How to Test Ignition Coils on a BMW 328
by William Zane
On recent models like the 328 3 Series, BMW switched from the use of spark plug wires to individual coil packs that install on each cylinder’s spark plug. This is an efficient and reliable way of delivering spark to the engine. Like any other mechanical component, coils can fail over time and lead to rough running and loss of power, particularly at higher RPMs. Testing a coil is possible with an ohmmeter, which tests the amount of ohms that the coil is producing.
Step 1
Open the hood. Remove the cabin microfilter cover by turning the three knobs that hold it on 90 degrees. Lift the cover out and set it aside. Remove the cabin microfilter and disconnect the wiring harness from the filter housing. Remove the four T30 screws that hold the filter enclosure in place. Set the enclosure aside.
Step 2
Pry the small, round plastic plugs from the top engine cover and that conceal the screws for the engine cover. Remove the 10-mm nuts that hold the engine cover on with a socket. Set the engine cover aside.
Step 3
Unscrew the oil cap on the lower engine cover. Pry out the plastic covers. Remove the 10-mm nuts with a socket. Set the cover aside.
Step 4
Locate the coils. These are in a row down the engine and are bolted to the tops of the spark plugs. They each have a wiring harness going to them. Remove the two small bolts that hold the coils onto the spark plugs. You can then pull the coil up and off of the spark plug.
Step 5
Examine the end of the coil where it fastens to the spark plug. There will be three contacts. The outside two are the primary contacts. Using an electric ohmmeter, test the contacts. Place the ends of the ohmmeter testing cables on the contacts to see how many ohms the coils are producing on the readout. According to BMW, the acceptable range is 0.4 to 0.8 ohms.
Step 6
Replace the coil if the ohm reading is not within the acceptable range. It is recommended to change all of the coils at the same time, since one failing may be a sign that the others may be failing soon as well.
Step 7
Reinstall the coils by sliding them back over the spark plug. Reinstall the two small bolts with a socket. Reinstall the upper and lower engine covers. Reinstall the cabin filter enclosure, the cabin filter and the lid for the enclosure.
References
- Linquist: Removing BMW Coils and Spark Plugs
- "BMW 3 Series Service Manual 1999 to 2005"; Robert Bentley; 2005
Things You'll Need
- Screwdriver
- Socket set and ratchet
- Ohmmeter
Writer Bio
William Zane has been a freelance writer and photographer for over six years and specializes primarily in automotive-related subject matter among many other topics. He has attended the Academy of Art College in San Francisco, where he studied automotive design, and the University of New Mexico, where he studied journalism.
