BMW No-Start Diagnostic Guide – DIY
This answer is applicable for many BMWs in addition to the one listed below.
I have a 1989 BMW 325i convertible that is giving me problems starting at times. This has been an ongoing problem for about a year. I initially ran the fault codes and on your recommendation replaced the “Throttle Position Switch”. This problem is still occurring but is intermittent. When this problem occurs, the car turns over with plenty of power but takes several attempts before it starts. Is a tune up in order?
Of course, not knowing the history of the vehicle’s service, it certainly may be time for a general tune-up. Have you tested or replaced the engine temperature sensor (in thermostat housing, with blue connector plug)? Have you done any spark and fuel testing, when it won’t start? This can give us a better idea of what may be going on. See the document below, outlining the tests to perform when in a no-start situation;
DIY – BMW – No Start
One of the most frequent “calls for help” that Bavarian Otto receives is “My BMW won’t start”. Of course, this is a frustrating situation for the BMW owner, whether the car is sitting in the garage or in the shopping center parking lot. However, before we can make any suggestions on what may be the cause of the no-start, we must perform some basic and simple diagnostic tests. The results of these tests will lead us in various directions, depending on what the results show. Follow along with Otto as we run through the initial diagnostic tests.
In order to perform these tests you’ll need a few basic diagnostic tools:
- Basic digital multi-meter that can function for DC voltage and resistance Ohms testing … or … a dedicated circuit tester like the Power Probe.
- Mechanic’s Stethoscope (Bavarian Autosport Part # 52750).
- Inductive Spark Tester:
Applicable Bentley repair manual.
- If the testing indicates that you have circuitry/wiring problems that must be traced, and you need more detail that what is offered in your Bentley repair manual, we can order the applicable BMW ETM (Electrical Troubleshooting Manual) for your BMW. The ETM is the vehicle’s complete wiring diagrams.
Finally, before we start testing, Let’s define some terms that we commonly use when talking about a no-start situation:
- Cranking – This refers to the starter motor actuating and turning … and “turning over” the engine (the engine is rotating).
- Turning over – Typically, this term is used to note that the engine is rotating as the starter is cranking, as in; “the starter is cranking and the engine is turning over, but will not start”.
- Firing – Typically used to note that the engine is attempting to start, that some or all of the cylinders are “firing”, as in; “the engine will fire once in a while, but will not stay running”.
NO START – Diagnostic Flow
BEFORE STARTING: If the vehicle is a 1988 or later model it is wise to use the appropriate engine management fault code reader tool to check for any stored fault codes before we start further diagnostics. Whatever is at fault may have generated a fault code that will tell us where to go next…… saving much time and effort. If no codes are present, that would have a relation to the no-start, continue on to step # 1.
1: Does the starter crank, and turn the engine over? If the starter is cranking but the engine is not turning over (rotating), you likely have a faulty starter motor or solenoid and the starter/solenoid assembly likely needs to be replaced.
YES – go to step 7
NO – go to step 2
2: Do you hear a “click” at the starter, when the key is turned to the START position?
YES – go to step 10
NO - go to step 3
3: Check battery condition. Do the headlights come on, nice and bright, blower motor works at full speed, horn honks nice and loud? If you have a digital voltmeter, check battery voltage. The voltage should be at least 12.6 volts.
Battery OK – go to step 5
Battery LOW – go to step 4
4: Place a charger on the battery or apply jumper cables and wait 10 to 20 minutes or replace the battery ….. as applicable. Return to step 1.
5: Check for voltage at the starter solenoid “turn-on” wire. This will be a small gauge wire that is typically connected to the starter solenoid via a spade type push-on terminal. On BMWs through the mid to late ‘90s, this wire is typically black and yellow. On later models the color is typically black. Remove the wire from the terminal and check for 12-volts between the wire and ground, when the ignition key is turned to the START position.
YES, 12-volts – Replace starter & solenoid
NO, 12-volts – go to step 6
6: With no voltage at the solenoid “turn-on” wire, we must diagnose from the ignition switch START terminal, through the wiring, to its ultimate end at the solenoid connection. This will require wiring diagrams for the specific year and model of your BMW. These are available through the Bentley repair manuals and the BMW ETMs (Electrical Troubleshooting Manual), which are the complete vehicle wiring diagrams. Both of these resources are either in-stock or available by special-order through Bavarian Autosport.
7: Test for ignition spark and fuel injector pulse signal.
Spark - The spark can be tested by using a spare spark plug or a specific spark tester tool. If a spark test tool is not available, unplug one of the spark plug wires from one of the spark plugs. Insert the spare spark plug into the end of the removed wire. Have a helper hold the side of the plug (the metal part) against the top of the engine (again, against a metal part like the valve cover or intake manifold) … use a pair of pliers with insulated handles. Crank the engine and see if there is a spark occurring at the tip of the plug. On later model BMWs that have Direct-Ignition systems (coil on plug), this can be accomplished by removing one coil and connector boot assembly, inserting a spark plug and assuring that the plug is grounded (as noted above) and also that the metal part of the coil is also grounded (a jumper wire can be used to ground the coil).
Injector Pulse Signal – To test that the fuel injectors are opening and closing, you will need either a mechanic’s stethoscope (available through Bavarian Autosport) or a long screwdriver. With a helper, touch the end of the stethoscope or the screwdriver to the body of one of the injectors. If using a screwdriver, push the handle into your ear (this makes a very effective stethoscope). Crank the engine and you should hear a very distinct… tap, tap, tap …. as the injector is opened and closed.
Spark OK and Injector signal OK – go to step 11
Spark OK, no Injector signal – Follow the ECU harness plug input/output testing as outlined in the appropriate Bentley repair manual. There is likely a wiring problem or the ECU may be faulty.
No Spark, Injector signal OK – go to step 15
No Spark, no Injector signal – go to step 8
8: In most cases, if there is no spark and there is no injector signal, we have a problem with the main engine management system …. it is not sending an ignition signal nor an injector signal. This may be due to a lack of main input power to the ECU (Engine Control Unit), in other words, it is not turning on …. Or ….. one, or more, of the critical input signals is faulty and the ECU is not able to generate the spark and fuel outputs. Of course, we could also have a faulty ECU, but this is not as common as the “parts replacers” would have you believe. We will now test some of the critical ECU input signals.
8a: Main input power to ECU – Using the Bentley repair manual or the applicable wiring diagrams, determine what pin(s) is the main power input for the ECU (the “turn-on” wire). Check for 12-volts from this pin in the ECU connector plug to ground, when the key is in the RUN and the START positions.
YES – go to step 9
NO – go to step 8b
8b: Using the wiring diagrams, determine where the ECU main input power is originating. Test for 12-volts at the origination point for the ECU main input power. On BMWs up through the mid-‘90s there is a main ECU power relay. The applicable Bentley repair manual will detail the location of the relay. Test the output side of the relay socket (the wire that feeds the ECU main power input) for 12-volts when the key is in RUN and START, typically terminal # 87 or 87a.
Typical early BMW Main ECU relay:
YES – You will now need to trace the wiring from the relay socket to the ECU connector harness to determine why the power is not arriving at the ECU connector harness (likely an open circuit or a short in the wiring, at some point).
NO – Determine if there is constant 12-volts at terminal # 30, in the relay socket. If not, trace the circuit for the wire at terminal #30. This should be a constant 12-volts regardless of key position. If you do have 12-volts at terminal #30, determine if there is 12-volts at terminal #86, in the relay socket, when the key is in RUN and START.
YES – The relay is likely faulty.
NO – Trace this circuit back to the ignition switch, including the switch itself. The switch should be sending 12-volts to terminal #86 when in RUN and START. You likely have a bad switch or a short or open circuit in this wire run.
9: If we have 12-volts to the main power input for the ECU, but no spark or injector signal, we may be missing a critical input signal for the ECU. As noted above, there is also the possibility of a faulty ECU. Before replacing an ECU, we must diagnose all other possibilities. If all else is proper, the last move is to install a known good ECU. A critical input, for the ECU to generate the spark and injector output, is the crankshaft position sensor and/or camshaft position sensor signal. Perform the sensor testing as outlined below. If the sensors test as OK, we will have to perform more in-depth diagnostics, and these tests will be outlined in the Bentley repair manuals. The crankshaft and/or camshaft position sensor design and mounting varies through the years and models of BMW production. We will give the general differences and diagnosing:
Early Motronic – 6 cylinder, single and double cam, Motronic models (Distributor cap mounted directly to the front of the cylinder head) up through the mid to late ‘80s, such as; 325e/es & 528e thru 11/86, 533i, 535i thru ‘88, 633csi, 635csi & L6 thru 5/87, 733i, 735i & L7 thru 87, and also the M3, M5 and M6 thru 93. On these applications, the crankshaft position sensor is mounted in the transmission bell housing, on the driver’s side. There are two sensors mounted in this area. The sensor that is more toward the front of the car is the RPM sensor and reads the ring gear teeth. The more rearward sensor is the position sensor and this reads a single pin that is on the side of the flywheel, behind the ring gear. We may have a faulty sensor or the pin may be missing from the flywheel. Follow the sensor’s wire up to the connector plug. Unplug the connector and test the sensor by checking the internal resistance of the sensor. The three terminals in the sensor’s plug housing are numbered 1, 2 and 3. Terminals 1 & 2 should have 960 +/- 10% ohms, all other combinations should have 100,000 to infinity ohms (see Bentley manual for additional detail). If the sensor fails the ohm test, replace it. If ohms test is OK, remove the sensor and look through the sensor’s mounting hole. Have a helper slowly turn the engine (using the front crankshaft hub nut/bolt) and look for the reference pin. If the pin is missing, the flywheel must be replaced. If the pin is present, the sensor may still be faulty (even though it tested OK in the resistance test). Swap the position sensor with the RPM sensor, both in the bell housing and at the connector plugs (both sensors are the same part). Check for spark and injector signal (step 7). If signals are present, does the engine start now? If so, purchase a new sensor and replace the one that is now in the RPM position (was originally the position sensor). If no spark or injector signal, after the swap, the ECU may be faulty.
Late Motronic – 6 cylinder, single cam, Motronic models, such as; 325i/is/ix thru 91, 325ic thru 92, 325 88 thru 91, 528e 12/86 on, 535i & 735i/il 89 on, 635csi 6/87 on. On these applications the crankshaft position sensor is mounted to the front of the passenger side of the timing cover and reads a toothed wheel at the front of the crankshaft. Similar to the Early Motronic, above, we must test the resistance of the sensor. The sensor’s plug has two terminals. The resistance should be 540 +/- 10% ohms. If the sensor fails this test replace it. Does the engine start now? If not, check for spark and injector signal (step 7). If no spark or injector signal, after replacing the sensor, the ECU may be faulty. If the sensor tests OK in the resistance test, check the full length of the sensor’s wiring, up to the plug. It is very common that the insulation may be separating at the sensor or at the plug, or for the wire to be chaffed by the water pump pulley. If you find potential faults in the harness, replace the sensor.
Twin Cam engines – The twin cam 4cyl., 6cyl. and V8 engines use combinations of crankshaft and camshaft position sensors. For these applications, it’s best to refer to the applicable Bentley repair manual for mounting position and test parameters. Additionally, these applications will typically generate fault codes that can be accessed by using one of the fault code readers available through Bavarian Autosport.
10: If you do hear a “click” at the starter, when the key is turned to START, this means that the circuit from the ignition switch to the starter solenoid is OK. The “clicking” is indicating that the solenoid is, at least, attempting to energize the starter. Either the solenoid or starter is faulty or there is not enough battery power getting to the main starter motor input terminal. We will now test the solenoid, the starter motor and the battery power to the motor.
Go to step 3, to determine if the battery is OK. If step 3 determines that the battery is OK, remove, clean and re-install both the positive and the negative cable clamps, at the battery. Also check the connection of the negative cable where it is bolted to the chassis (near the battery). Remove, clean and re-install the positive cable connection at the starter. If the positive cable is coming from the battery in the trunk or under the rear seat, locate the junction block under the hood (the applicable Bentley repair manual will detail the location), and perform the same remove, clean and re-install of the cable connections. If the starter solenoid still just clicks, when the key is turned to START, replace the starter.
11: We will now test for the fuel pump operating. (see SAFETY NOTE, below, prior to proceeding) Remove the fuel hose where it connects to the fuel rail (or to the pressure regulator assembly or fuel filter …. which ever is more convenient. See the applicable Bentley repair manual for detailed locations). If there is fuel under high pressure at the connection (when you open the connection), you can assume that the pump is running. As a 100% determination, place the open fuel line in a suitable container and turn the key to RUN (Ignition). The pump should run for a couple of seconds and pump fuel into the container. Note that the pump should not continue to run, if the engine is not running. Re-connect the fuel line after testing.
SAFETY NOTE: If the fuel pump is operating, there will be up to 75psi of fuel pressure in the system. Assure that there are no open flames or potential ignition sources in the work area. Wear safety glasses and wrap fuel hose connection with a large rag, to catch the pressurized fuel.
YES, fuel pump is running – Remove the spark plugs and verify that they are clean and dry. Re-install and test to see if the engine will start on starting fluid. If so, perform fuel delivery and pressure tests as outlined in the applicable Bentley repair manual. You may have a faulty fuel pressure regulator.
NO, fuel pump is not running – go to step 12
12: Test for 12-volts at the fuel pump electrical connections. Using a voltmeter, test for 12-volts between the positive and negative wires for the fuel pump (on most external pumps this can be done with the wire terminals still connected to the pump, on applications that use a harness plug, remove the plug and test the wires in the plug terminals). You should have 12-volts, for a few seconds, when the key is turned to RUN (Ignition). If you repeat the test, you should wait a minute or two between tests, for the fuel pump output to reset (this is because the fuel pump output will turn-off if the engine is not running, as a safety measure).
YES, 12-volts – You likely have a faulty fuel pump. Apply 12-volts directly to the pump to verify that it does not run.
NO, 12-volts – go to step 13
13: Verify that the fuel pump fuse is not blown (the Bentley repair manual will detail the fuse locations). As a 100% test, check the fuse with an ohmmeter. Use the Bentley repair manual to locate the fuel pump relay. Jumper the relay as outlined in the Bentley repair manual (terminals 30 & 87 in the relay socket) and repeat the testing in step 12, but without turning on the ignition key. There should be a continuous 12-volts as long as the relay jumper is in place. Remove jumper wire when testing is complete.
YES, 12-volts – Fuel pump relay is likely faulty.
NO, 12-volts – go to step 14
14: Test for 12-volts at terminal 30 in the fuel pump relay socket. There should be continuous 12-volts regardless of key position.
YES, 12-volts – You likely have a short or open circuit between terminal 87 in the relay socket and the fuel pump connection. Further wire/circuit testing will be required.
NO, 12-volts – You likely have a short or open circuit between terminal 30 in the relay socket and the battery power connection at the origination of the input wire . Further wire/circuit testing will be required.
15: On single coil models, test for 12-volts at the coil positive input wire (typically green, see Bentley manual for specific models) when the key is in both the RUN and START positions.
YES, 12-volts – The coil may be faulty, the ECU may be faulty or there may be a wiring problem between the negative wire from the coil and the ECU. Follow the ECU harness plug input/output testing as outlined in the appropriate Bentley repair manual (centering on the negative coil wire). There is likely a wiring problem or the ECU may be faulty. If the harness plug testing is OK, you may wish to replace the coil as a first step. If this does not cure the problem, the ECU may be faulty.
NO, 12-volts – You may have a faulty ignition switch, start relay (automatic transmission only), neutral safety switch (automatic transmission only) or a wiring problem in the circuitry between the switch and the coil input wire. Use the wiring diagrams in the applicable Bentley repair manual for further circuitry testing.
NOTE: On multi-coil models, test for coil input using the wiring diagrams in the applicable Bentley repair manual.