Outside Fuse Box
The engine compartment fuse box on the 4.0/4.6/P38 is positioned behind the battery, so in addition to the usual heat cycling stress in the engine compartment, it is subject to corrosion problems due to corrosive vapors from the battery making their way into the fuse-box. Keeping your battery terminals clean and the battery maintained helps to avoid the corrosion problem. Another problem can be spilled coolant getting into the fuse-box. Photos below show the effect of this. We recommend covering the fuse-box with plastic before filling the header tank with coolant.
Although fuse-box problems can lead to a variety of weird symptoms (sometimes even leading you to think there are BECM issues), the most common problem is HEVAC related due to burning out of Relay 7 and its mounting terminals. There are different theories about why this happens; I have heard from several Rover techs, and ex-Land Rover mechanic, that the most frequent immediate cause of burned out terminals on the fuse box is clogged pollen filters imposing additional load on the climate control system blowers.
They advise changing the pollen filters much more frequently than the recommended 30,000 miles to prevent this. (See the Pollen Filter Replacement page to find out how dirty they get even in normal street use). , who is an electronics technician, and I (an electronic engineer) have doubts about this as reduced airflow normally reduces the load on any blower or pump.
Another Engineer found a more logical cause -- a loose primary wire on the Left Hand blower motor which, when bouncing during offroading, would intermittently make contact, creating not only an arcing condition at the blower motor but on/off surges at the RL7 relay. If your fusebox does eventually give up, instead of replacing it you can repair it using the following ingenious illustrated procedure supplied by Ron Beckett.
Symptoms and Diagnosis
Note that fuse-box problems can cause a variety of symptoms. Although the most common are in the Hevac system or can also get strange things happening that look like BECM failure (see below). Even experienced mechanics can be fooled by this, thinking the BeCM needs replacing -- every 4.0/4.6/P38 owner's nightmare. Instead the problem can often be cured at minimal cost.
Hevac Problems and RL7 Failure
The most common problem in the fuse box is failure of the relays that power the electric fans -- particularly RL7. You can inspect the relays at the front of
Note on Causes: Clogged pollen filters are commonly blamed for this RL7 problem, but that makes little sense as it would reduce the load on the blower motors. Also Thomas Dirksen has astutely observed that if the pollen filter hypothesis was correct, both left and right blower motors would be equally affected, ie RL7 and RL6. However nearly all problems seem to be with the left one (RL7) alone. In Thomas's case the cause was a loose wire on the motor itself; this would be well worth checking on to avoid a repetition of the problem. For access to the blower motors, see the pages on HEVAC Blend Motor Replacement and Dashboard Removal and Replacement.
Dismantling & Repair
To remove the fuse-box, disconnect the battery leads (always remove the earth/ground lead first - then it won't matter if your wrench shorts across from the positive terminal to the body. Remove the fuse-box cover and undo the nut holding the battery lead onto the stud (see below).
Remove the three bolts holding the fuse-box in place and carefully turn it over to access the connectors underneath. Depress the protruding clip on the connectors and gently work them off the box. (Note these pix are for an early fuse box. Later ones changed but Ron is unsure of the changes made to the fuse box.)
Unbolt the three BeCM power cables from the bottom of the fuse box, noting their colors and to which stud they are attached. The nuts (see arrows) are captive on the power leads so you won't lose them.
Remove 5 screws (yellow arrows); release clips (brown arrows).
Once the board is out, you'll soon see the damage - see pix below. Note also the discoloration of the PCB near RL12, the fuel pump relay. Close examination of the pin showed that the gap (arrowed) in the pin for the blade of the relay it was slightly wider so it hadn't been making good contact and had overheated. Note the corrosion from coolant that had been spilled into the fuse-box
Unbolt battery lead from fuse box.
Underside of fuse box - power leads arrowed
With the fuse box out and on the bench, note the location of every relay and fuse. If possible, take a photo for reference. You'll note from the pix below thatthe fuse box is not fully populated with relays and fuses as Australian delivery HSEs were not fitted with heated windshields. SE models may have different relays, too, depending upon the options fitted.
Only RL7 (lower right) has been removed. Note broken left hand side of fusebox.
All relays and fuses removed to allow removal of the cover panel over the printed circuit board.
Now come the delicate bits - removal of the printed circuit board (PCB) from the case, which may be embedded by years of high under-hood temperatures. Refer to the pix below. Remove the 5 Phillips head screws and carefully lift off the panel that you can see - the one with the relay numbers and the green and yellow stripes.
Removal of the lower case requires gently levering the clips located around the sides of the case and prising the case off the PCB whilst simultaneously pressing on the three BECM power studs and the pins of the connectors on the bottom of the case. Once the PCB has started to move you can gently work it out of the case by pressing on the studs and pins underneath.