By Brad Bergholdt, McClatchy-Tribune News Service –
QUESTION: Thank you for the column on cars with manual transmissions and clutch use. My father never ever had a vehicle with an automatic transmission. His view of the world was that anybody who had to drive a car with an automatic transmission shouldn’t be allowed on the road. I learned to drive on a 1936 Ford 5 window coupe with, of course, a manual tranny. In high school, it was a big deal to “double clutch” and smoothly shift back into low gear at about 20 mph with no gear grinding. No worries about stress on parts.
Now here is my question: In a manual transmission, what exactly is the “synchronizer” and how does it work? I know that in my ’36 it worked in 2nd and high gears, but not low gear.
—Bob Hughes
ANSWER: I received several neat messages with old car-shifting experiences such as yours. A modern manual transmission’s gears are actually in constant mesh. When shifting, a brass cone clutch, or synchronizer, promptly grabs the tapered side of the next gear, bringing it to the same speed as an adjacent hub, which is splined to the output shaft. A toothed outer collar is then permitted to slide over both parts, locking them together, completing the shift. On older vehicles without this synchromesh feature, the driver would “double clutch” when shifting. The first press of the pedal assisted disengagement of one gear, and the second press, with a well practiced dose of throttle and properly timed shifter movement, brought the two rotating parts to approximately the same speed, minimizing gear clash.
Q: I have a 2007 Mercedes CLS 550 with 33,000 miles. I love the car but every time I fill the gas tank, there is very strong fuel odor in the garage until the tank is 2-3 gallons used. I do not top off the fuel. The shop had it twice under warranty and said it was fixed each time, but the odor was back in the next fill-up.
Being fed up after wasting three days with them, I took it to another dealer. They fixed it, saying the seals on top of the gas tank and the filler-line sensor were bad, and it was good for a few months. Now the smell is back again. I cannot understand the issue and why it recurs. I tried calling Mercedes, and they were totally unhelpful. Any suggestions or fixes would be most appreciated.
—Kiran Rai
A: I can’t offer much help regarding the exact cause of the leak, but it sounds like the second shop has at least rattled the door. A strong fuel odor infers the car would fail its OBD-II evaporative emissions on-board test as well, which might still be a warranty issue.
Have you also encountered a check engine light? There are several hoses and components connecting to the top of the tank in addition to the fuel sending unit and fuel pump seal — any of which could be the cause. An emissions analyzer, set to manual mode, works great for sniffing out leaks; hydrocarbons go off the scale as the probe tip is brought near the source of the leak. Or a smoke machine can be used to gently pump diagnostic smoke into the fuel tank and associated hoses for a visual leak check.
Fill it up just before taking it back to that second dealer. This will make diagnosis easier but the repair a bit more unpleasant. And don’t forget a box of donuts, atop the passenger seat.
