Demystifying the BECM (4.0/4.6/P38A)



Introduction
Overcoming Intimidation
Basic Principles
BECM Functions
BECM Design and Operation
BECM Connectors
Diagnosis
Access and Removal
Dismantling the BECM
Replacing a Power Transistor
Replacing a Relay
More Information

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.


Overcoming Intimidation
The BECM is nothing more than a bunch of fuses, relays, transistors, microprocessors and other electronic components. All modern cars have them -- the BECM just concentrates a lot of them in one place. What little information is available on the Internet about the BECM seems designed to intimidate and discourage the do-it-yourself owner. Car Electronic Services' page on the BECM cautions you against even taking the lid off due to the possibility of damaging the CMOS circuits with electricity, and Andy Reavell's report on changing a transistor refers to dealing with this component as "rocket science" and provides warnings and caveats about attempting repairs. My attitude is a bit more cavalier  -- most of the BECM is very well built and rugged, much of it using  good old fashioned parts such as relays. It does contain CMOS logic, said to be very vulnerable to static electricity, but in my 30 years as an electrical engineer I have never had a problem with it. If it was that delicate it would not be used in the harsh automotive environment. In my opinion, the unit is ruggedly built and should withstand a healthy degree of poking, prodding and testing. About the worst that can happen if you mess up is having to get a new one, which you would have had to do anyway. It is true that you need a bit of experience and skill to start messing with the surface mount components on the unit's signal processing circuit board, but those are much less likely to fail than the power handling components on the other, more easily accessible circuit board. Besides, what have you got to lose? <g>.


BECM Principles
The basic principle of operation is simple. Several 60 amp maxi-fuses in the engine compartment fuse box feed power into the BECM via its own fuse box and a handful of relays inside the unit. The BECM receives (low-powered) input signals from various systems in the vehicle, processes the information in its logic and generates control signals. In the BECM power output section, a bunch of power transistors are used as electronic switches to take these small control signals and use them to switch large currents on and off to control and operate the various systems. As in any electronic system, most failures occur in the high power components such as the power switching transistors.

The sections below go into more detail about the specific functions of the BECM, diagnosis of problems, and how to dismantle and fix it.


BECM Functions
Surprisingly, there is more information on the BECM in the regular shop manual than in the Electrical Troubleshooting manual. The following list, drawn from the former, summarizes the things the BeCM controls, supplies information to, or 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 box) and distributes them to the various vehicle systems via the upper row of connectors around the BECM enclosure.

Photo at right: BECM with top cover removed showing top circuit board with BECM fuse box at top of photo, relays on right and power transistors on left.

Direct Connections and Serial Data Buses
Communication between the BECM and the various vehicle systems is mostly by direct connection. However some circuits -- the instrument pack, seat outstations, door outstations, and center console use digital serial data buses for information (not power) communication with the BECM, allowing connection to several points via the same wires. In theory this reduces the number of wires needed, but in practice each data bus comprises no less than ten wires (!) as each of its five connections is duplicated for reliability. The five connections are as follows:
• Feed wire: Battery voltage supply
• Earth wire:  Vehicle earth
• Clock wire:  Reference signal
• Signal wire:  Transmits digital signals
• Direction wire: Identifies direction of signal

Sleep and Activation Modes
When the vehicle is left and all timers have timed out the BECM goes into "sleep" mode which reduces current draw to about 40 mA with the alarm armed. When an input is sensed (eg unlocking the vehicle), it returns to activation mode with a current draw of about 1 amp. This is the cause of some of the battery drain problems some owners have experienced when parked in areas of high radio interference -- the BECM keeps getting "woken up" by the alarm system's RF receiver. If this happened constantly the battery could easily be dead within 24 hours.


BECM Connectors
In order to tap into all the above vehicle functions, there are 19 electrical multi plug connectors on the BECM, in addition to the screw terminals for the more substantial connections to power and ground. Ten of the multi plug connectors are on the front, and four on the right hand side -- all these are easily accessible by removing the seat base trim. There is also one connector on the rear end of the unit, reachable under the back of the passenger seat. The only ones that are really tricky to access are the four on the left side of the unit.

As mentioned above, inside the BECM are two printed circuit boards, arranged one above the other. The upper one contains the power handling semiconductors needed to drive the various circuits. The connectors are also 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.

Diagnosis
Most BECM problems are not especially fatal except those relating to alarm functions, which can lock you out and/or disable the ECM and hence prevent the engine from starting. engine.  For a treatise on these problems see the alarm system operation and diagnosis page.

Most BECM failures relate to simple things such as certain lights or other electrical devices in the vehicle ceasing to operate. Note that before deciding the problem lies within the BECM, it is worth checking carefully that the power supplies and the ground are good. For example, make sure the battery ground cable is not corroded and causing a bad ground. Low battery voltage due to failure of a cell can also cause weird symptoms. Intermittent non-crank can also be a BECM problem. A malfunctioning engine compartment fuse box can also look like BECM failure, as this supplies power to the BECM and many other components. According to Car Electronic Services, if the engine compartment fuse box is OK, erroneous fuse failure codes can be symptomatic of BECM problems.

The most common fault symptoms include mis-operation of headlights, door locks, electric windows, fuel filler flap, indicators, sun roof and especially rear wash/wipe. These faults may be intermittent. Since nearly all these functions 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.


BECM Access and Removal
You can access most of the connecctors on the BECM (except those on its left side) by simply removing the seat base trim and, in the case of the front ones, lifting up the carpet. However if you want to take the lid off to perform internal repairs (see below) you need to remove the unit from the vehicle. This involves taking the seat out -- not too bad a job actually. If the seat base trim is already off, you just have to remove the cover from the seat belt bolt beside the transmission tunnel and unclip the seat belt. Then undo the four bolts that secure the seat to the 47-way adjustment mechanism. After that you can lift the seat up a bit for access and disconnect the multiplugs, allowing complete removal.

Once the seat is out of the way you can remove the heater rear outlet duct and gain full access to the mountings. Of course all 19 connectors have to be removed including the 3 power supply wires and the one ground wire before the unit can finally be withdrawn.


Dismantling the BECM
The lid of the BECM is held on by umpteen small screws with miniature torx heads. The ones around the periphery are tapered metal/plastic screws and the others are machine screws. Once you have found a torx bit small enough and removed the screws, the top panel simply lifts off and you can see the top (ie power distribution) board of the BECM. If you are at all familiar with electronics, the sight of this board will do much to demystify the BECM, which turns out to use a lot of good old-fashioned components including some antique 7400 series logic chips. Dominating the board are the rows of power transistors taking up most of one half, and the ten relays taking up most of the other half.

To follow the traces from the connectors, and to replace components, you need access to both sides of the board. If you unplug the four ribbon connectors located around the periphery of the board (these provide communication with the lower or signal processing board), you can easily flip the power board up vertically as shown in the photo, providing convenient access. 


Power Transistor Replacement
The power transistors used are actually "intelligent" MOSFETS made by Phillips with 5 pins each  -- input, ground, battery, load, and "status". The status pin gives an internally calculated 5 volt logic output based on the device's operating condition. In particular, when the device is supposed to be on, it can sense a low current condition (ie an open circuit) or a short circuit (in which case it shuts down the power MOSFET).  This signal is processed by the BECM lower (logic) board to detect if a headlight bulb is burnt out  etc, and generate appropriate messages on the Message Center. (The transistor can also detect supply under- and over-voltage and overheating, but I do not think those subtleties are used by the BECM). If you are interested, full specs are available on the Phillips website at this link.

Part numbers are BUK202-50Y (20 amp rating) and BUK203-50Y (4 amp rating). However alert reader Brian Ruder reports that the BUK203 50Y has been discontinued and replaced by the BUK 219-50Y (6 amp rating). There are four rows of MOSFETS -- mounted in two sets of back-to-back mountings with a heat sink in between (see photos below). The transistors are held against the heat sink by spring clips.  If one of them is defunct you might be able to see blackening or other signs of failure but not necessarily.

If you are used to working with printed circuit boards, the replacement of one of the MOSFETS presents no special challenges except that the pins are soldered on both the top and the bottom of the board. First remove the corresponding heat sink clip (and the ones nearby to improve access). Heat up each top and bottom joint and suck the solder off them as best you can. Then, when you wiggle the transistor while applying heat to all pins at once (as best you can), it should come out.

If you are not accustomed to these procedures, a surer way of removing a transistor without overheating and ruining the board traces is to cut off all 5 of the pins above the board, releasing the body of the transistor. Then you can work on removing the pins from the board one by one, and clearing out the holes so you can insert the new transistor.


Relay Replacement
The relays are all Siemens and NEC parts (unfortunately about 6 different types), and are soldered on the board instead of using sockets. This does not seem to be the brightest idea as sooner or later they are bound to fail, but probably not before your warranty has long ago run out!!

The relays appear to be soldered only on the underside of the board, and their removal is less of a challenge. Unsolder the joints (here again a solder sucker is useful but not essential) and yank them out. Clear out the holes so the new relay can go in.                                             

More Information
Meanwhile here are some related pages and links:

Alarm System Operation & Diagnosis
Remote/key Handset Problems and Solutions
Replacing the BECM, ECM, Remotes etc
Ultrasonic Alarm Sensor Problems and Repair
Diagnostic Scanners for Range Rovers
Car Electronics Services (UK) page on BECM problems (they offer BECM rebuilding for countries outside North America & Australia)
Andy Cunningham's page with Andy Reavell's report on replacing a blown transistor on the BECM power board