After several days of desert dirt road driving the 3d gear of my manual impreza transmission strated having problems shifting: I always get a grinding noise when I stick the 3d in. Two separate mechanics say it's the 3d gear synchronizer and that the whole transmission needs to be rebuilt, with quotes above $3K? I did not expect that much, is it what it costs for Subarus? Can I just keep driving until the car dies (it got 120K). I found that if I bring the RPMs as low as possible I can shift the 3d fine, but this creates a delay in the shifting.
You can try double-clutching, as was done in the days before synchros. Basically, when you shift into third (either up into it or down into it), you would pause while in the neutral position, release the clutch pedal and rev the engine simultaneously, then push the clutch pedal out again and complete the shift into third. I have to do this occasionally on my MGB as the second-gear synchro is a little weak.
Transmissions have been constant mesh since the 1930's, except for reverse (exclude Aston-Martin and similar). That is, there is an input shaft, primary shaft and a secondary shaft, with the pairs of gears for 1,2,3, and
The one piece primary shaft has the gears cut on it. The secondary shaft has separate gears with the synchro cones/rings. In neutral, the input (from the engine, via the clutch) spins and there's no connection to the output. Select a gear and one of the sets of secondary gears is connected to the primary shaft.
Now, the idea of double declutching is to perform what synchro rings do when not worn out. The input gear speed, governed by the motor, has to match the output gear speed, driven by the road wheels. When the speeds are matched the desired gear can be selected without crunching.
So ... changing say 2nd to 3rd: release the accelerator, depress clutch, select neutral, release clutch. The engine revs will slow down. When the engine revs match the road speed for 3rd (this happens in a couple of seconds), depress clutch and get it into 3rd. If you let the revs drop to idle, you won't select 3rd without a crunch, and you'll have to give it a blip in neutral.
In fact, you can change gears without a clutch, with care. I had to do this once during a forest rally when the clutch cable broke. Getting started was entertaining: select 1st gear and start engine... drove the car 200km back home clutchless too.
If you hold some de-select pressure on the stick in 2nd, when you release the accelerator, the load comes off the transmission, and the stick will glide perfectly safely into neutral. Keep some 3rd position select pressure, and when the revs drop to the right speed, it will glide into 3rd, at which point you get on the gas again.
So ... changing 3rd to 2nd ... always harder, but the same technique. As you pass through neutral, with the clutch out, give the engine a rev. This synchronises 2nd road speed with engine speed for that speed.
All this does is speed up the engine: when the clutch is in, it does not cause the input side of the transmission to speed up. In fact it slows down due to oil viscosity.
I was experimenting today with the shifting 2->3 and 4->3 with the double clutching. I had to find the desired RPMs by trial and error. So it indeed sounds like between 1.5 K and 2.0 K is the right range of RPMs to get it to shift smoothly 2 -> 3, and that's at around 25 mph. Shifting from 4->3, and 2->3, required different RPMs, consistent with your explanation of the RPMs of the wheels. Ugh!
Assuming that I can learn this while keeping my eye on the traffic, I would have to start doing this for every gear combination, not just the 2->3 and 4->3, to keep it simple for my brain to map. A rhetoric question: what's better - to learn double clutching or to spend the
3K? Also, I wonder how long it will take for the transmission to get ruined from the fact that I slightly grind it every time I do it wrong (50% of times so far).
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