Each spark plug in a 6.0L Diablo has its own
ignition coil directly on top of the plug itself. This arrangement known
as "Coil On Plug" or COP allows for a very efficient control of engine
timing. A low 12v lead goes to each COP where it goes to an induction
coil. The secondary coil lead goes directly to the
socket. Because there are no high voltage leads going along the engine
top to each coil higher voltages and sparks can be obtained.
|Figure 1. Diablo Coil On Plug Coil.
The only problem is that sometimes these coils short
out. They are buried deep within the engine (see below). Normal coil
testers will not work because the probe cannot be placed along side the
To test each coil you need to remove the complete COP from the
engine. Since there are 12 of them this will take some time.
Figure 2 shows the COP for cylinder #6. As you can see it is buried
deep within the engine. To remove the COP first unclip the 3 pin
electrical socket. Be sure to label each socket wire 1-12. Next
you must slide the aluminum clip sideways as shown in figures 3 and 4.
Them gently pull the COP straight up rotating it slightly from side to
side along the way. These COP's are coated with a silicon grease
so the come up slowly. Figure 1 shows a removed COP and Figure 5
shows the actual spark plug deep down in the engine.
To checkout the COP first do a visual inspection looking for
any burn marks or cracks. Check there are no carbon deposits in the
rubber shoe that fits over the spark plug itself. Next
examine the electrical socket on the top of each COP. There are three
pins numbered 1,2 and 3. With an meter measure the
resistance across leads 1 and 3 (fig 7). It should be about 1.2 ohms.
There should be an infinite resistance between leads 1 and 2, 2 and 3, and the
outer metal jacket and to lead that goes to the plug inside the
bottom rubber boot.
The rubber boot peels off from the base of the coil unit and is shown
in figure 8. The body of the COP unit is wrapped in two brass metal
sheets that slide off the unit (fig 9). These should be checked
for corrosion. And should be covered in a thin layer of silicon grease.
The inside of the rubber boot should be clean, free of carbon deposits
and dry. The spring should be as shown in figure 9.
Next the plugs themselves should be examined. They are buried deep
within the engine. A socket wench as shown in figure 10 is required with
a long extension bar. Figure 10 shows an isolated plug. As with all car
plugs they should be free of oil, soot and other deposits. The
spark gap should be 22mm. It is often helpful to use some fine
sand paper to lightly clean the spark gap surfaces as shown in figure
12. Getting the plugs out and in requires that you use a
long wire grab tool with prongs as shown in figure 13. These are
available at most auto stores. The plugs should be torqued th
about 180 lbs.
If you are getting a check engine light due to misfires and suspect a
coil on one side of the engine, switch the coil with another on the
opposite side of the car. The L or R check engine light should also
As far as putting the coils back into the engine, nothing special
just reverse the instructions given above. Be sure the clips are
installed correctly. The COP cannot be pulled up if the clips are in
correctly. This in fact is the hardest part of the whole
operation. The clips go in only one way. The COP leads should be at 8
o'clock, the clip back surface is at 2 o'clock as shown in figure 2.
Coil On Plug for Cylinder #6
Remove electrical connection
||Fig 4. Slide
out retainer clip
Plug deep within the engine
Isolated Coil On Plug unit
Measure primary coil resistance.
Coil rubber boot removed.
Brass bands around coils slide off
||Fig 10. Socket to
Lightly clean plug with sand paper
||Fig 13. Flexible grab
tool to remove plugs