5. Ignition HT coils: removal. refitting and testing
Remove the ignition HT coil cover (where fitted) and make a note of the connections before disconnecting the HT leads and the coil wires.
On 75 models remove the coil bracket top mounting bolt and swing out the assembly until the bottom mounting bolt can be
removed. Withdraw the complete assembly; the individual coils can be unbolted if necessary.
On refitting, ensure that there is clean metal-to-metal contact between the coils centre poles and their mounting
bracket, also that the mounting bolts are securely fastened. If there is any doubt about the wire connections, note that the HT lead length
will show which coil is connected to which spark plug and that the coil for cylinder number 1 (front) has the black/blue wire, number 2 has
the black/red and number 3 (rear) has the black/green wire.
On 100 models remove the coil mounting bolts and nuts and withdraw the coils. Check that all connections are clean and
all mountings securely fastened on reassembly.
If there is any doubt about the wire connections note that the front HT coil serves cylinders 1 and 4, 1 being on
the outside (left-hand side); the black/blue wire is connected to its terminal 1, the green/yellow wire to its terminal 15 and the brown
wire to its (centre) terminal 31. The rear HT coil serves cylinders 2 and 3, 2 being on the outside (left-hand side); apart from the black/red
wire being connected to its terminal 1, all other connections are similar to the front coil.
The coils can be tested fully only on a spark-gap tester, which means that they must be taken to an authorised BMW dealer
or a similar auto-electrical expert who has the necessary equipment if they are thought to be faulty. First of all, however, a suitable meter
should be used to check that full battery voltage is available when the ignition is switched on. Connect the meter between the coil terminal 15
and a good earth point to make the test. If no power is found, check back through the system (as far as is possible) to trace the fault.
A quick check which will give a reasonable idea of a coil's condition is to measure the resistance of its windings, but
this can be applied to 75 models only due to the lack of equivalent data for 100 models. Disconnect the coils completely and connect the meter
between terminals 15 and 1 to obtain a reading for the primary winding. To test the secondary winding connect the meter between terminal 15
and the HT lead terminal. In both cases the resistances recorded should be close to those specified; if not the coil is suspect but should be
checked very carefully by an expert using the correct equipment before any action is taken.