Guide: BMW M30 Motronic "No Spark" Troubleshooting

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TimU
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Joined: Tue Dec 14, 2021 2:16 am
Location: Sydney

Guide: BMW M30 Motronic "No Spark" Troubleshooting

Post by TimU »

This guide is a modified and updated version of Chris Wright’s excellent “No spark” troubleshooting guide that has been shared throughout the years.

I reorganised, reformatted and updated the content to be more "roadside friendly", and included some extra steps gleaned from my own experience plus information from the Bentley manual.

All feedback, updates and corrections are welcome.
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BMW M30 Motronic "No Spark" Troubleshooting

This guide was written for BMW E24 and E28 cars from '83 to '87.
'88/'89 cars have their main relay wired slightly different, and a different reference sensor configuration.

Break-down of components needed to get spark to the engine:
Battery, Ignition Switch, OBC Relay, Main Relay, Coil, Reference Sensor, ECU

This document assumes:
1) The engine cranks over under its own power.
2) Fuel delivery systems are functional.

For a complete "no start" troubleshooting guide, refer to HPSI Motorsport’s excellent Bosch Motronic Fuel Injection Tech Article.
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First check for spark:

• Option 1) Pull the main Coil plug wire from the distributor cap and hold its conductor close to a grounded point, e.g., the engine block.
• Option 2) Pull a plug wire and get an extra spark plug and plug it in. Position the plug so that the metal shell is grounded.
• Option 3) Use a spark tester (from any auto parts store). You may also need an extra plug wire to plug in and test at the coil tower.

Try to start the car. Watch the contact point or plug to see if you are getting a spark.

If you are getting a nice strong blue spark, congratulations, you don’t have a no-spark problem. Check Fuel Delivery if you still have starting issues.

If you get spark at the coil but not the spark plug, check the Distributor Cap and Rotor. A wet or damp Distributor Cap or Rotor can prevent spark.
If you get a weak or orange coloured spark, check your battery, coil and plug leads for deterioration.

Symptoms:
No spark observed during spark tests

Possible Causes and Corrective Actions:
1. Low Battery Voltage
a. Check battery voltage. Min 10V needed during cranking

2. Faulty Main Relay
a. Bypass with a jumper wire, or replace as needed

3. Faulty On-Board Computer (OBC) Relay
a. Bypass with a jumper wire, or replace as needed

4. Poor Earth/Grounds
a. Inspect, clean and tighten all (5?) ground points in the engine bay

5. Faulty Ignition Switch
a. Test, bypass and replace as needed

6. Failed Ignition Coil
a. Test and replace as needed

7. Faulty Reference or Pulse sensors
a. Test and replace as needed

8. Missing Reference Pin on flywheel.
a. Test, inspect and replace flywheel (sorry)

9. Faulty Motronic Unit
a. Replace or repair 

Step by step "No Spark" procedure:

If no spark from either the Coil or Spark Plug:

Bypass the Main Relay and OBC Relay

This test also bypasses the OBC relay and force-energizes both Main Relay pin 87’s needed for Fuel delivery and Injector function.
• Pull the Main relay.
• Insert a 3-way jumper wire from socket 30 to both 87 sockets
• Try to start the car.

If no start, skip to the Speed and Reference Sensor checks below.

If the car starts, the fault will be either the Main Relay or the OBC relay.

Note:
The 3-way jumper wire can be used indefinitely but should be removed when the car is safely home, otherwise the battery will drain.

Bypassing the OBC Relay:

• Find the OBC relay box under the LHS dash.
o On a RHD car, it will be up above the glove box.
o On a LHD car, it will be under the driver’s knee panel.
• Jump (connect) the two green wires (pins 3 & 4) in connector C2.
• The plug can stay removed from the box with the jumper wires connected.

Testing for an OBC Relay failure without bypass

• Pull the Main relay
• With the key on, check for 12V at pin 86 in the main relay holder (the numbers are moulded into the bottom of the relay).

If there is no voltage at pin 86, the OBC Relay may be faulty. Inspect the OBC Relay box under the dash.
If there is no voltage (12V) at pin 3 of C2, the ignition switch may be bad.
Both terminal 15 on the coil and pin 3 of C2 are powered from pin 15 of the ignition switch.
If 12V is present at main relay pin 86, the OBC relay is fine.

Check for Main Relay failure

(It’s usually easier to just bypass or replace the main relay and skip to the Speed and Reference Sensor checks below, otherwise check the main relay as in the next steps.

Testing the Main Relay:

• Clean the relay prongs and the sockets in the holder base and plug the Main relay back in.
• Check the little circuit diagram on the side of the Main Relay to see if it has two separate contacts for the output pins 87 and 87b or if it just has one contact going to two pin 87's.

o If there is just the one contact: With the key on and the Main relay plugged back in, check for voltage at the RD/WH wire in a fuel injector plug (or use a "noid" light on the injector plug).
o If there are two contacts: With the key on, check for 12V at BOTH terminals 87 and 87b of the Main relay. (You have to wrap a wire around each pin and plug the relay back in and test for voltage at each wire. Don't short between the pins!!) One terminal can be good, suppling power to the fuel pump, and the other bad, not suppling power to the ECU, or vice versa.
o Alternately, to check both relay contacts and the Start Input: With the key on, check for voltage at Pin 4 (Start Input) and Pin 35 in the ECU harness plug, and at the RD/WH wire in a fuel injector plug (or use a "noid" light on the injector plug).

If there is no voltage in the tests above: replace the Main Relay.

As stated, it might be easier to just replace the Main relay out of hand in the first place, instead of doing the testing. It is also a good idea to keep a spare Main Relay (and Fuel Pump Relay) in the trunk Tool Kit and just replace it out of hand if the car stops out on the road.

Check the Earth/Grounds

Inspect, clean and tighten all ground points in the engine bay

1. Side wall near the battery
2. Back of engine at the top of the Valve Cover
3. LHS of engine, low to the sub-frame
(Where else?)

Checking the Ignition Switch and Coil:

• With the key on, check for 12V at terminal 15 (+) on the coil.

If no voltage, the ignition switch or wires to it may be bad.
Wiggle the ignition switch to see if it is intermittent.

• With the key off, check the Coil resistance
o Terminals 1 (-) and 15 (+) should be 0.50 Ohms
o Terminal 15 (+) and the high-tension centre tower should be 6000 Ohms.

Replace the coil if it has high or infinite readings (Open) or little or no resistance (Short).

• To start a car with a bad ignition switch:
o Run a jumper wire direct from the battery (+) to term 15 (+) of the coil.
o Briefly jump pins 11 and 14 in the diagnostic plug to engage the starter.

Checking the Speed and Reference Sensors:

(82-87) uses two sensors located at the rear manifold brace.
The Crank Position Sensor (aka Reference Sensor) (Grey) is required for spark (timing). The car will not start without this signal.
The Flywheel Speed Sensor (Black) is required for the fuel pump to continue running after the engine starts.

The ECU energizes the fuel pump (via the Main Relay) while cranking, but shuts it off if there is no Speed Sensor signal when the key is released.

• Inspect the 3-pin plugs and harness of the sensors first. The plastic can get brittle with age and they can crack around the wire locking bail leaving them loose.

• Disconnect the two connectors.
o Grey is the Crank Position Sensor (CPS).
o Black is the Flywheel Speed Sensor (FSS).

• Check their resistance as below. Check both hot and cold if the car's hard starting is temperature sensitive:
o Pin 1 to Pin 2 (Yellow to Black): 960 ohms, +/- 96 ohms
o Pin 1 to Pin 3 (Yellow to Shield): >100,000 ohms (no connection)
o Pin 2 to Pin 3 (Black to Shield): >100,000 ohms (no connection)

88-89 Cars use a single Pulse Sensor at the front of the engine on the Crank Pulley, and a coil pickup on one of the ignition wires.
Test the resistance using the same process as above:
o Pin 1 to Pin 2 (Yellow to Black): 540 ohms, +/- 54 ohms
o Pin 1 to Pin 3 (Yellow to Shield): >100,000 ohms (no connection)
o Pin 2 to Pin 3 (Black to Shield): >100,000 ohms (no connection)
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• Check the sensor mounting into the bellhousing.
o The bellhousing is marked with B and D stamps.
o B is for the Grey CPS
o D is for the Black FSS
o The car will not start if the sensors are reversed in the bellhousing or at the connector plugs.
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Check for missing Flywheel Reference Pin

The resistance test above is simpler, and gives you info on the condition of the crank sensors, but the AC voltage output test below is a bit more conclusive, as it also checks for the rare instance of a missing flywheel reference pin:

1. Pull the main relay to disable the FI system.
2. Pull rubber boots back off the plugs. Leave the plug connected to the harness.
3. Set your voltmeter to AC volts
4. Back-probe the yellow & black (pin 1 & 2) wires while cranking:
o Speed Sensor (Black Connector) -- 4.0 AC volts max at crank.
o Ref. Sensor (Grey Connector) ------ 0.4 AC volts max at crank.

If the AC voltage is not within spec, and you’re sure that the reference plugs themselves are functional, you’ll need to get under the car and check the flywheel reference pin is still intact.
You’ll need someone to hand-crank the engine to slowly rotate the flywheel while you check for the reference pin.
If the reference pin is missing, a new flywheel may be required.

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Replace Motronic ECU / DME

If the tests above have passed, the Motronic ECU is the final component needed to produce spark. The ECU controls the spark timing to the Coil.

The ECU is located under the RHS dash.
On a RHD car, it will be above the driver’s knee panel.
On a LHD car, it will be above the glove box.

• Replace the ECU with a known good unit.
• Open and inspect the ECU for cracked or blown solder joints.
o Repair / resolder if possible.

Acknowledgements:
• Chris Wright’s various “No spark” troubleshooting copy & pastes at BigCoupe.com
• Bentley Manual for BMW E28
• HPSI Motorsport’s Bosch Motronic Fuel Injection Tech Article
04/1986 635csi Euro spec
Australian Delivered, RHD
Pod
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Location: usually in the barn, Kent, England.

Re: Guide: BMW M30 Motronic "No Spark" Troubleshooting

Post by Pod »

Very well presented! Thank you for taking the time to post this 🙂
songzunhuang
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Re: Guide: BMW M30 Motronic "No Spark" Troubleshooting

Post by songzunhuang »

I hadn't seen his post before and I agree with "Pod" that is is very well presented.
It also happens to be exactly what I was looking for as I troubleshoot a very poor running condition.
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Song Huang
1984 633CSi
Last 7 of VIN: 6997383
emac
Posts: 178
Joined: Thu Feb 06, 2020 2:57 am
Location: SC

Re: Guide: BMW M30 Motronic "No Spark" Troubleshooting

Post by emac »

Excellent write up!

I will add.... if the machine shop knocks the raised pin off of the flywheel, the car will not start! Trust me, I know. :mrgreen:
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