Building a Set on a Wet 3 Percent Grade with One Pair of Chocks
You are at a grocery distribution center outside Harrisburg building a set for a 240-mile run to Pittsburgh. Your tractor and 48-foot lead trailer are already coupled and the landing gear is up. You still have to add a converter dolly and a 28-foot pup loaded with 9,000 pounds of canned goods. The yard slopes about 3 percent down toward the dock, and the pavement is wet. You are working alone; no spotter is available and a second driver is waiting for your spot. The dolly's spring brakes are set and its air lines are not yet connected. The pup's wheels are not chocked. Your tractor's parking brake holds on the slope when you test it, and you have a set of wheel chocks behind the seat but only one pair.
1. In what order will you couple the pup and the dolly, and which brakes and chocks do you set at each step? Explain why the 3 percent slope changes the procedure.
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Order and holds:
- Chock the pup first. The single pair goes on the downhill side of the pup's tires. The pup's spring brakes are currently set (no air line connected), so the chocks are a backup to a brake that can be released - not the primary hold.
- Keep the dolly's spring brakes set. Back the dolly under the pup and lock its fifth wheel on the pup's kingpin. Do not connect the dolly's emergency line yet: charging that line releases the dolly's spring brakes and takes away your only brake on a free-rolling dolly.
- Back the lead trailer to the dolly. The tractor's parking brake holds and the dolly is still on its spring brakes, so nothing in the set can roll while you line up the pintle.
- After the pintle is latched and pinned, connect the dolly's lines, charge the system, and only then release the dolly's spring brakes.
Why the slope matters: the dolly is the one piece with no chock and no driver. On flat ground an early line connection is sloppy; on a 3 percent grade it lets the dolly roll into the dock or into you.
2. Which checks must pass before you release the dolly's spring brakes and move the set, and what does each check catch?
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Checks, in order:
- Visual at the fifth wheel: jaws closed around the kingpin, release handle in, safety latch down, no light or gap between the upper and lower plates. A gap means the kingpin is riding on top of the jaws instead of locked in them.
- Tug test #1: with the pup's spring brakes still set, ease forward against them. If the coupling is not locked, the tractor moves and the trailer does not - you find out at 2 mph instead of 60.
- Connect and charge, then tug test #2: set the pup's brakes with the hand valve and pull gently. This tests the coupling with air on and trailer brakes released, which is the condition you will be driving in.
- Dolly connections: pintle latch closed and pinned, safety chains crossed under the drawbar with enough slack for turns, air lines clear of the drawbar, lines not crossed, electrical cord secured, dolly ABS lamp working.
The pattern is deliberate: each check is done while the set is still held by something you control.
3. After you creep 10 feet, the dolly's ABS lamp stays lit. What does that mean, and what do you do about it?
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The lamp should come on at key-on and go out once the dolly's wheels turn above roughly 5 mph. Staying lit means the dolly's ABS is not functioning.
- The brakes still apply. That is the trap: the set feels normal in the yard, and the defect only shows up in a hard stop on wet pavement, when a locked dolly axle can fold the combination or swing the dolly.
- Treat it as a defect. A converter dolly with an ABS fault does not go on the road. Have the shop check the dolly's ABS cable at the left side, the connector plug, and the ECU; a corroded plug or a pinched cable is the usual cause.
- If the shop cannot fix it in your window, swap dollies. A 30-minute delay is cheaper than explaining a folded set on the Turnpike.
Do not sign off the inspection with the lamp lit and do not let a dispatcher talk you past it.