Combination Vehicles: Combination Inspection

13 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 393.42, 49 CFR 393.43, 49 CFR 393.55, 49 CFR 393.70, 49 CFR 393.71, 49 CFR 396 Appendix A, 49 CFR 571.121, TC 21-305-20 para 13-24, TC 21-305-20 para 13-26

What you need to know

393.70(a) sets the tracking standard at 3 inches to either side.

A combination that tracks worse than this is telling you something is bent, worn or wrongly adjusted in the coupling or the running gear. None of the figures offered is the one the rule gives.

When two or more vehicles are operated in combination, the coupling devices connecting the vehicles shall be designed, constructed, and installed, and the vehicles shall be designed and constructed, so that when the combination is operated in a straight line on a level, smooth, paved surface, the path of the towed vehicle will not deviate more than 3 inches to either side of the path of the vehicle that tows it.

49 CFR 393.70

393.43(a) sets the 20 to 45 psi band.

The test exists because this valve is what preserves the tractor's air when a trailer line bursts or the trailer breaks away. That confirms the tractor keeps enough air to stop itself if the trailer fails.

For air braked towing units, the tractor protection valve or similar device shall operate automatically when the air pressure on the towing vehicle is between 138 kPa and 310 kPa (20 psi and 45 psi).

49 CFR 393.43

FMVSS 121 S5.2.3.1(a), applied by 571.121(c)(2), requires direct control of at least one axle on a semitrailer or converter dolly.

Other axles may be indirectly controlled.

Each semitrailer (including a trailer converter dolly) manufactured on or after March 1, 1998, shall be equipped with an antilock brake system that directly controls the wheels of at least one axle of the vehicle.

49 CFR 571.121

393.55(e) requires an exterior ABS malfunction indicator lamp on trailers built on or after 1 March 1998, placed by FMVSS 121 on the left side as viewed from the rear.

Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998, and subject to the requirements of paragraph (c)(2) of this section, shall be equipped with an ABS malfunction indicator lamp

49 CFR 393.55

393.43(d) excepts trailers with three or more axles from the requirement that every brake apply on breakaway, though they must still have brakes that apply automatically and immediately and hold for at least 15 minutes.

Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle. With the exception of trailers having three or more axles, all brakes with which the trailer is required to be equipped must be applied upon breakaway from the towing vehicle. The brakes must remain in the applied position for at least 15 minutes.

49 CFR 393.43

393.42(b)(3) requires both: 1,361 kg, which is 3,000 lb, or less gross weight, and the towed axle weight not exceeding 40 percent of the sum of the towing vehicle's axle weights.

Both conditions, not either. Both limits apply together.

Any semitrailer or pole trailer (laden or unladen) with a gross weight of 1,361 kg (3,000 pounds) or less which is subject to this part is not required to be equipped with brakes if the axle weight of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle.

49 CFR 393.42

393.71(a)(1) limits a combination to three saddle-mounts.

The last truck of a triple saddle-mount combination is also among the vehicles not required to have operative brakes under 393.42(b)(2).

No more than three saddle-mounts may be used in any combination.

49 CFR 393.71

Dummy couplings cap the trailer's air connections while it stands uncoupled, keeping dirt and water out.

They come off before connecting and go back on when uncoupling.

Remove dummy couplings from the air connections on the semitrailer.

TC 21-305-20 para 13-26

396 Appendix A item 2 treats any missing or ineffective fastener as a defect.

Ineffective covers a bolt that is present but not holding, which is why fasteners are checked for tightness and not just presence.

Any fasteners missing or ineffective.

49 CFR 396 Appendix A

396 Appendix A names cracked or broken mounting angle iron.

The whole coupling load passes through that structure into the frame.

Any mounting angle iron cracked or broken.

49 CFR 396 Appendix A

396 Appendix A fails any cracked weld or cracked parent metal.

A crack in the parent metal next to a weld is often the more serious of the two, because it means the base structure is failing.

Any welds or parent metal cracked.

49 CFR 396 Appendix A

396 Appendix A fails the pin for being missing OR not secured.

A pin that is in place but free to work its way out is treated the same as one already gone.

Pivot bracket pin missing or not secured.

49 CFR 396 Appendix A

Para 13-24 keeps the same both-directions principle on the flat.

Level ground is rarely perfectly level, and a trailer with no tractor under it has nothing else holding it.

On level ground wheels should be blocked on one side in front and one side in rear.

TC 21-305-20 para 13-24

Check yourself

• How far may your towed vehicle's path deviate from your towing vehicle's path on a straight, level, smooth paved surface?

• At what pressure should the tractor protection valve activate, and why does you test it?

• What ABS coverage is required on your semitrailer built on or after 1 March 1998?

• What should you look for on the trailer during a walk-around to check ABS?

• Which trailers are excepted from having ALL required brakes apply on breakaway?

• A light semitrailer may go without brakes under what two conditions?

• The maximum number of saddle-mounts in a driveaway-towaway combination is ______.

• What must be removed from the trailer air connections before your hoses are attached?

• What fastener condition fails your fifth wheel at periodic inspection?

• Besides fasteners, what mounting defect is listed for your fifth wheel?

• What weld condition is a defect on your coupling device?

• What pivot bracket pin condition fails an inspection?

• On level ground when uncoupling, how should your wheels be blocked?

Answers are in the practice test for this topic.