City buses, intercity coaches, articulated transit vehicles and school buses share a pattern that separates them from trucks: the electrical system runs the length of a body that is mostly occupied by people, and one whole layer of it is operated by passengers rather than by the driver. Youlai supplies these programmes from four product families — power distribution, smart control modules, switches and sensors, and displays — under an IATF 16949 certified quality system in Changsha, China. This page is the horizontal view of how those families fit together on one vehicle; the buyer guides linked at the end of each section go deeper into individual selection.
1. How the electrical system on a bus is organised
The topology looks like any commercial vehicle — battery, protection, distribution, control layer, bus, cluster — but two things stretch it. The first is length: a 12-metre rigid or an 18-metre articulated body cannot sensibly be served from one box behind the dashboard, so distribution becomes several nodes along the vehicle. The second is that the saloon is an input domain of its own — stop-request points, accessibility requests and occupant emergency devices, operated from dozens of positions by people the driver cannot see, whose behaviour has to be agreed before the panel layout rather than after it.
System architecture · teal = the body power trunk and the local zones it feeds
Power path
- Battery and isolatorMain feed, master switch and high-current protection
- Main distributionHigh-current protection and the handoff into a trunk that follows the vehicle body
Body power trunk One protected longitudinal feed; branches leave near the loads, not back at the dash
- Front zoneCab and front doorDriver equipment, front entrance, accessibility devices and local lighting
- Centre zonePassenger saloonStop-request receivers, saloon lighting, HVAC and articulated-body interfaces
- Rear and roof zoneRear body and auxiliariesRear doors, destination and exterior lamps, roof equipment and rear HVAC
Signal path
- Body-function requestsDash switches from the cab; stop-request and accessibility devices from the saloonWireless request → receiverThe 433 MHz hop ends at a receiver; only the receiver's output becomes controller or bus content
- HubBody controllerHolds the latch and annunciator logic behind the request points, drives body outputs, one node on the bus
Shared vehicle bus — CAN, commonly J1939 on the driveline side · a backbone, not a stage
- Engine or traction controlPublishes speed, load, temperatures and faults; on an electric bus the drive and battery systems take this role
- Door, HVAC and ramp controllersUsually the body builder's or door supplier's modules — integrated across an interface rather than supplied
- Cluster and saloon annunciatorRenders what other nodes publish; originates almost nothing itself
Two boundaries decide most of the cost, and only one of them is electrical. The first is where the chassis ends and the body begins: these platforms are frequently bodied by one company on a chassis from another, so ownership of the harness, the network content and each function is split across two engineering organisations before a component supplier is chosen. Settle early which side owns the body network, which signals cross that interface and who integrates the door, ramp and HVAC controllers — left until after harness release, that question lands in the loom, the software and the cluster at once. The second is service class: a low-floor city bus with standing passengers and a duty measured in stops per shift is a different electrical specification from an intercity coach with seated passengers and long continuous runs, even where the two share a chassis.
Beyond those two, where on the vehicle a part lives decides its enclosure as much as its function does:
Dry, enclosed
Driver cab
Cluster, dash switches, head-up display, receivers and cab-mounted controllers. IP53 or IP54 is defensible here because the cab structure carries the sealing duty — but only inside the trim, away from the door aperture.
Occupant-facing, cleaned
Passenger saloon
Stop-request points, accessibility devices, saloon lighting and annunciators. The environment is mild; the duty is not. These parts are pressed by untrained hands hundreds of times a day, so cycle life, actuation force and marking durability outweigh ingress class.
Wet, sprayed
Underfloor and chassis
Distribution nodes, the battery and isolator compartment, wheel-end sensing and the runs between them. Road spray, kerbside water and depot washing all reach here, so sealed enclosures, sealed connectors and gland exits belong in this zone — not a cab-grade box relocated underfloor.
Weathered, hot
Roof and exterior
HVAC packs, destination signs, exterior lighting, aerials and, on an electric bus, the traction and charging equipment. Long runs, direct sun and — on an articulated vehicle — a cable path across the turntable that flexes on every turn, which makes routing and strain relief specification items.
2. Distribution across a long body
Distribution hardware is specified first and changed least. On a bus the question is not only how many ways a box has, but how many boxes there are and where the split falls — the alternative to a second node is another twenty metres of copper down the vehicle and back. The NBX series covers that range by way count and by sealing class, with NBX-957 as the reference for the body central position: 9–32 VDC, so one platform serves 12 V and 24 V programmes. Circuit map, fuse ratings and relay count are always quoted against your harness drawing rather than read off a listing.
| Model | Sealing | Where it belongs on a bus body |
|---|---|---|
| NBX-957 | IP54 (IP65 variant) | The body central position. Specify the IP65 variant if the chosen locker is not genuinely dry. |
| NBX-970 / NBX-958 | IP54 | 42 and 32 circuits — interior or panel-protected positions on high-feature coach and large-bus platforms. |
| NBX-981 | IP54 | 15 ways: a dash-side group, not a body node. |
| NBX-952 / NBX-954 | IP65 | Sprayed or semi-exposed positions. NBX-952 keeps CAN body logic in the box; NBX-954 is the passive twin for programmes where a separate controller owns it. |
| NBX-969 / NBX-971 | IP65 / IP67 | Underfloor and chassis-mount positions where a cab-grade enclosure cannot be justified; configurable contents, sealed connectors, gland exits. |
Size each node from the frozen load list plus 10–15 % spare ways, then match sealing to the mounting zone rather than to the vehicle type. The step most often skipped is agreeing the depot cleaning regime: an IP54 box is correct in an interior locker and wrong in a wheel-arch position that meets a lance every night, and those two can be a metre apart on the same vehicle.
What changes on a battery-electric bus?
The 24 V side does not go away; it is fed from the traction pack instead of an alternator, which turns the converter from an accessory into a primary component. EBX-2314 is the reference at bus scale — a 36 kW HV DC/DC onto the 24 V auxiliary bus that also runs the high-voltage pre-charge sequence. Treat the enable path, not just the power rating, as a specification item: it carries a hard-wired line alongside CAN so the auxiliary bus can come up when the network is not available. The rest of that domain — pre-charge ownership, the pack boundary, what an EV cockpit has to display — is set out in new-energy commercial vehicle electronics; everything on this page still applies on top of it.
Protection on the pack side is a separate discipline. JDK-2509 is a 1500 VDC battery-pack fuse in the aBat utilisation category of IEC 60269-7, with a mechanical striker that shows fuse state without electrical interrogation in the depot. Where a 12 V branch coexists with the 24 V system, EBX-2407 handles step-down and dual-battery balancing in one IP67 unit; smaller isolated branches are served by EBX-2515.
Going deeperFuse box guide for what goes inside the box, relay vs fuse vs junction box for which enclosure type a position needs, IP67 distribution boxes for sealing construction, and the selector for every NBX model side by side.
3. The passenger request layer
This layer has no equivalent on a truck, and it is where a bus programme most often runs late. A stop request is not a switch closing a load: it is an input from an untrained user that has to be captured, latched, annunciated to both the driver and the saloon, and cleared at the right moment. The button is only the first of those four; the other three belong to a controller that is frequently specified after the buttons have already been counted.
JDK-2306 is the wired reference: a self-return momentary device on 9–36 VDC, so one part covers both 12 V and 24 V platforms, rated for 200,000 operations. What makes it a bus part rather than a switch is the rest — green in standby and red on press, so confirmation reaches the passenger without looking towards the driver; a flush or stanchion mount, which decides how many request points a saloon layout can carry; and tactile vibration with braille and a raised STOP legend for passengers who cannot see the indicator.
Where running a cable is the expensive part — a stanchion mid-saloon, a folding-seat area, a retrofit — TDK-2406 and TDK-2407 are the battery-powered pair, transmitting at 433.07 MHz and rated for 300,000 operations. TDK-2406 is the general request; TDK-2407 is the accessibility variant.
Why does the accessibility request need its own channel?
Because the driver's response is different in kind, not just in urgency. A general request means stop at the next station; an accessibility request may mean kneel the vehicle, deploy the ramp and hold the door longer, and the driver needs to know which one arrived before the vehicle comes to a stand. Keeping the two on separate physical paths from button to annunciator is what makes that distinction survive integration.
EBX-2404 is built that way: the receiver carries the general request and the accessibility request on separate output pins, so the cluster or annunciator subscribes to each independently, and it pairs up to 16 transmitters. The fleet-facing behaviour matters more than the headline numbers — pairing times out, a long learn-button press clears all records for a fresh provisioning cycle, and a debounce window stops a crowd of near-simultaneous presses chaining into repeated requests.
Note what none of these devices do. They are all momentary; the latch, the “stopping” annunciation and the clear-on-door-open behaviour live in a controller. EBX-954 is the logic reference for that role, but it is catalogued as a heavy-truck body controller — agree the actual module and its network content per programme.
Occupant emergency devices are a separate function, easy to conflate with a stop request and best kept apart. TDK-2419 is a bus EDSS unit: an emergency-stop trigger under a protective cover with status and warning indication on the same body. It sends the request; the vehicle's EDSS control performs the controlled stop, which is why its ratings are built to the vehicle's EDSS definition rather than published as a catalogue envelope. Where the requirement is cab or chassis isolation instead, JDK-2425 is the direct-acting device — a mushroom-head emergency stop and cut-off at IP67.
Going deeperThe stop request button guide sets wired, wireless and accessible options side by side, the TDK-2406 / TDK-2407 page covers the 433 MHz link and pairing in detail, and the switches and sensors guide covers the range.
4. Doors, cab controls and what stays hard-wired
Start with a boundary, because stating it saves a round of clarification later. The passenger service door itself — the leaf, its drive, its obstacle detection and its emergency release — sits with the body builder or door supplier, not with us. What surrounds it does not: the distribution node feeding the door controller, the driver's switch inputs and indicators, the interlocks routed through the body network, and the harness interface between the two.
In the cab, the parts are the same family a truck uses. EDK-908 is the driver's-door master switch — window lift, central locking, mirror adjust, select and heat in one 24 V panel at IP54, switching on its own outputs with a LIN line so it can also sit on the cab's low-speed network. Where those functions move into a module, EBX-2163 is the door control module built for it, on CAN with a LIN port reserved. Both are cab hardware, and their datasheets are worth reading precisely: “passenger window” means the nearside cab window, not a saloon door, and IP53 / IP54 puts them inside the trim rather than in the door-aperture wet zone.
Which bus functions should not depend on the network?
Every bus has a short list, and it separates into two tiers that should not be written as one. The statutory and safety tier — occupant emergency devices, emergency door release, battery isolation — has its behaviour, independence and category set by the vehicle's approval rather than by convenience; the emergency door release in particular is a construction requirement of the vehicle, not an electrical feature. The availability tier — hazard lighting, saloon emergency lighting, a ground-level stop — is hard-wired so it stays usable when the network is not, a different justification with a different acceptance criterion. Say which tier each function belongs to, because that decides whether the answer is a rated device or simply a separate wire.
For the rest of the dash, the choice between hard-wired, LIN and CAN switching is decided by switch count and diagnostics rather than by preference. EDK-907 keeps a hard-wired backup path alongside its CAN pair, a useful hedge while body content is still being defined, though at IP53 it is a dashboard part that belongs inside the trim. EDK-2507 packages four buttons and a rotary input at IP66 on CAN — sealing a bus dash rarely needs, but a door-pillar or exterior position might.
Going deeperCAN vs LIN vs hard-wired switching is the interface decision in full, door switch panels the LIN panel a cab door needs, door control modules the module side including anti-pinch, and CAN switch panels multiplexed keypads.
5. Driver information and indirect vision
The display layer is specified last because most of what it shows is decided elsewhere: the cluster is agreeing content with the driveline, the body controller, the door and HVAC modules and the request layer at the same time, so its specification is really a set of agreements about who publishes what. Camera video is the exception, reaching the display on its own path rather than as bus content — which is why coverage and display are specified together instead of inherited from the CAN matrix.
PBX-2301 is the 8-inch combined cluster: an IPS panel between two analogue dials, 9–32 VDC, two CAN channels and a built-in heater so the LCD starts readable in sub-zero conditions. Its two AV inputs each carry a switched camera supply, which is the practical reason camera coverage and cluster are specified together. Read its catalogue position before quoting it: it is listed in plateau and new-energy configurations, so the transit or coach variant is agreed at RFQ rather than assumed.
Where the programme wants speed, navigation or lane content in the driver's line of sight, PBX-961 is a windscreen head-up display with content dispatched over the vehicle bus and image geometry customised per cab and windscreen rake; its listed target classes include coach cabs. Catch the electrical boundary early — an 18–32 VDC input makes it a 24 V part. PBX-2203 is the combiner variant for cabs whose windscreen geometry or dash depth will not take a windscreen unit.
When does a coach need a camera-monitor system rather than mirrors?
Approval usually settles this before preference does. Under UN R46, a camera-monitor system may replace a mandatory mirror only where the device holds R46 approval for the mirror class it replaces, and the required field of view has to be demonstrated on the body it is fitted to, with its own camera geometry. Coverage on a long or articulated body is therefore measured on the vehicle rather than copied from a generic figure, and the deliverable is a validated installation, not a screen. What an operator buys is blind-zone reduction; what the engineer manages in exchange is monitor position, image latency and the driver's scanning pattern. That is also why our own camera-monitor hardware, PBX-955, is quoted as a project-configured system rather than a catalogue line: camera count and placement, monitor format and the field of view to be demonstrated all follow the body it is fitted to, so a baseline configuration is a starting point for the discussion rather than a specification. The first question to settle on any such enquiry is which role the system is being bought for — a supplementary view alongside the mandatory mirrors, or an approved replacement for them — because that decides the approval route and the evidence package long before it decides the hardware.
Tyre pressure monitoring is the other stream worth settling early, because it is cheap at design and awkward as a retrofit. EBX-957 is the IP67 receiver, reporting pressure, temperature and wheel motion over CAN; YDK-902 is the valve-stem sensor it pairs with, and YDK-903 the clamp-mount alternative catalogued for off-highway rims rather than transit. One integration detail surfaces only when wireless stop-request and TPMS share a vehicle: the two receivers sit on adjacent parts of the 433 MHz band (433.07 and 433.92 MHz), so antenna position and the coexistence check belong in the harness drawing rather than at vehicle build.
Going deeperThe instrument cluster guide covers cluster selection, HUD vs digital cluster compares formats, commercial-vehicle HUD the head-up layer, mirror systems powered mirrors through to camera monitors, and heavy-truck TPMS the sensing side.
6. What makes a bus and coach specification different
A functional match is not enough on a bus. The same module can be correct behind the dashboard and wrong two metres away in the doorway, and a part that suits an intercity coach can be under-specified for an urban duty that works it three hundred times a day. Most of what separates a bus specification from a truck one lands in the rows below.
What has to be validated for a bus body?
| Constraint | How it lands in the specification |
|---|---|
| Duty cycle | The most under-specified item on transit programmes, and it is a count rather than a number of hours: stops per shift, door cycles per stop, request presses per stop, kneels and ramp deployments per day. The published cycle figures on occupant-facing parts exist for exactly this. State the daily count and the service life in years, and let the two numbers meet. |
| Occupant-facing hardware | Parts a passenger touches are judged on properties that rarely appear on a datasheet for an underfloor box: actuation force and travel, tactile and visual confirmation, marking legibility after repeated cleaning, and housing flammability. Interior material and fire-behaviour requirements are set at vehicle level by the body's approval, so confirm which schedule applies before the material is frozen. |
| Ingress, in the right code | Saloon and cab parts here are quoted in the ISO 20653 automotive code — IP5K2 on JDK-2306 and EBX-2404 — not IEC 60529, and the two notations are not interchangeable. Give the code as well as the number, name the depot cleaning method, and say whether any position is jetted: an IP67 claim covers temporary immersion and says nothing about a lance at close range. |
| Voltage and auxiliary load | Commonly 24 V, and much of the catalogue reflects that — EBX-954, EBX-2163 and PBX-961 are 18–32 V parts, while distribution and the request layer are wider. A 12 V programme is workable but has to be declared rather than inferred, alongside the isolator arrangement, cranking and jump-start expectations and the continuous load with HVAC running. |
| Accessibility and vehicle approval | Buses and coaches are approved as complete vehicles. In the UNECE framework UN R107 covers the construction of category M2 and M3 vehicles, and its 11 series of amendments, in force since May 2026, was adopted specifically to improve accessibility on class I urban vehicles; applicability differs by contracting party. The approval belongs to the vehicle manufacturer — what a component supplier owes it is defined behaviour and evidence: a separate accessibility path, published force, travel and cycle figures, and the feedback and marking features. |
| Indirect vision | UN R46 governs devices for indirect vision. A camera-monitor system stands in for a mandatory mirror only where the device holds R46 approval for the mirror class it replaces, with the field of view demonstrated on the body it is fitted to — hence settled per programme, not per catalogue line. |
| EMC | Buses and coaches are road vehicles, so the route is UN R10 rather than the machinery route used off-highway. The 07 series has applied since 12 June 2025, with 06-series approvals accepted transitionally until 1 September 2029, and it distinguishes residential from non-residential operating environments with different conducted emission limits — a live decision for a fleet working inside a city, so agree which environment approval is sought against before the limit lines are fixed. Certified testing is performed at third-party accredited laboratories when the programme requires it. |
| Service life and change control | Buses stay in service for a decade or more and are maintained by depot staff rather than a dealer network. Parts availability over that window, connector continuity, change control on MCUs and materials, and the diagnostic route a depot technician will actually use all belong in the specification rather than in the relationship. |
Route conditions are a specification input rather than a boundary on where we work — the Middle East / GCC and Southeast Asia pages show how one catalogue is filtered for Gulf dust and heat versus mud and humidity. See sourcing OEM vehicle electronics for how IATF 16949, APQP and PPAP divide responsibility, and IP65 / IP67 protection for how sealing claims are validated.
7. Specifying a bus or coach programme
Quotes come back accurate when the request carries the facts that decide the hardware. For a bus or coach programme that means:
What should a bus or coach electronics RFQ include?
- Vehicle and body — service class, length, rigid or articulated, floor height, door count and positions, capacity, and whether chassis and body come from the same manufacturer.
- Electrical limits — nominal voltage, isolator arrangement, cranking and jump-start requirements, and the continuous load with HVAC running.
- The load list by zone — every circuit with its current, duty and switching method, grouped by zone rather than as one flat list, because the zone decides the enclosure.
- The passenger interface schedule — how many request points and where, wired or wireless at each, how many accessibility points, the annunciation expected, and which module owns the latch.
- Door, ramp and interlock ownership — who supplies the drives, which interlocks cross the chassis-to-body interface and in what form.
- Network architecture — CAN channel count and application layer, any LIN sub-nets, the DBC or signal list if one exists, and any RF bands already in use.
- Mounting position and evidence per part — the zone, cleaning regime, temperature, vibration profile and orientation, and the reports required at each validation gate.
- Approval route — which framework and edition the vehicle is approved under, which functions carry a safety or accessibility requirement, and who holds the approval.
- Connector preference and the harness drawing — on a long body most re-work traces back to a connector or sealing assumption made without the drawing in front of both parties.
- Commercial inputs — markets, annual and lifetime volume, sample stages, tooling ownership, PPAP level and SOP date, which decide validation depth and timing.
The OEM RFQ checklist puts this in a form you can send. Manufacturing is IATF 16949 certified with in-house environmental and EMC pre-compliance testing, and PPAP deliverables and the submission level are agreed against the customer-specific requirements and the released drawing revision.
Send the load list, the passenger interface schedule and the harness drawing and we will come back with a proposed hardware split across the four families. Use the contact page or message +86 134 6767 4786 on WhatsApp — typical reply within one business day.