Another car questions + opinions

[compu_85]

Speedometer cables are driven by the output of the transmission. The cable spins, and rotates a magnet in the speedometer. It works a lot like a voltmeter... faster rotation makes more force and moves the needle further against the clock spring.

Cars with mechanical speedos, but with cruise control, will have an impulse sensor someplace... either on the back of the speedometer or in a little box in line with the cable. Then the cruise control unit counts pulses.

Electronic Taxi meters work the same way On newer cars they just tap into the vehicle speed signal.

Regarding selecting gear ranges in an automatic, that will do different things depending on the car.

In the 2003 F350 we had, the shift pattern was PRND321. From a stop, if you shifted into 2, the truck would move off in 2! And if you shifted into 3, it would move off in 3! Nice to have that control. It had a "5" speed automatic. Technically, it could make 6 different ratios: It was a 3 speed with an overdrive on the end. But 2 of the ratios ended up being very close. The "2" gear was actually 1st + overdrive, and 3 was really 2nd.
As the truck was warming up it would shift 1, 1OD, 2, 2OD, 3OD; when warm it would shift 1, 1OD, 2, 3, 3OD. 2OD was slightly shorter than 3, so it made the engine run 200 rpm or so faster.

In a friends 76 Mercedes, the shift pattern is PRNDSL (Drive, Slope, Low). It's a 4 speed automatic, but it moves off from a stop in 2nd in drive. If you shift into S / 3, it will move off in 2nd and then shift to 3rd only. If you put it in L / 2, it will move off in 1st, then shift to 2nd. From a stop in D, if you floor it it will kick down to 1st as the car starts moving.

In another friends 85 Mercedes, the shift pattern is PRD32. When you come to a stop in D, the transmission actually rests in 2nd gear. As you touch the accelerator, before the throttle opens, the transmission kicks down to 1st gear so you move off in 1st. This makes the car "pull" less as you're sitting at a light.

Some 78-80 Mercedes accomplished the same thing, but in a more rube goldberg method... they had a small computer box that read the pulses coming from the cruise control pickup, to know when the car was stopped. It looked at the brake light switch too. Like my friends 76, the transmission rests in 2nd gear in D at a stop. But the little box would look for the brake lights turning off after the car was stopped (ie, you took your foot off the brake), and energize the kickdown circuit to shift the transmission into 1st.

-J