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23 Posts
Discussion Starter · #1 ·
Hi gang-

Has anyone experienced their P38 BECM staying "awake" from the RF receiver. I know that some on the earlier models had some problems. It appears my battery drain may be related to this issue. Has anyone experienced this on a later model P38? Short of moving, any ideas what kind of household wireless devices may be keeping my BECM awake? I do not live near radio towers or cell towers. Any ideas??

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490 Posts
Do you know for sure your 4.6 BeCm is "staying awake?"
The following is a test I did on my 95 4.0 and the car passed the test.
I got the test from
Battery drain on the P38A can often be caused by the BeCM being kept awake by one of a multitude of inputs. These include:
Alarm RF receiver - anything on the right frequency will cause the BeCM to awake. There are known issues with early receivers being prone to interference, and all but the latest can be kept awake by other equipment on the same frequency band.
Bonnet or door open switches.
Pretty much every cabin switch
To test for sleep mode:

Switch off interior lights (hold central button on light above centre of dash for several seconds)
Open Drivers door window and close door.
Insert Ignition Key and turn to position 2
Turn to off and leave the key in the barrel
Open the drivers door and the key-in message will be displayed on the instrument pack
The interior lights should still be off.....
Wait for 2 minutes (sometimes closer to 2 minutes 30 seconds) for the BeCM to go to sleep.
If you can still see the Key-In message then the BeCM is awake.
If the message has gone, and you do something to wake the BeCM (try opening the bonnet), you'll get a bleep from the dash. This indicates that the BeCM has been awakened.

If you do this with an ammeter in series with the battery lead you'll see the current draw around 0.6 to 1A when the BeCM is awake and more like 0.05A when it's asleep.

Of course, you are now left with *what* is awakening the BeCm The following inputs will wake the BeCM:

The Radio RF receiver is a favourite for picking up interference. More on this here.
Another favourite is the bonnet close switch - make sure the bonnet is closed properly.
Armed with this information you can at least get some idea of whether the BeCM is awake and what might be causing it.

Good Luck!

· Registered
19 Posts
Becm staying awake

I have encountered battery drain through an alternator and on another car
a faulty CD changer.
when you do the Becm sleep test pua an ammeter in the circuit from the battery you will also see the voltages drop as the things shut down I can not remember the sleep mode voltage reeding but i think it is about 250ma if it is above that you can isolate circuits by removing fuses to find the faulty circuit
good luck

· Registered
490 Posts
To echo MG-France, the '92 RR we have with factory CD will drain the battery in a few days or even two days in real cold if the car sits. The CD constantly "remembers" the position of the disc so it starts there when you turn it back on. Our 4.0 doesn't seem to have this problem though. Let us know how the sleep test comes out when you get a chance.
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