​Technical Data BecM  aka: more data

Demystifying the P38 BeCM

Photo at right: BeCM removed from position under left front seat to show connectors and fuse box. Note that most of the connectors (but not all) are in the front. The BeCM fuse box is on the right hand side of the unit and is accessible via a removable plastic panel in the seat base trim.

Introduction

With the introduction of the Range Rover 4.0/4.6, Land Rover took a bold step into the electronic age, and concentrated a majority of the vehicle electrical brainpower and horsepower in one box -- the Body Electrical Control Module (BeCM, sometimes alluded to in the manuals as BeCM). Later models have retreated slightly from this ideal, with brainpower distributed more evenly around the vehicle according to function, but the BECM days stand out as a bold experiment. In this section we try to demystify the BeCM, summarize its functions, and help with diagnosing and repairing the kind of problems people have experienced with it.

Receives input from

1. Instrument pack
2. SRS Diagnostic Control Unit (DCU)
3. Selector lever display panel (Automatic transmission vehicles only)
4. Engine compartment fusebox
5. Maxi fuse 1 - Power supply
6. Maxi fuse 4 - Power supply
7. Maxi fuse 5 - Power supply
8. Cruise control ECU
9. Transfer box ECU
10. Engine Control Module (ECM)
11. Electronic Automatic Transmission (EAT) ECU
12. ABS ECU
13. Electronic Air Suspension (EAS) ECU
14. HEVAC ECU
15. Center console switch pack
16. RH door outstation
17. LH door outstation
18. RH seat outstation
19. LH seat outstation
20. ICE unit
21. Diagnostic Socket

BECM Design and Operation

Signal and Power Handling
The BECM, located under the right front seat, consists of an enclosure containing two main circuit boards arranged one above the other. Broadly speaking, the bottom one is a low power board that accepts inputs from the vehicle's systems (via the lower row of connectors on the front and right of the BECM) and uses its computing power to do the "thinking".

Its decisions are sent to the solid state switches on the upper board, which gets its amps from the battery terminals on the right side of the BECM (via the adjacent BECM fuse arranged in rows near the top or bottom of the unit. Thus the upper connectors (those on the left, rear, and top front of the unit, plus the BECM fuses on the right) tap into this upper board and are mainly power outputs. The bottom row of connectors on the front and right of the BECM tap into the lower board containing the signal processing and logic circuitry, and are mainly inputs from sensors and switches, plus low-current output signals to other ECUs. The following pictures and listings indicate the locations and functions of the different connectors.

Top row connectors provide power to the following circuits:
C325/1283: Rear lights & indicators, door locks, aerial amp, RF receiver.
C323/1284: Mirror dip, RH doors, RH rear window & ICE amp; sunroof anti-trap, trailer tail lamp, fuel flap.
C324/1292: Sunroof back/forward, rear screen heater, power to sun visors.
C120/1281: LH Front lights & Indicators, SRS, alarm sounder, front and rear screen wash, horn, ignition switch position 2.
C113/1282: RH Front lights & indicators, starter relay, window & headlamp wipers.

Photo: Connectors on Front of BeCM.
Top Row of connectors provide power output to the circuits listed above.
On 1999 and later models these were renumbered as follows (left to right):
C1283, C1284, C1292, C1281, C1282.

Bottom Row of connectors are inputs to the BECM, or output signals to other ECUs and systems in the vehicle. See listings below.
On 1999 and later models these were renumbered as follows (left to right):
C1288, C1289, C1285, C1286, C1287.

C112/1283: Inputs: Trans oil temp, engine speed, ABS pressure & operation, cat overheat (Japan).
Outputs: Parking Brake On, Door/Tailgate Open, Ignition switch position 1, Road Speed, Fuel Level; Security Code to ECM.
C114/1289: Inputs: Washer fluid level, inertia switch tripped, front wiper parked, MIL on, low oil pressure, EAS warnings, fuel level, road speed,
ETC warnings, hood open, alternator charge, low brake fluid, coolant temp.
Output: Engine Speed.
C326/1285: Inputs: Rear wiper park, front courtesy lamp switch, tailgate open, sunroof anti-trap, RH rear window, RH front door data bus, sunroof closed,
RH mirror position, RH rear door open, RH front window anti-trap.
Outputs: RH front door data bus, reverse gear selected.
C362/1286: Inputs: LH front door data bus, LH windows position and anti-trap, LH rear door open, ultrasonic sensor.
Outputs: Ultrasonic sensor power, LH front door data bus, LH rear window anti-trap power supply.
C626/1287: Inputs: Transmission diagnostic, center console data bus, gearbox selector switch, transfer

Photo: Connectors on Right Side of BeCM.
Top Row of "connectors" (really just terminal posts) provide the power supply to the BeCM from the battery via Maxi Fuses 1, 4 and 5 in the engine compartment fuse box. They provide power via the BeCM fuses listed above.
On 1999 and later models these terminals were renumbered as follows (left to right in the picture): C1356, C1336, C1337, C1338.

Bottom Row of connectors are inputs to the BeCM, or output signals to other ECUs and systems in the vehicle. See listings below.
On 1999 and later models these were renumbered as follows (left to right): C1288, C1278, C1280,

Bottom Row of Connectors:
C907/ 1288: Inputs: Left and right seat positions Outputs: Left and right front seats serial data buses
C257/1278: Inputs: On/Off for front & rear screen washers, rear wiper, right & left indicators, headlight flash, hi & lo beam, front fog lights & side lamps, fuel filler flap release, horn. Front wiper switch position, ignition switch position III, front fog telltale. C256/1280: Inputs: Instrument dimming supply, ignition switch position I
Outputs: Instrument pack serial data bus (8 lines) & audible warning, engine & road speed to instrument pack, rear fog telltale, security LED. C255/1279: Inputs: On/off for rear fog light, key in ignition, clutch depressed, seat heaters on/off, alternator charging/engine running, rear screen heater, hazard warning, cruise control, diagnostic connector K & L lines.
Outputs: Diagnostic connector K & L lines, hazard warning telltale lamp, road speed to trans & cruise control ECUs, power to cruise ECU, telltale light & inverter.

Rely on power from the BECM it is possible that their malfunction is due to a BECM failure. If you have a copy of the Electrical Troubleshooting Manual, you can trace any circuit to see where the problem is.

For example, if the front fog lights are not getting power from the BECM even though the BECM input from the fog light switch is OK, first make sure power is getting to the BECM via the 3 maxi fuses in the engine compartment fuse box. If so, the problem must lie within the BECM itself.As in all solid state circuitry, by far the most likely failures are going to be in the power handling parts of the circuit, namely the high powered transistors that directly feed the upper row of BECM connectors. The next most likely possibility is failure of one of the relays that populate a large section of the power board.

From the information above you can tell which connector is affected, and the ETM will even tell you which pin. Then by taking the lid off the BECM (see below) you can trace the connection from the output connector back to the corresponding power transistor or relay.