Battery drain help please

Fri Feb 27, 2015 8:40 pm

Hi i have just bought a g reg 318i touring, the battery drains overnight. Did some checks today, the white dme relay is causing the drain. Just need to know why it would stay energised with the key out, was thinking poss faulty ignition switch?

Fri Feb 27, 2015 8:50 pm

The DME relay isn't switched directly by the ignition switch - the ignition switch feeds power to a pin of the ECU, which tells the ECU to earth one of the pins of the coil of the DME relay (which has the other end of the coil connected to 12 volts permanent live.
Does the relay click when you pull it out and again when you plug it back in with the ignition on?
Does it still click if you pull it out and plug it back in with the ignition off?

Fri Feb 27, 2015 9:00 pm

Hi Brian, have not tried it with ignition on (the car does start and drive fine), but with ignition off it does click when taken out of its holder and click again when put back in.

Fri Feb 27, 2015 10:08 pm

The test with the ignition on was only to ensure you knew what the 'click' was like. If it clicks when the ignition is off, then it's being powered on, rather than a faulty relay.
One problem we have here, if this car is new to you, is that we don't know if something has failed, or someone has been fiddling.
Check for power between the ignition coil + terminal and body earth (top of RH suspension turret) with the ignition both on and off. Preferably use a 12 volt bulb of at least a few watts to check, rather than a meter.

Sat Feb 28, 2015 10:53 am

Thanks Brian will check that in a day or two and report back. Its probably the case that someone has been fiddling around....

Mon Mar 02, 2015 9:08 pm

Hi had a quick look today, the coil has no feed with the ignition off, the dme relay plug has permenant live at pins 30 & 85, is this normal or should one of these lives be switched? Need a wiring diagram i think. Another fault is the only instrument that works in the cluster is the speedometer, all the others are dead and the only warning light that works is the alternator light. Any help appreciated.

Tue Mar 03, 2015 12:00 am

Permanent live at the two pins of the DME relay is correct - both are connected to plain red wires that come directly from one of the smaller wires attached to the battery + terminal. The relay is turned on by the ECU earthing the brown wire on pin 86 when instructed to do so by a feed of 12 volts + on a plain green wire to the ECU, but for reasons as yet unknown, the ECU on your car appears to be permanently earthing pin 86.
ECU is in the cabin, directly above the driver's right knee. Remove the under dash panel, remove the plug from the ECU, and confirm that the DME now switches off.
Wiring diagrams for the engine electronics are in short supply. All the printed diagrams I have are for US models, and the M40 engine didn't make it over there.
The M42 is a useful guide, although far from accurate in detail, and completely different in the TPS and ignition primary areas. http://www.autolib.diakom.ru/CAR/BMW/19 ... AMS/02.pdf

Tue Mar 03, 2015 11:15 pm

Hi Brian, did try with ecu unplugged and the earth is removed and the relay remains off, remembered i had a spare ecu, off a l reg cat equipped m40 touring, plugged that in and all is well, problem solvedâ˜º. The previous owner had broken the small posi terminal off the battery with the feeds for that relay and wedged it in place, so perhaps this loose connection may have strained or spiked the circuit and damaged the original ecu? Also the lack of working instruments was due to fuse 10 missing completely! Wonder what delights/bodges i will find next.... Just need a non cat m40 ecu now as mine is g reg and not sure if it will run correctly with a cat ecu, although it does start and run for now.

Tue Mar 03, 2015 11:35 pm

M40 cat. ECUs and non cat. ECUs are the same, although a newer car might have an upgraded (or cheaper to make) version with a different type number.
There is a grey/black wire in the engine loom, on the bulkhead, near the centre of the car, that has a single pole plug and socket in it. If your M40 has a cat. (or more specifically, a lambda sensor), this plug and socket should be unplugged, leaving fuel trim in the hands of the lambda sensor. If there is no cat. fitted to the car, the plug and socket should be pushed together, which connects a pin of the ECU to a potentiometer on the side of the air flow meter. This potentiometer is then adjusted to trim the idle fuel mixture to give the required CO level in the exhaust.
Late M40s, produced when cats. and lambda sensors were standard equipment, use a different AFM with the potentiometer deleted, the grey/black wire redundant, and possibly deleted completely.

Tue Mar 03, 2015 11:41 pm

The usual reason for fuse 10 blowing on manual gearbox cars is damage to the wiring loom to the reverse light switch that clips along the gear linkage to the switch on the gearbox.
With fuse 10 blown, you should also not have a heater fan or door mirrors. If these worked, then the wiring has been 'got at'.