Gearbox whining in 5th gear

Question

I bought an old VW Golf II recently. The gearbox was whining when driven in 5th gear between 75 and 90 KPH then it would stop. I suspected gearbox oil, so I had it changed. I was there when the mechanic drained it out and it was totally black and had low viscosity. It was most probably used engine oil that previous owner had put in. The good news is that there were no filings. Now with the new oil (proper gearbox oil) the whining is not as loud, but it can be heard from 70 KPH to over 100 (haven't driven it faster). It disappears when I lift of the throttle or press the clutch. The volume of the sound changes with the amount of engine load. No other gears make this noise.

So, it's probably a bearing, it might be under side (axial) load. I know I'm looking at transmission rebuild some time in future, but should I be worried about this right now. It's not very loud. Should I be avoiding loading it in 5th gear, that is, should I be going down to 4th for hills and overtaking?

If it's a bearing it should become louder if it heats up during a long trip, right?

What is the worst (realistic) case scenario?

Answer 1

Realistically, you could run it this way indefinitely without anything more than the whine. It will probably drive you nutty after a while and you'll automatically figure out ways to avoid it making the noise (whether you do it intentionally or not ... human nature!).

Bearings tend to last way beyond the point where they start making noise. If a bearing becomes too worn, though, it could displace a gear enough to cause wear on the bear faces (it would take a lot of wear to make this occur, though). You have fresh oil in there and if it seems to be running okay, it probably is. If the person intentionally put heavier viscosity oil into the transmission than it called for, I'd assume they knew about the issue and were trying to mask it (but you probably already figured this out).

Really, anything anyone says is going to be an opinion, because we don't know the true problem (we're assuming it's a bearing) and we don't know the true damage which has already been done. It could last for the life of the car or it might last until next week. If I were you, I'd drive it as needed, but start planning for replacement or overhaul of the transmission at some point. Hopefully you'll get to the point where you can afford it prior to when it becomes a true issue.