BMW Coupes and Sedans 1970-1988 Repair Guide

Troubleshooting the Electronic Ignition System

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See Figure 1

  1. Pull the high tension wire out of the center of the distributor cap and hold it within about 3 / 8 inches (9mm) of a good ground.
  2.  
  3. Crank the engine and check for a spark. If there is a spark, proceed to Step 2; otherwise, follow the steps below:
  4.  



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Fig. Fig. 1: Early electronic ignition diagram

    1. Pull the black/red wire off starter terminal 6 and connect a voltmeter between the terminal and a good ground. Operate the starter and read the voltage on the meter. Then read the voltage across the battery terminals while the starter is operating. If the voltage of the black/red wire is significantly below battery voltage, replace the wire or connectors as necessary.
    2.  
    3. Unfasten the transmitter wires at the control unit and measure the voltage coming from the transmitter (in the distributor).
    4.  


The voltmeter should be set on the most sensitive scale.

  1. The connector going to terminal 7, and the terminal to 31d. at cranking should register a voltage of at least 0.05 volts. If not, check the rotor gap. It should be 0.01-0.3 inches (0.35-0.68mm).
  2.  
  3. Replace any defective parts as necessary. Replace the transmitter wiring, if defective, or replace the pulse transmitter, if necessary. f
  4.  
  5. Inspect the spark plugs, distributor cap and rotor, and ignition wires. Replace any defective parts as necessary.
  6.  
  7. Measure the voltage between the coil primary terminal leading to the control unit and ground. If there is less than 2 volts, proceed to the next step. Otherwise:
    1. Check for voltage at the plug going into control unit terminal 15. If there is not voltage, replace the ignition switch.
    2.  
    3. Check for voltage at the plug going into control unit terminal 16. If there is no voltage, repair/replace the wire leading from ignition coil terminal 1 to terminal 16.
    4.  
    5. Check to see that the control unit is properly grounded (terminal 31). If not, replace the ground connection.
    6.  

  8.  

If none of these checks reveal a problem, or if making the required repairs still results in poor voltage output, replace the control unit.

  1. Measure the voltage between ignition coil terminal 15 and ground. It should be more than 3 volts. If not, replace the resistor of the ignition coil.
  2.  
  3. If the above teste are performed and the engine starts, adjust the ignition timing as described in Engine Performance and TuneUp Connect a dwell meter between coil terminal 1 and ground. Check the dwell angle at 1500 rpm and at 6000 rpm. At 1500 rpm, the dwell angle should be 32-53°. At 6000 rpm, the dwell angle should be 43-56°. If dwell is incorrect, substitute a new or known good pulse transmitter. If dwell is still incorrect, replace the control unit.
  4.  

DIGITAL MOTOR ELECTRONICS (DME) ENGINE CONTROL SYSTEM

The DME engine control system used on many 1984-88 models is a complicated system. Troubleshooting is best left to a trained BMW service technician who is familiar with this system.

 
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