems
22 January 2009, 12:48 AM
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Centre Diff / Drop Gear Problems
I've got a Legacy GTB with a TY754VBAAA gearbox (same as some Impreza STIs I think).
Since I've had the car on the road (only the middle of December) I've had problems with front end of the car hopping / jumping when on large amounts of steering lock, but only when the car is warm. Reading around I decided this was most likely a problem with the centre viscous coupling.
On Monday of last week I also started hearing a rhythmic whining noise, related to road speed. It's not a constant whine, but comes and goes regulary. The frequency the noise comes and goes at and the pitch of the noise increase with road speed. I think it's louder when under load, but once the noise started I stopped driving the car.
At the weekend I decided to investigate / replace the viscous coupling, but it wasn't as straightforward as I'd hoped... I drained the gearbox oil, which came out a very strange grey colour. There were some metallic fragments on the magnetic sump plug too. Eventually I removed the rear most housing of the gearbox and a pair of gears that take drive from the output of the viscous coupling to the propshaft (I believe these are called the drop gears?). They're obviously dead anyway
What is likely to have caused this? Is it related to the viscous coupling problem or something completely separate? None of the bearings attached to these gears are obviously damaged, run smoothly and don't seem to have a huge amount of play (although I've only checked that by hand).
Where the propshaft enters the housing I've removed there's a seal and what looks like 3 bearing races. Those races are quite badly scored.
As I understand the viscous coupling should simply pull out now. Mine won't, it's quite well stuck. Is there anything else I need to do first?
I've already got a replacement viscous coupling, but where do you think I should go from here? Are the gears easy enough to source and replace? What about the bearing type thing the prop runs through? Is there likely to be damage to the rest of the box too (it shifts smoothly and otherwise seems fine when in use)?
Sorry if some of the terms I've used are wrong / confusing - not really sure what lots of these bits are called!
Thanks
Since I've had the car on the road (only the middle of December) I've had problems with front end of the car hopping / jumping when on large amounts of steering lock, but only when the car is warm. Reading around I decided this was most likely a problem with the centre viscous coupling.
On Monday of last week I also started hearing a rhythmic whining noise, related to road speed. It's not a constant whine, but comes and goes regulary. The frequency the noise comes and goes at and the pitch of the noise increase with road speed. I think it's louder when under load, but once the noise started I stopped driving the car.
At the weekend I decided to investigate / replace the viscous coupling, but it wasn't as straightforward as I'd hoped... I drained the gearbox oil, which came out a very strange grey colour. There were some metallic fragments on the magnetic sump plug too. Eventually I removed the rear most housing of the gearbox and a pair of gears that take drive from the output of the viscous coupling to the propshaft (I believe these are called the drop gears?). They're obviously dead anyway
What is likely to have caused this? Is it related to the viscous coupling problem or something completely separate? None of the bearings attached to these gears are obviously damaged, run smoothly and don't seem to have a huge amount of play (although I've only checked that by hand).
Where the propshaft enters the housing I've removed there's a seal and what looks like 3 bearing races. Those races are quite badly scored.
As I understand the viscous coupling should simply pull out now. Mine won't, it's quite well stuck. Is there anything else I need to do first?
I've already got a replacement viscous coupling, but where do you think I should go from here? Are the gears easy enough to source and replace? What about the bearing type thing the prop runs through? Is there likely to be damage to the rest of the box too (it shifts smoothly and otherwise seems fine when in use)?
Sorry if some of the terms I've used are wrong / confusing - not really sure what lots of these bits are called!
Thanks
25 January 2009, 08:33 AM
#2
Same box as StiV/Vi saloon and P1. Is that the original fit for your car i.e does your car run 4.44 diffs front and rear?
Unusual for the transfer gears to have gone as they are usually bullet proof unless you're running humungous power. Sounds like the viscous may have died and then rooted the transfer gears
I've got a the same 5 speed sti/p1/gtb box and the same 1:1 transfer gears, but mine has a dccd centre diff that just bolts on instead of the centre diff as a straight swap (I swapped them two years ago- in fact the transfer box and centre diff were swapped together as one unit although they do unbolt) so it runs a 65R/35F split instead of the 50/50 that the viscous diff runs.I've just taken the whole box,transfer gears and centre diff out of my P1 as I'm now running a 6 speed dccd box in it instead. It's up for grabs if it's of any use.
Unusual for the transfer gears to have gone as they are usually bullet proof unless you're running humungous power. Sounds like the viscous may have died and then rooted the transfer gears
I've got a the same 5 speed sti/p1/gtb box and the same 1:1 transfer gears, but mine has a dccd centre diff that just bolts on instead of the centre diff as a straight swap (I swapped them two years ago- in fact the transfer box and centre diff were swapped together as one unit although they do unbolt) so it runs a 65R/35F split instead of the 50/50 that the viscous diff runs.I've just taken the whole box,transfer gears and centre diff out of my P1 as I'm now running a 6 speed dccd box in it instead. It's up for grabs if it's of any use.
26 January 2009, 01:14 PM
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Don't know what the diff ratios are, but yes its the standard box for the car.
I'd really like to know why the viscous coupling won't come out now, haven't had a chance to look at it since but I thought it should just pull off.
I'm trying to keep things standard (and if possible avoid swapping the whole box over - car is on axle stands on my drive and that wouldn't be fun) at the moment. So I know I need a pair of gears and the bearing type thing the prop runs through. Is that bearing a pain to change? Might be easier to find the whole housing if it is as mine has some internal damage from the bits of gears too.
I'd really like to know why the viscous coupling won't come out now, haven't had a chance to look at it since but I thought it should just pull off.
I'm trying to keep things standard (and if possible avoid swapping the whole box over - car is on axle stands on my drive and that wouldn't be fun) at the moment. So I know I need a pair of gears and the bearing type thing the prop runs through. Is that bearing a pain to change? Might be easier to find the whole housing if it is as mine has some internal damage from the bits of gears too.
26 January 2009, 07:08 PM
#4
You do have the same diffs.
Re viscous - as far as I know it should just come out.
If it's any use, I also have the original standard viscous centre diff that can be put back on my box, instead of the current Dccd, so you could unbolt your whole box + centre diff and just replace with mine. Box and centre diff weigh in around the 50kg mark - not a problem between 2 blokes or with a trolley jack or whatever.
Re viscous - as far as I know it should just come out.
If it's any use, I also have the original standard viscous centre diff that can be put back on my box, instead of the current Dccd, so you could unbolt your whole box + centre diff and just replace with mine. Box and centre diff weigh in around the 50kg mark - not a problem between 2 blokes or with a trolley jack or whatever.
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