Mercedes W140 S

8/18/2019 Mercedes W140 S

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Mercedes W140 S-Class | Intermittent misfire

Background information

The Mercedes W140 S-Class V engine had de!elo"ed an intermittent misfiring fault# The misfiring could $e o$ser!ed during idle s"eed %hen the engine %as

%arm & ho%e!er it %as not a serious issue# Within a month it $ecame a""arent that as the engine coolant tem"erature reached '0 (C) the *intermittent+ fault

$ecame "ermanent# ,t this stage the car %as still dri!a$le) $ut had significantl less "erformance than e."ected and higher fuel consum"tion than normal#

The engine has a Bosch /-etronic 2*Luftmasse-Hitzdraht +3 inection control %ith hot %ire air mass meter) 5 ignition coils and 5 distri$utors# The ignition is

controlled $ a se"arate 67/ 2*Elektronische Zündanlage+3 i#e# ignition control module# There %ere no 8TCs stored#

The in!estigation

,s an initial test) the inectors %ere cut off one $ one in order to determine the clinders at fault# It %as found that %ith clinders 1) 4) ') and 9 cut off) the

idle s"eed did not change# These clinders are all fed $ the distri$utor mounted on the right clinder $ank#

Inector ' %as dou$le checked %ith the sco"e and %as found %orking $ut unsettled# The inector o"en time changed :uickl 2;1 ms3 and in turn the engine

idle s"eed also changed<

The king lead %as checked and sho%ed t%o t"ical %a!eforms


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The s"ark "lug leads %ere then checked at clinders four and se!en) and the kV %as found too lo% and unsta$le returning %a!eforms such as the t%o $elo%


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The "rimar ignition %as measured and com"ared to the king lead# The "rimar !oltage %a!e 2Ch ,3 looked normal) $ut there %ere a$normal signals on the

king lead 2Ch B3# ere is an e.am"le of a good %a!eform


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6!er three or four firings the !oltage %as um"ing in the same manner as %hen the secondar kV %as measured at the s"ark "lugs "re!iousl# In addition

the engine %as uddering in the same manner as the follo%ing unsta$le %a!eform<

o%e!er) the "rimar current %as sho%ing normal signals


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The solution

The first ste" to sol!ing this issue %as to re"lace all the s"ark "lugs# This %as done as the %ere all close to their 5=000 km life s"an# In addition to ne%

s"ark "lugs) one su""ressor %ith a ne% V 2igh Voltage3 lead %as re"laced tem"oraril# o%e!er) neither change sol!ed or im"ro!ed the "ro$lem %e %ere

e."eriencing# Through a "rocess of elimination) the "ro$lem could no% $e identified as $eing %ith either< the coil) the distri$utor) or the ignition controlmodule 267/3#

The interference at the "rimar ignition %as misleading %hen !ie%ing the diagnostics results# This %as "ro!en $ecause the tem"orar connection of a good

coil did not change the recordings# 8ue to this) %e mo!ed our attention on to the distri$utor#

>nl once the distri$utor %as remo!ed) %as it "ossi$le to see %here the high !oltages had $roken through on the $ack side and um"ed against the engine

$lock# This resulted in all energ $eing lost under the ca") and nothing $eing deli!ered to the "lugs# The "ro$lem had started %hen the misfiring at idle s"eed

%as first noticed# Various small cracks in the casing) starting from the distri$utor centre "in) %ere gro%ing in length o!er a short time# The distri$utor %as

later cut for in!estigation to identif that cause of the "ro$lem#

The high !oltage shot out at the glass fi$re 1 kilohm resistor and made a conical hole in the casing#

n the a$o!e left image) the distri$utor is sho%n still in "lace? ou can clearl see the discolouration in the middle# Both the centre and right images sho% the

micro cracks#

The left image $elo% sho%s the rear of the distri$utor) re!ealing the outlet for the high !oltage) %hile the right image sho%s the distri$utor cut in half)

dis"laing the e.it taken $ the high !oltage at the resistor#


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The result

>nce the distri$utor had $een re"laced the %a!eforms sho%ed the e."ected signals#

The conclusionThe ca"ture of the %a!eforms did not take long) $ut the interference on the "rimar side %as misleading) due to the ground signal $eing distur$ed $ the

high !oltage going into the engine $lock# >ther than that no "art %as unnecessaril re"laced and the fault %as found e!en %ithout stored 8TCs#

The reason the distri$utor %as not considered to $e at fault earlier in the in!estigation %as $ecause it %as almost ne%) ha!ing $een re"laced in 5009 and

onl $een used for 5=000 km#

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Mercedes W140 S