Ignition Coil Failure, Diagnosis &Replacement (4.0/4.6)


Introduction

On the Range Rover 4.0/4.6, the all-electronic ignition system is usually trouble-free compared with earlier models. There are very few components to fail, but one is the coil pack, which consists of four coils mounted together in one unit. Failure can occur and cause misfiring. Fortunately, because there are four coils, failure of one of them will not strand you, but misfiring is bad for the catalytic converters so the problem should be fixed as soon as possible. Walter Gates kindly contributed the information and photos below, documenting his experience diagnosing and replacing a perplexing misfire which turned out to be due to one of the coils failing. (Photo at right: Single burnt coil after removal from the four-coil pack)

Symptoms and Diagnosis Procedure

Symptoms:
Walter had a 98 4.0 Range Rover with 80,000 miles. He felt a slight engine missfire for a couple of days during routine in driving. After a couple of days the Check Engine light began blinking while driving, and the engine began to run very rough and lose power.

Diagnosis by OBD-II Fault Codes
Walter used OBDII scanning software which showed a number of pending trouble codes and two certain trouble codes -- #1313 and 1314 -- both indicating catalytic converter damage. (Note: the shop manual lists no separate trouble codes for the ignition coils). Upon deleting the codes and rescanning, the engine would not start. Checking the engine compartment fusebox for the cause, Walter found Fuse #26 was burned.

Fuse 26 Failure: Narrowing Down Problem to Coil
After replacing Fuse #26 (a yellow 20A) in the engine compartment, the engine restarted, but ran very rough and had to be kept above 2,000 RPM. Rescanning for trouble codes again showed #1313 and #1314, and after shutdown restarting was again imposssible. Fuse 26 was then found to be burned again.

According to the Electrical Trouble Shooting Manual (ETM), Fuse #26 protects three things:     1) The  four ignition coil drivers;   2) The four 02 sensors mounted on the catalytic converters; and,   3) The Engine Control Module. All these items are quite expensive to replace.  If the catalytic converters were damaged they, too, were expensive.  To narrow down the problem, Walter removed and tested all ignition leads with a digital ohm meter. He visually inspected all four ignition coils for cracks; none were noticed.  I checked all four coils where the input leads connect, and they all tested at 12.2 volts. 

To eliminate the O2 sensors Walter disconnected them (easily done by squeezing the top end of the connector and gently pulling and wiggling it), and turned the ignition on. After waiting 10 to 15 seconds for all systems to check, he attempted to start the engine-- it would not start, and fuse 26 was burned again. This implied the sensors were most likely not the problem.  Walter decided not to reconnect them but again replaced fuse #26. 

Next, Walter disconnected the grey coil pack connector behind the left (driver side) rear corner of the plenum. He then turned the ignition switch to "ON" and waited 10 to 15 seconds for all systems to check (he did not try to start the engine because the ignition coils were disconnected.)  He then checked fuse #26 -- this time it  was NOT burned.  Walter concluded the ignition coils were most likely the problem, so did not have to implement his final diagnosis plan of disconnecting the engine control module to see if fuse #26 would or would not burn.
 
Closer Inspection of Coils
At first Walter could not see anything obviously wrong with the coils, and all tested the same on the ohm meter. So Walter reconnected the 02 sensors and the coil pack.  He turned the ignition to "ON" and waited for all systems check and then tried to start the engine, but it would not start. Once again, Fuse #26 was burned.  After sevaral more burnt fuses, Walter found he could restart the engine by hitting "START" as quickly as possible but then  had to keep it running at 2000 RPMs or higher or it would die and fuse # 26 would burn.  After keeping the engine running for 15 to 20 minutes and then letting it idle down until it died, Walter rechecked the coils and noticed one of them had something on top of it that was black and light brown in color (it looked like a tree leaf).  There was also a distinct oder of an electrical burn.  Walter poked at the "tree leaf" but found it was hard and would not brush off the coil.  Upon closer inspection, he saw that the top of one ignition coil had ruptured (see photos). 

Each coil has two electrical connection points on the bottom (visible in the photo above) which are not accessible until the coil pack is removed. Walter notes that he tested each coil at those points, too, with a digital ohm meter.  Only the failed coil showed very little resistance and triggered the alarm on the testing instrument. However, on the top where the ignition leads are inserted, the failed coil tested the same as the other three coils that were not ruptured!!!  Thus, it is mandatory to test each coil at its electrical connections to identify the failed coil if its case has not ruptured.  

Coil Removal and Replacement
The coil pack is located in a slightly awkward position at the rear of the engine behind the plenum but is clearly visible. First, disconnect the leads.  Second, disconnect the coil pack connector (grey plastic at rear or plenum on the driver's side).  Third, remove the two nuts (one on each end of the coil pack mounting bracket).  Lift the coil pack and remove it from behind the plenum on the passenger side where there is plenty of open space.  Installation is the reverse. 

The cylinder number for each lead and coil in the pack is clearly marked on the coil pack mounting bracket.  Walter put white paint in each of the stamped numbers to make reading them easier after disconnecting its leads. Although only one of his four coils had failed, he had to replace the entire coil pack at a cost of $750 as he was unable to find a single Lucas coil.


Although the diagnosis took about four hours, it took less than 10 minutes to replace the coil pack.  (Ignition coil fault codes would have been nice).  Now Walter has three spare Lucas coils and will know what to do when Mr. Lucas shows his presence through fuse #26!!! If one of yours fails, contact Walter to see if he can spare one!!

Walter reports that he had a repeat of the ignition coil rupture in May 2004 -- and again had to replace the coil pack at considerable expense. This time, however, the source of the problem was 3 wires being rubbed through the insulation where the wiring harness went under the plenum.  On the underside the wires were shorting against the intake manifold.   It was an easy fix.