129 sl starting problem

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New Member
Mar 23, 2008
hello need some help on starting a 1990 300sl m103.
the car has been parked up for over a year and will now refuse to fire up.
before it was taken of road it began to idle very badly and misfire.
i have replaced the following parts this week.
distributor cap.
new ign leads.
spark plugs.
rotar arm.
ovp relay.

i now realise i am not getting any power to the fuel pump,and as this has a

mas module i removed module and jumped pins 1 and 2 which turned the pumps on.
. the car would start after 10secs of cranking but was very rough and missfiring badly.

i also noticed that the rev gauge would not move .

would this be a faulty mas relay or is the crank sensor needing replaced or any other item i should look at,as pluging the mas module back in does not turn on fuel pump.

any help would be great .thanks.
The fuel pump relay has 2 stages
first it runs briefly on ignition switch on to pressurise the system- from cold you should hear it running briefly
second stage is it needs a signal from the rev counter or ECU or ignition unit [not sure on your car] to tell the relay the engine is running- if it doesn't give this signal the relay won't power the fuel pump as a safety measure.

Sounds as if the relay isn't getting that signal or the relay itself is faulty [ they are not cheap!]

I think the misfiring is a separate issue

= poor fuel quality- its been laid up for a year
damp in the ignition distributor body can migrate to the cap as the engine heats up. [ the wife's hairdrier is your friend!] OR you haven't got the firing order correct on the leads!
sorry for delay on reply .the mas module was not connecting with connector plug.

the plug wires were put on wrongly on no 4/6..running well now.

thank you.

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