BMW 5251 Service Manual page 355

120-7
Ignition System
Spark plug electrode gap /Too small
/TOO
big
Table b. Ignition secondary voltage diagnostics
Secondary
voltage low
Fuel air mixture
I I
Too lean
Secondary
voltage high
Spark plug electrode Worniburnt
4
The illustration shows normal scope trace of spark at idle.
1. Start of ignition voltage peak
2
Level of ignition voltage
3. Level of combustion voltage
4. Period of combustion
5. Combustion curve characteristics
6.
Start of decay process
7. Termination oscillations
Spark plug electrode
temperature
Engine compression
Spark plug wires
4
Short combustion period (indicates large spark plug gap).
Too high
Too low
I""
9 , 10
I" to I!,
r)
i " 21
10 il
bW,rn?7G
Too low
Too high
Faulty
4
Ignition spark at idle speed.
A
-
Normal ignition voltage peak (spark plug is good).
B
-
L o w voltage peak is an indication of a closed plug gap.
C
-
High voltage peak indicates an open plug gap.
<
Long combustion period (indicates small spark plug gap).
~i,+i!,~,i~tiii~iit~~,t1!lc,',l~~4~l~,~,l,~+~{+',,l
1 W W Sl , @ 10
:o
9 20 2 1 10 0