Lift Axle Schematics
Air control and service brake diagrams for one, two, three and four auxiliary lift axles. The regulator, control switch and reverse relay are cab mounted; the control module sits at the axle. Port assignments and line sizes are called out.
11-page PDF · lift control and service brakes for 1–4 axles, the truck system context, and the in-cab console · US Letter landscape · vector, prints clean in black & white
Lift control
Single Lift Axle — Air Control
Reservoir through a pressure protection valve to the control module. The regulator, gauge, lift switch and reverse relay are in the cab — regulated air pilots the module and a return line feeds the gauge actual load pressure.
Reservoir volume
The lift axle reservoir must hold the volume of air the axle requires under DOT rules. Main supply comes off it downstream of the pressure protection valve.
Regulator and gauge in cab
The regulator sends regulated air to the control module pilot, and a second line returns from the module to the cab gauge so it reads actual load pressure.
Exhaust must face down
Mount the control module with its exhaust vent downward. Any other orientation traps contaminants and fails the valve.
2 Lift Axles — Air Control
Common reservoir and pressure protection valve feeding 2 independent modules. Each axle has its own cab regulator, switch and control module, so it can be raised and lowered on its own.
Independent per axle
Each axle gets its own cab regulator and switch plus its own control module, so axles can be dropped one at a time as the load builds.
Shared supply line
One reservoir and one pressure protection valve feed every module, so only the branch runs change with axle count.
One bag pair shown
Each axle is drawn with a single lift and load bag for clarity. The opposite side of every axle is identical.
3 Lift Axles — Air Control
Common reservoir and pressure protection valve feeding 3 independent modules. Each axle has its own cab regulator, switch and control module, so it can be raised and lowered on its own.
Independent per axle
Each axle gets its own cab regulator and switch plus its own control module, so axles can be dropped one at a time as the load builds.
Shared supply line
One reservoir and one pressure protection valve feed every module, so only the branch runs change with axle count.
One bag pair shown
Each axle is drawn with a single lift and load bag for clarity. The opposite side of every axle is identical.
4 Lift Axles — Air Control
Common reservoir and pressure protection valve feeding 4 independent modules. Each axle has its own cab regulator, switch and control module, so it can be raised and lowered on its own.
Independent per axle
Each axle gets its own cab regulator and switch plus its own control module, so axles can be dropped one at a time as the load builds.
Shared supply line
One reservoir and one pressure protection valve feed every module, so only the branch runs change with axle count.
One bag pair shown
Each axle is drawn with a single lift and load bag for clarity. The opposite side of every axle is identical.
Service brakes
Truck Brake System — Lift Axle Tie-In
Where the auxiliary axle joins the chassis brakes: foot valve, both service reservoirs, steer axle, drive axle spring brakes and the SR-7. Circuit colours follow the manufacturer convention — primary green, secondary orange.
Dedicated reservoir
The lift axle brake reservoir is fed from the secondary tank, keeping the auxiliary axle on the secondary circuit.
Secondary (front) signal
The signal is teed into the existing secondary line running from the foot valve to the spring brake relay valve. Tubing follows the manufacturer convention — primary green, secondary orange.
Service brakes only
This sheet shows the service circuit. It does not cover spring/park brakes on the auxiliary axle.
Single Lift Axle — Service Brakes
Fed from the secondary tank. The lift axle relay valve is signalled by a tee into the existing secondary line between the foot valve and the spring brake relay valve. Tubing follows the usual colour code — primary orange, secondary green.
Dedicated reservoir
The lift axle brake reservoir is fed from the secondary tank, keeping the auxiliary axle on the secondary circuit.
Secondary (front) signal
The signal is teed into the existing secondary line running from the foot valve to the spring brake relay valve. Tubing follows the manufacturer convention — primary green, secondary orange.
Service brakes only
This sheet shows the service circuit. It does not cover spring/park brakes on the auxiliary axle.
2 Lift Axles — Service Brakes
Fed from the secondary tank, with one relay valve per axle. The tee into the existing secondary line passes through a quick release valve. Tubing follows the usual colour code — primary orange, secondary green.
One relay valve per axle
Every lift axle gets its own relay valve so delivery time to each set of chambers stays short.
Quick release valves
The teed secondary signal runs through a quick release valve — two for four axles, one per pair — so the brakes release locally rather than back through the foot valve.
Arched hops are crossings
Where the signal bus arches over a supply drop the lines cross without connecting — they are separate circuits.
3 Lift Axles — Service Brakes
Fed from the secondary tank, with one relay valve per axle. The tee into the existing secondary line passes through a quick release valve. Tubing follows the usual colour code — primary orange, secondary green.
One relay valve per axle
Every lift axle gets its own relay valve so delivery time to each set of chambers stays short.
Quick release valves
The teed secondary signal runs through a quick release valve — two for four axles, one per pair — so the brakes release locally rather than back through the foot valve.
Arched hops are crossings
Where the signal bus arches over a supply drop the lines cross without connecting — they are separate circuits.
4 Lift Axles — Service Brakes
Fed from the secondary tank, with one relay valve per axle. The tee into the existing secondary line passes through two quick release valves, one per pair of axles. Tubing follows the usual colour code — primary orange, secondary green.
One relay valve per axle
Every lift axle gets its own relay valve so delivery time to each set of chambers stays short.
Quick release valves
The teed secondary signal runs through a quick release valve — two for four axles, one per pair — so the brakes release locally rather than back through the foot valve.
Arched hops are crossings
Where the signal bus arches over a supply drop the lines cross without connecting — they are separate circuits.
In-cab console
Console Pneumatic Schematic
Regulator and gauge inside the console, with both gauge feed options drawn — back from the control module gauge port, or teed off the regulator outlet.
Two sides, two sheets
The console carries a pneumatic side (regulator and gauge) and an electrical side (switch and reverse relay), drawn separately so each can be followed on its own.
Gauge feed is a choice
The gauge can read back from the control module gauge port, or be teed off the regulator outlet. Connect one, not both.
Check relay terminals
Terminal numbering on the reverse relay varies by part. Confirm it against the relay actually supplied before wiring.
Console Electrical Schematic
Ignition feed, lift switch and the reverse relay that raises the axle when reverse is selected, with the relay terminals called out.
Two sides, two sheets
The console carries a pneumatic side (regulator and gauge) and an electrical side (switch and reverse relay), drawn separately so each can be followed on its own.
Gauge feed is a choice
The gauge can read back from the control module gauge port, or be teed off the regulator outlet. Connect one, not both.
Check relay terminals
Terminal numbering on the reverse relay varies by part. Confirm it against the relay actually supplied before wiring.
Control Module Port & Line Size Reference
Connection points on the lift axle control module. Line sizes are the manufacturer's recommendation for adequate airflow.
| Port | Connects to | Line size |
|---|---|---|
| Main supply | Axle air reservoir, downstream of the PPV | 1/2 in |
| Pilot (control) | Downstream of the load bag regulator | 1/4 in |
| Load delivery ×2 | One line to each load (ride) bag | 1/2 in |
| Lift delivery ×2 | One line to each lift bag | 3/8 in |
| Gauge | Return line to the cab gauge — common with load ports | 1/4 in |
Mounting: the control module must be mounted with its exhaust vent facing downward. In any other orientation contaminants cannot clear the valve, which will eventually fail it.
Axle Weight Calculator
Work out how an auxiliary axle changes your legal weight distribution before you spec it.
Speccing a lift axle?
Petra can help you match the axle, control kit, and brake plumbing to your chassis and the weight limits you run under.