Thread: overrun noise - g/box or t/case?

Problem : a bit noisy going down hill in gear (especially 4th) with no throttle. The noise goes when the drive is from the engine, not the wheels.

Vehicle 300 Tdi Defender.

[1] I can't ellict this noise in neutral.
[2] I've checked the uni's and replaced one that had slight play (no change)
[3] I removed the sump from the t/case, nothing 'obviously' wrong. The t/case has the extended sump and runs synthetic gear oil. I renewed the oil and added some Moreys (no effect).
[4] I have a Roverdrive fitted. Engaging overdrive eliminates the noise.
[5] The diffs run synthetic oil. The front diff is overfilled (I fill it, then fill the swivel housings, as I run oiled wheel bearings) The rear diff has the ARB cover, so can take extra oil, so I added some Moreys (no effect).
[6] The gearbox has a radiator type oil cooler fitted, the feeds are at the top, so that it holds it's oil when not circulating so I can fill the main box to the plug. It runs Synthetic Nulon gear oil. The oil level is to the plug.

The transmission has 330,000km.

I measure g/box temp at the casing, it has not been getting hot.

Any ideas about which box is going to need attention soonest?

Thanks in advance, Michael2.

I sent a query to Ray at Global Roamer, who make the overdrive. It looks like it's a $10 bearing and I can remove the O/D in 15min. Now I just need to know how to strip and rebuild it. It's been on for 6 years, and probably 150K km, so a little maintenance won't go astray.

Hello Michael

I'm sorry your overdrive is grumbling.

When the unit is engaged the sun shaft in the the centre of the unit stops rotating so if the noise stops with the overdrive engaged the cause (as you surmise) is the bearing at the rear of the unit.

As demands on the overdrive have increased due to the use of chipped TD5s we have changed the size of that bearing and if you have access to a lathe you can make a simple modification to change the existing 6005 bearing to the larger 6205.