position against th e open ing forces applied to its
pr essu re sta ge (Fig. 11).
Injector opens (start of injection)
The solenoid valve is en ergized with the pickup
curren t which ser ves t o en su re that it open qu ickly.
The force exerted by th e tr iggered solen oid now
exceeds th at of the va lve spr ing and the armatu re
opens the bleed or ifice. Almost imm ediat ely, the high-
level pick-up current is reduced to the lower holding
curren t requ ired for th e elect romagnet. This is possi-
ble due to the magnet ic circuit’s a ir gap now being
sm aller. When the bleed orifice opens, fuel can flow
from t he valve control chamber into th e cavity situ-
ated above it, an d from there via the fu el retur n to
the t ank. The bleed orifice prevents com plete pres-
su re balan ce, and th e pr essure in the valve cont rol
chamber sinks as a result. This lea ds t o the pressur e
in the v a lve-contr ol cham ber being lower th an that in
the n ozzle’s ch amber volu me which is still at the
sa me pr essu re level a s th e rail. The redu ced pressure
in t he valve-con trol ch amber cau ses a redu ction in
the force exert ed on t he control plu nger, th e nozzle
needle open as a resu lt, and injection star ts (Fig. 1 1).
Injector opens fully
The contr ol plunger reach es its upper stop where it
rema ins support ed by a cushion of fuel wh ich is gen-
erat ed by the flow of fuel between th e bleed and feed
orifices. The injector nozzle has now open ed fully,
and the fuel is injected int o th e combu st ion chamber
at a pressure alm ost equal to that in the fuel rail
(Fig. 11).
Injector closes (end of injection)
As soon as th e solenoid valve is no longer trig-
gered, t he valve spr ing for ces the a rmature down-
wards and t he ball closes th e bleed orifice. Th e
armatur e is a 2–piece design. Here, although th e
armatur e plate is guided by a driver shou lder in its
downwar d movemen t, it can “overspring” with the
retu rn spring so that it exert s no downwards-act ing
forces on th e a rmature an d the ba ll. Th e closing of
the bleed orifice lead t o pressure build u p in the con-
trol ch amber via th e input fr om t he feed orifice. This
pressure is the sam e as tha t in th e ra il and exert s a n
increased force on t he con trol plu nger through its
end fa ce. This force, together with th at of the spring,
now exceeds the for ce exerted by the cha mber volume
and the nozzle needle closes. Injection ceases as soon
as th e nozzle needle comes u p again st its bottom stop
aga in (Fig. 11).
STAN DARD PROCEDU RE
STANDARD PROCEDURE - INJECTOR CLASSI-
FICATION
NOTE: Fuel Injectors have different flow rates.
When ALL injectors are removed, re-enter all injec-
tor six digit codes.
The cla ssificat ion of injectors in to 3 cla sses
descr ibes t he quan tity ch ara cteristic of the in ject or.
This will ma ke it possible in the fu ture to match the
engine softwa re to t he t olera nces of the injector
with in a m ore narrowly gr aduat ed ra nge. Classifica-
tion ca n be clearly recognized, a nd assigned only by
mean s of a DRBIII!.
Classified injectors can be recognized by the six-
digit alph anu meric code or pa rt number and identifi-
cation on th e magn etic head (circle with a num ber
between 1 and 3 inside) (Fig. 12). The number corr e-
sponds to the classification stage.
These genera l condition s equ ally apply if, as a
result of replacing an engine, car rying out r epa irs to
the cylinder head etc., the cylinder selective assign-
ment of the injectors or the en gin e cont rol module
assign ment may have cha nged. If proper attention is
not paid to the classification on these vehicles drive-
abilit y and smokin g concer ns could result.
If an injector is repla ced, it is then necessar y to
assign th e classification n umber to t he corresponding
cylinder with the DRBIII! in the contr ol module.
Fig. 12 INJECTOR CLASSIFICATION MARKINGS
1 - ELECTRICAL CONNECTOR
2 - SIX-DIGIT ALPHANUMERIC CODE
VA FUEL INJECTION 14 - 37
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