Bmw Troubleshooting

8/3/2019 Bmw Troubleshooting

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For the engine to run the following conditions must be met:

Power on DME pins:

27 Start Input

18 Un-switched Power input37 Power Input from Main Relay

Ground on DME pins 2, 14, 19, 24

Timing data from the CPS on DME pins 47 & 48 from a rotating engine

To have spark power must be present at the coil positive and ground

pulses

from the DME's pin 1 must reach the coil negative. Power to the coil iscontrolled by the ignition switch via C101. When checking for spark,

use the

output lead from the coil to eliminate the distributor, rotor and plug

wires.

To have injector firing power must be present at each injector and

ground

pulses from the DME's pin 16 (Bank1) and pin 17 (Bank2) must reach

therespective injector bank. Note that the injectors are wired as two

banks of

three. With cylinder 1,3,5 being bank 1 and 2,4,6 being bank 2. Power

to the

injectors is controlled by the main relay.

The fuel pump relay must have power on pin 86 (relay coil) from the

main relay

output (pin 87) and power on pin 30. The DME will ground pin 85 to turnon the

relay and power the pump(s) via pin 87. Of the above, only the fuel

pump power

is fused. So if the there's power at pin 87, but not at the pump, check

fuse

11.


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The main relay and DME pin 18 receive power from the smaller of the

to wires

that connect to the battery's positive terminal. That wire incorporates

anin-line fuse. When the DME is presented with a start signal, it grounds

the

main relay pin 85 and furnishes power to the fuel pump relay,

injectors, and

DME.

Troubleshooting:

Disconnect the battery and the DME cable. Then:

1) Disconnect the coil negative and check continuity from that

connector to

DME pin 1. Also verify that from DME pin 1 to ground is an open circuit.

2) Check the resistance across DME 47 & 48, which should be 500-560

ohms. If the CPS is dismounted, the resistance can be seen to change

from about 500 to 540-540 when a ferrous object is brought to the face

of thesensor. Neither pin should be grounded.

3) Check for continuity from DME 36 to main relay 85 and from DME 3

to fuel

pump relay 85.

Reconnect the coil, remount the CPS (air gap should be 0.040"), plug

the

relays back in, reconnect the DME, and connect the battery. Then dothe

following checks:

1) With the key off, verify that power is present at DME pin 18 and

main relay

86 & 30.


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2) With the key on, verify that power is present at DME pin 27 and pin

18. Power to pin 18 is from the main relay and there should be power

to the

injectors and fuel pump relay.

3) With the key on, verify that no voltage is present at the DME

grounds (2,

14, 19, 24).

4) Verify that power is present at the coil positive and at fuel pump

relay

pin 30. Those get switched power from the ignition switch via C101.

The engine will start and run (if poorly) with only those connections to

the

DME in place. The other signals from Cylinder ID, AFM, temp sensor,

etc., are

necessary for proper operation. But they won't prevent the engine from

firing.

IMPORTANT:

A power check means seeing a voltage within about a tenth of a volt

of what

you measure across the batter terminals, which should be at least

12.6v on a

charged battery.

A continuity check means seeing less that 1 ohm of resistance.

An open circuit means seeing a resistance of at least 100k ohms.

A good quality auto-ranging Digital Multimeter will make these tests

much

easier.

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Bmw Troubleshooting