I’ve had some tough times over theyears diagnosing problems on sev-eral European vehicles. You’veprobably experienced the same

difficulties. If so, I think I can help.Following are ten of the toughest

diagnostic dilemmas I’ve faced—inno particular order—with solutionson how to follow them through tosuccessful repairs. These tips on ser-vicing vehicles from Audi, BMW,Mercedes-Benz, Volkswagen and Vol-vo should make your life at the shopjust a tad easier. Let’s get started.

1Audi—Cranks But Doesn’t Start.This fix not only works on older

Audis (particularly turbos), but it ap-plies to newer models, as well. In 1984,Audi introduced the 5000 turbo. Onthese models, there was a need forknock control, so Audi introduced anElectronic Ignition Control Unit with aknock sensor input to retard timing dur-ing periods of detonation. Unfortunate-ly, the Hall-effect sensor used was tooslow to update the control unit as towhen and how much to retard the tim-ing, so faster sensors were also required.

As a result, Audi installed two newsensors in the bellhousing of the trans-mission. One of these sensors—an igni-tion timing sensor—is used to indicatewhen No. 1 cylinder is at top dead cen-ter. The other sensor—an rpm sendingunit—reads off the ring gear teeth. Be-tween these two sensors, a faster up-date could be made to the ignition tim-ing to compensate for the inputs fromthe knock sensors. With these sensorsadded, however, the system becamemore complex to diagnose, especiallyduring no-start situations.

Let’s say you have one of these carsin your shop and it cranks but won’tstart. The first thing to do is check forspark. No fire? If you have experiencewith Bosch-equipped vehicles, youknow to check battery voltage whilecranking, because if it’s too low (below11 volts), the engine probably won’tstart because the computer is notproperly powered up due to the lowsystem voltage. You also may want tocheck for any blown fuses so you don’toverlook anything simple.

Once you’re past that stage, youneed to start checking sensors. Let’sbegin with the Hall-effect sensor in the

distributor. First, turn the ignition keyon and hook up the positive lead ofyour voltmeter to the “R/B” wire andthe negative lead to the Br/W wire ofthe sensor connector. You should see10 to 11 volts. Now check the harnessside of the G/W wire. You should read5 volts here because this voltage iscoming directly from the ElectronicIgnition Control Unit.

It’s the job of the Hall-effect sensorto ground the reference voltage to in-dicate camshaft position, so you’ll alsowant to check it while cranking. Usinga scope, you should see a nice, crisp 5-volt square wave. With a voltmeter,look for about 2.5 volts, with a 50% du-ty cycle.

If the Hall-effect sensor is workingproperly, turn your attention to the igni-tion timing and rpm sensors in the bell-housing. These sensors are AC pulsegenerators, and can be easily checked atthe computer. First, perform a quickcheck for resistance. Connecting yourmeter between pins 11 (yellow) and 29(black), then between pins 12 (yellow)

and 13 (black) should get you the resis-tance readings for the rpm and ignitiontiming sensors, respectively. Look forabout 1000 ohms in both cases. Next,connect your scope or meter, with thepositive lead on the black wire and thenegative lead on the yellow wire ofwhichever sensor you want to check.The black wire carries the signal, theyellow wire is the ground. For the igni-tion timing sensor, you should get a me-ter reading of about .1 to .3 volt at nor-mal cranking speed. On the rpm send-ing unit, you should see about 2 to 4volts. Using a scope, both sensors willexhibit similar sine waves, although thefrequency on one will be very slow (2 to3 Hz) and the other extremely fast (afew hundred Hz).

Having all these signals on the mon-ey doesn’t mean you have a bad controlunit, though. So take a test-light, clip itto battery positive, then tap pin 22 ofthe control unit to see if you can trig-ger the power stage (ignition) moduleto fire the coil. If you get spark, boththe module and coil are good.

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Suppose everything at this pointchecks good, but you still have the no-start problem? What you may haveoverlooked is the fact that the Hall-ef-fect sensor and the ignition timing sen-sor must be in alignment in order forthe computer to process the crank sen-sor signal. If the cam belt has jumpedtwo to three teeth, the Hall-effect sig-nal will be out of sync with the bell-housing-mounted sensor. A quick rota-tion of the distributor should get sparkback, but with a noticeable lack ofpower. When you align the “OT” markon the flywheel with the pointer on thebellhousing, the Hall-effect sensorshould be just switching states. If itisn’t, you know the timing belt is off afew teeth. By the way, this also holdstrue for 1989 and later 80 and 90Quattros with multiport injection. Ei-ther of these systems is easily identi-fied by the power stage modulemounted near the coil.

2Audi—Intermittent No-Start,Hard Starting in Cold Weather.

Audi has used some version of CIS on

every one of its vehicles, so it’s no won-der its models share common problems.Need I remind you that CIS is basicallya bulletproof system, and only afteryears of service will it show its flaws.Usually these involve intermittent no-starts and hard starts in cold weather.

There are a few factors that con-tribute to these symptoms. First, re-member that this is still a mechanicalsystem, even though there are someelectronic controls to fine-tune things.That means the mechanicals have to beright. One common problem with theCIS system is bad injector O-rings. Asthese O-rings shrink, dry up or crackwith age, they can let enough air in toprevent the airflow meter (AFM) flapfrom opening. And if the flap doesn’topen, you won’t get enough fuelthrough the injectors to keep the en-gine running after the cold-start injec-tor has stopped spraying. A simplecheck is to push on the AFM flap whilecranking, to see if the engine stays run-ning on its own. By the way, any smallvacuum leak can cause this problem.

If the engine won’t stay running afterthe above test, there may be a more in-depth problem with the fuel system.With CIS, a second pressure—calledcontrol pressure or lower chamberpressure—is balanced against main sys-tem pressure, and that’s what controlsthe mixture ratio. Low control pressureand normal main system pressure leadsto a rich mixture; high control pressureand normal main system pressure leadsto a leaner mixture. So if the controlpressure is too high on a cold engine,you’ll have trouble keeping it running.

The component that controls thecontrol pressure is known as a frequen-cy valve. This valve buzzes with anon/off signal. The more it buzzes, themore it drops control pressure. OnCIS-E and -III systems, a new compo-nent was used to control lower cham-ber pressure. It’s called a DifferentialPressure Regulator (DPR). This com-ponent doesn’t use a frequency signal,but rather a milliamp signal to raiseand lower the control pressure. Thehigher the milliamp signal, the lowerthe control pressure, and vice versa.You’ll need a multimeter hooked up inseries to measure this signal, unlessyou have a low-amp inductive probe.

On a normal motor during a coldcrank, you should expect to see about75 to 125 milliamps (mA) from theDPR. This number will drop to about10mA during cold enrichment. Thesignal will be somewhere between 3and 8 volts DC, but this voltage won’ttell you anything other than that there’sa signal present. Chances are one ofthese two items will help you out onmost hard-start problems.

3BMW—No-Start. On the BMWfront, we’ll start off with 1990 and

later 3- and 5-Series no-starts, particu-larly those cars having spark but no ora weak injector pulse. Like the Auditurbo and Quattro models mentionedpreviously, these cars use two bellhous-ing-mounted sensors to give TDC po-sition and engine speed info to the ig-nition control unit. The resistance andAC voltages should be the same, justthe pin locations at the control unit aredifferent. Pins 8 and 27 make up thespeed sensor, and pins 25 and 26 makeup the TDC sensor.

If the sensor readings are good,

43December 1998

Does European diagnosticwork have you miffed? Ifso, here are ten commonproblems, with solutionsto help make working onthese vehicles less of a

brain-teaser.BY KERRY JONSSON

check for battery voltage at the injectors,particularly while cranking. You maynotice a drop in voltage to the injectors,or a weak pulse. Try jumping from pin30 to pin 87 of the main relay mountednext to the airflow meter. It’s the lowerof the three relays on the 3-Series andjust outside of the fuse/relay panel, onthe engine side, on the 5-Series.

If you look under the rear of the in-take manifold of the 3-Series, and un-der the throttle body of the 5-Series,you’ll see a round plug connection thatgoes up to the injection harness. Thisharness carries power and pulsedgrounds to and from the injectors andis prone to corrosion, especially if thehead gasket is leaking coolant right on-to the connector.

4BMW—Idle Surge. On mid-’80sBMWs, you may encounter an

idle surge that you’ve seen before butcan’t remember how you fixed it. Startby checking for voltage on the bottompin of the throttle switch. You shouldsee zero volts with the throttle at idle,and 12 volts when you crack the throttleopen. Remember, if the ECU doesn’tthink the throttle is closed, it won’t tryto control idle speed.

The next thing to look at is the IdleAir Motor itself. First, try blocking thehoses leading to and from the motor.The idle should drop if it’s the motoradding the additional air that’s causingthe surge. If it does, check the com-mand to the motor. This is a milliampsignal from a separate Idle Speed Con-trol Unit. Therefore, it must be mea-sured in series with your meter. Youshould see about 480mA on a cold mo-tor with a fast idle, and about 450mAon a warmed-up motor. If you don’tget those numbers, you need to re-place the idle speed control unit; if youdo, a new idle speed motor is required.Be careful with this diagnosis; any vac-uum leak can cause an idle surge.

5Mercedes-Benz—Coolant Tem-perature Sensor Diagnosis. No

matter how reliable Mercedes-Benzesare, they’re not immune to putting youin a diagnostic quandary. Here’s what Imean: Mid-’80s to mid-’90s Benzes usean EZK ignition control system forspark control and Bosch CIS-E or -IIIfor fuel control. Both control units

need a coolant temperature input, andget it from one Coolant TemperatureSensor (CTS). Some models use a two-wire sensor; on others, a four-wire sen-sor is used.

The two-wire sensor is differentfrom most such sensors in that eachwire is its own 5-volt reference—onefrom the ignition control unit, the oth-er from the fuel control unit. Thatmeans each pin of the CTS has to haveits own thermistor, with the sensorshell supplying the ground for both cir-cuits. In the case of the four-wire sen-sor, each 5-volt reference has its ownground, as well. The resistance values

for the CTS should be equal on eachleg—about 3000 to 4000 ohms on acold motor (about 70°F) and 300 to400 ohms on a fully warmed-up motor.Because both sensors have the same 5-volt reference and the same resistancevalues, you’d think both circuits wouldhave the same signal voltage. Not so!

Backprobing the sensor on a coldengine will give you about 1.5 to 2.0volts on the EZK side. Move over tothe CIS-E side and you’ll get about 2.3to 2.7 volts at the same temperature.As the sensor warms up, both signalvoltages will drop, to about .5 for theignition system and .7 for the fuel sys-

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This is the location of the main relay on a 1988 BMW 528. It’s the green one. A voltagedrop across the contacts of this relay can lead to no-starts, especially after extendedstorage or cranking of the vehicle while it’s in your shop.

Here’s the typical location of the CTS (arrow) on a Mercedes-Benz straight-six. Notice howthe connectors are separated into two identical plugs. This makes swapping them thatmuch easier after you’ve determined that the CTS is not giving you the readings you want.

Ph

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: Ker

ry J

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tem. A word of caution: Sometimesthe CTS for the fuel control leg of thecircuit does not drop in resistance, giv-ing a cold indication to the controlunit, even though the engine iswarmed up. Result: Rich-running. Aquick way to check the CTS is toswitch the 5-volt references betweenthe EZK and CIS units. If the problemclears up, the sensor is shot.

6Mercedes-Benz—Surge, Hesi-tation. A mid-’80s to mid-’90s

Mercedes-Benz may not run richwhen it’s warmed up, but it may surge.It may also have a slight hesitation offidle. In a case like this, the first thingyou want to look at is the throttleswitch. This is a difficult undertakingon V8s, considering the switch is underthe airflow sensor plate due to thedowndraft setup Mercedes uses.

There should be a three-wire con-nector on the driver’s side of the intakemanifold, next to the valve cover. Lookat the signal voltage on the Br/Y wire atthis connector. You should see zerovolts at idle and 12 volts once you openthe throttle. If the throttle switch con-tacts have carbon on them, the signalvoltage won’t go to ground and thecontrol unit won’t know that the throt-tle is closed, so it won’t control the idlespeed motor properly. Hence the hesi-tation on throttle tip-in.

7 Volkswagen—No-Start. ForVolkswagen, some unusual prob-

lems have made my list. One is the mid-’80s Digifant system found on Jettasand Golfs. You may have had one ofthese cars that wouldn’t start. Afterchecking for spark and fuel, you foundthe plugs soaked with fuel. Upon fur-ther inspection, you noticed a noid lightstaying on, even though the key was inthe On position. You then check thepower and ground circuits at the controlunit and everything seems to be fine.What gives? I mentioned earlier thatBosch control units are not fully turnedon until they have better than 11 volts.The control unit leaves the injectorsgrounded with the ignition key in theOff position. With the key on, the con-trol unit ungrounds the injectors. But ifthe control unit doesn’t get proper volt-age, it won’t unground the injectors.

Look for a drop in the battery volt-

age feed during a crank, as well asacross the contacts of the Digifant relayin the fuse relay panel. The relay mayprovide enough voltage to power upthe injectors, but not enough to havethe control unit unground the injectors.The injectors will then spray with thekey on. Try jumping pins 30 and 87 ofthe Digifant relay and see if the condi-tion goes away. If it does, you need arelay; if it doesn’t, you probably need acontrol unit.

8Volkswagen—Oil Pressure Di-agnosis. Volkswagen put a Dy-

namic Oil Pressure Control Unit in itsinstrument cluster to notify the driver

that oil pressure is low for agiven engine rpm. If this sys-tem sounds complex, imag-ine diagnosing it if there’s anoil pressure problem, or aproblem with the controlunit.

Understanding how thesystem works will make di-agnosis a lot easier. First, beaware that there are twopressure switches that go tothe control unit. There’s al-so a tach signal that goes tothe control unit. One of theoil pressure switches ismounted on the oil filterhousing. This is a 1.5 barswitch that goes to groundat about 22 psi of oil pres-sure. The second switch isin the cylinder head. Onsome older models it’s onthe rear of the head, and onlater models it’s betweenthe No. 3 and 4 cylinders.This switch is a .3 or .7 barswitch, which is normallyclosed and will open (notclose) at about 4 to 12 psi.These are good signals tothe control unit, and should

cause the light to go out.If the light comes on, here’s a quick

way to determine what component isat fault: Start the engine and let it idle.Since the high-pressure switch goes toground with pressure, simply groundthe signal wire, and that will indicateenough oil pressure at the filter. If thatswitch is the problem, the light will goout. If the light remains on, unplug thelow-pressure switch and leave it un-plugged. Since this switch opens withoil pressure, leaving it unplugged willindicate enough oil pressure and turnthe light off. If the light comes on afterthat, you probably have a problem withthe control unit itself.

Also, scope the tach signal. A signalwith a lot of “hash” in it will give a falseindication of high-rpm operation andpossibly trigger the light, but thisshould happen only when the pressureswitches are plugged in.

Another thing to check is the oillevel. If it’s too high, the crankshaft

45December 1998

On BMWs, I mentioned the importance ofbattery voltage making it to the computer.Well, Mercedes-Benzes are no exception.After years of service, the main relayspictured here also develop poor connec-tions. These relays are mounted next tothe control unit. By jumping pin 30 (con-stant battery power) to pin 87 (relayswitched output), you can bypass the re-lay and see if that improves driveability.

may splash the oil, causing it to foamup. This aerated oil will not properlypressurize the switches, resulting in afalse signal.

9Volvo—No-Start, Low FuelPressure. With Volvos from the

mid-’80s to the early ’90s, a persistentproblem has been the fuel injectioncontrol unit’s inability to ground thefuel pump section of the main relay.I’ve noticed it more on 700-series ve-hicles than on the 200-series. If youhave a Volvo that’s not starting due tolow fuel pressure, here are somethings to check out to make the expe-rience quick and painless. First, onfour-cylinders, look for constant bat-tery voltage at pin 4 of the fuel injec-tion control unit. When the ignitionswitch is turned on, this voltage shouldbe sent to pin 35 of the control unit.The control unit, in turn, grounds pin21, which activates the main relay.This provides power to pin 9 of thecontrol unit and the MAF sensor.

Once the engine is cranked, a crankspeed signal leaves the EZK ignitioncontrol unit from pin 17 and goes topin 1 of the fuel injection control unit.When the fuel injection control unitsees this, it starts to ground pin 18, toground the injectors. It should alsoground pin 20 to activate the fuelpump relay section of the main relay.

On six-cylinders, constant voltage isfound only on pin 21. With the keyturned on, voltage should end up atpin 18. With voltage here, pin 21should go to ground to activate themain section of the relay, and providepower to pin 9 to also power up thecontrol unit. Once a cranking speedsignal is on pin 1, the ECU should startto ground pin 13 for injection pulseand pin 17 for fuel pump relay activa-tion. If this doesn’t pan out and youhave good grounds at pins 17 and 5,look for a bad control unit.

10Volvo—No-Start, Weak orNo Injector Pulse. Some

late-’80s to mid-’90s Volvos are knownfor starting problems. You may noticethat you have good spark and fuel pres-sure, but weak or no injector pulse.Maybe you tried unplugging the CTSor the MAF, to see if the car would runin a fail-safe mode, but that didn’t

work. What else could you have tried?Well, remember, to get a good injectorpulse, you need to have good powerand ground pulses. So try checking thevoltage supply to the injectors, whilecranking if you can. If it’s significantlybelow battery voltage, check the radiosuppression relay mounted in front ofthe driver’s-side strut tower, up againstthe fender. This relay’s contacts get“carboned up” over time, leading to alarge voltage drop. And that’s whatcauses the no-start. Simply jump pins30 and 87 and you’re bypassing this re-lay and on the way to a fix.

I hope these sagas help make yourday in the trenches go a little easierwhen you come across these vehicles. Ialso hope your customers appreciatewhat you sometimes have to gothrough to give them the servicethey’ve come to expect. And in theend, isn’t that what it’s all about?

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Here is the connection Mercedes-Benz gives you to test the throttle switch. Since theybury the switch below the downdraft airflow meter setup, you cannot get to the switchitself. Checking the Br/Y wire for voltage can be done at this easily accessible connection.

This is a typical location of a Volvo ECU on the 200- or 700-series. It’s where you’ll check tosee if the control unit is grounding pin 20 on the four-cylinders or pin 21 on six-cylinders,energizing the fuel pump section of the main relay. This is easier to do than backprobing themain relay, which typically is located behind the center console on 700-series models.

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