DRIVEABILITY CLINIC

Lights Out, Somebody's HomeThe company I work for specializes in the manu

facture and sale of diagnostic equipment (most of it

for European vehicles). Our office is just around the

corner from an import repair shop that was having

trouble with a BMW. I've developed a reputation as

a problem-solver, so they gave me a call. I grabbed

my black bag and headed out. While they were

happy that I was able to help them find the prob

lem, they weren't too pleased when I revealed the

cause.

Sure, I know the saying is supposed to be, "The

lights are on, but nobody's home." Unfortunately,

the Bimmer I was staring at was behaving in just

the opposite manner. The engine was running, but

everything on the instrument panel was strictly

lights out. Oddly enough, this 1992 325i had come

in for a simple oil change and a coolant flush, a

routine service for just about any shop. But some

where along the line, something had gone haywire.

The maintenance work had been completed and

now the engine was misfiring constantly and the

instrument panel was dead to the world.

Before we get into our diagnosis on the 325, let's

acquaint ourselves with BMW's 88 pin Motronic

engine management system. BMW calls this system

DME (Digital Motor Electronics) M 3.1. It's used in

3- and 5-series BMWs equipped with six cylinder

M50 engines. Crankshaft and camshaft sensors pro

vide the control unit with engine speed and posi

tion info. A hot wire airflow sensor, throttle poten

tiometer, and heated 02 sensor provide additional

information. Sequential fuel injection and individ

ual coil-over-spark plug ignition finish off the heart

of this Motronic system.

—By Lester Bravek

28 August 1996 IM! FTT SERVICE

By the time I got there, the instrument cluster had

already been removed, so we couldn't access the

fault codes. The DME unit and instrument cluster

fuses had also been checked, so I made straight for

the DME control unit. I hooked up my Autodiagnos

Multitester+, and got into Monitor Mode to view

the ECU pin voltages on the M3.1 DME system.

A scroll through the list on the Multitester+ screen

with key-on, engine-off (KOEO) revealed nothing

unusual, so it was on to test two (Running Mode) to

check for faults with the engine running. That's

when I struck pay dirt: The Multitester+ told me to

"Check injection value, wire to MO#33 and signal

to corresponding ignition coil."

I removed the Multitester+, then connected an 88-

pin breakout box to the DME unit and harness con

nector. My oscilloscope indicated that I had battery

voltage on pin 50, which is the positive voltage sup

ply to the ignition coil for cylinder four. The rest of

the ignition coil drive circuits also checked out

okay, along with all six injector drive stages.

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Circle No. 108 on Reader Service Card

IM August 1996

ILights Out...

My next step was to remove the cover from the

ignition coils and check the resistance values for

all of the ignition primary circuits. The design of

the harness connector makes it difficult to test the

individual coil terminals when the cover is in

place. After all that digging, no problems were

found here, and it was time to dig deeper.

I switched the ignition coil plugs between cylin

ders four and five while monitoring pin 33 from

the breakout box. There were still no pulses from

this output—confirmation that the trouble was

within the ECU drive stage, not the ignition coil

itself. The ECU failure to cylinder four suggested a

possible link to the coolant-flush job.

Meanwhile, back to the instrument panel. I won

dered what could have sent a death-ray to the elec

tronics in the instrument cluster? As we snooped

about, we discovered that BMW had kindly joinedall six grounds for the high side of the ignition

coils together (arrow). From here, the ground wire

continued to a bolt above the thermostat housing.

If the engine had been started with this ground cir

cuit loose or disconnected, and a high-voltage dis

charge was reflected into the control unit through

the ignition-coil windings. The discharge had

surged down the primary circuit, breaking down

the ignition drive transistor, then continued

through the instrument cluster. Instant smoke.

30August 1996 IMPORT SERVICE

Lights Out..

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The DME control unit is connected to the instru

ment cluster via the engine TD signal (RPM) at pin

74. The frequency of this square wave signal

(shown here) is directly proportional to engine

RPM. The EGS (transmission control unit), is also

connected to this output line, but fortunately it was

not affected by the loose ground wires.

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4There were other paths from the DME control unit

to the instrument cluster, but the TD line was the

conduit for this high-voltage signal, due to its rela

tionship to the ignition circuit. Other connections

that escaped damage included the check engine

light, vehicle speed, Ti signal (injection on time),

RXD, and TXD (diagnostic link).

A word of advice—watch out for those ground con

nections. They can bite you big time. This car

required a brand-new DME control unit and an

instrument cluster, to the tune of $1500 from the

friendly neighborhood dealer. That's a lot of money

for an oil-change and a coolant flush, and you

know who paid the bill this time.

32

There's more than one way to destroy a control

unit, and it doesn't even have to be your fault. This

melted coil came out of a 1993 525i, which is also

equipped with the M50 engine and DME 3.1. The

coil had apparently shorted internally, which low

ered its resistance. The increased current flow

through the coil took out the DME control unit.

August 1996

The control unit had that expensive "burned electrical component" smell. I know you're not sup

posed to open these things up, but we had to see

what melted. The grounds to the coil primaries are

switched by the control unit. When the coil asked

for "more power," the switching transistor (arrow)

did the job as long as it could before it blew.

This system has sequential injection. If the control

unit determines that one of the spark plugs isn't fir

ing (for any reason), it will shut down the injector

to the corresponding cylinder. This keeps

unburned fuel out of the exhaust, which saves theconverter and keeps the emissions as low as possi

ble until the fault is corrected.

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Circle No. 110 on Reader Service Card

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Lights Out...

776 mV MAXIMUM

73B mV AVERAGE

656 mV MINIMUMQGQmV

680

48Q

£86

We checked the O2 sensor signal to see how theone cylinder miss was affecting emissions. At idle,the O? sensor showed no crosscounts and was

stuck nearly full rich. The control unit may havebeen fooled into thinking that the extra oxygen thedead cylinder was pumping represented a lean

mixture and was attempting to compensate.

There are two alternatives for fault code retrieval. Ifyou have a dedicated piece of equipment like theBosch "Hammer," the diagnostic connector can beused. If not, the CHECK ENGINE light on the dashcan also be used. To enter diagnostic mode for coderetrieval, snap the accelerator pedal open andclosed five times in five seconds (ignition on).

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Raising the idle speed to about 2500 RPM producedthe O2 sensor pattern shown here. The control uiiitwas apparently able to compensate for the missing

cylinder at that speed. This system is equippedwith a CHECK ENGINE light, but it will illuminate

only when the fault might affect emissions. Thecoil failure and miss never turned it on.

The CHECK ENGINE light will begin flashing thefault codes to you one at a time. Each fault codehas four digits,"and each digit will be flashed separately. For a Code 1223, you would see one flash,then two, then two more, then three flashes. Thecontrol unit can store up to 15 fault codes in itsmemory. Emission-related failures get top priority.

34August 1996 IMPORT SSRVIC6

After the cause of the fault has been corrected, the

fault code memory can be cleared from the control

unit's memory by disconnecting the control unit

harness connector (ignition OFF), disconnecting

the vehicle battery, starting and stopping the engine

60 times after repairs are completed (I don't think

so Tim), or by using dedicated service equipment.

One of the other changes that has been incorporat

ed into the DME 3.1 system is a hot-wire mass air

flow sensor. This replaces the analog airflow meter

that was used on earlier Motronic systems. An

advantage of this type of intake air measurement is

that changes in elevation, air pressure, and temper

ature are automatically compensated for.

Also on the intake side of the system, a throttle

position sensor is used. This replaces the idle and

wide-open throttle switches that were found on

earlier Motronic systems. The potentiometer in the

sensor gives the control unit a much better idea of

what the throttle opening is, allowing for quicker

response to changes in power demand.

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IMPOFTT August 1996 Circle No. Ill on Reader Service Card

Lights Out...

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The separate Reference and Engine Speed sensors

seen on earlier Motronic systems have been com

bined into a single sensor. This inductive sensor is

mounted at the front of the engine and reads off the

crankshaft. A missing tooth in the crank-mounted

sensor wheel produces the signal shown above.

This provides the control unit with TDC info.

Later Motronic 3.1 systems also include control ofthe valve timing. An electronically controlled

hydraulic unit (arrow) uses engine oil pressure to

advance or retard the valve timing to achieve thebest compromise between engine performance and

fuel economy. The ignition coil ground is visible at

the top of the valve timing controller (arrow).

The control unit keeps track of where the valvetiming is at at all times. A camshaft sensor at the

intake valve camshaft provides the control unit

with an alternating current signal, once per

camshaft revolution. This information is used by

the control unit to maintain the firing order and

advance for the ignition and injectors.

Here's one last look at the ignition coils before we

go. This engine uses plastic parts, so good groundsbecome extra important. Ground straps, like theone shown, assure that the coil bodies are properlygrounded. Bus bars connect one coil body to the

next. Make it a point to check these grounds (cleanand tight) during spark plug replacements.

36August 1996 IMPORTSEHVICE