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Old 04-16-2004, 06:07 PM
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Join Date: Feb 2004
Location: Northern California
Posts: 31
fuel system prime @ start

Hello everybody,
I have read this forum over the last two years on a daily basis. I have enjoyed and benefitted from the great amount of knowledge and experience. Using the search feature has always provided answers to my questions and I did not have reason to post any. So first of all, thank you for the answers!!! It has saved me a lot of money, I'm sure.

I tried this post on the SL site but did not get any replies. I need some help with a symptom I've been chasing for a while. I have an '84 380sl. When turning the key to the RUN position, I used to hear the fuel pump run for a second or so. When activating the starter, the engine would fire immediately. I haven't heard this "fuel prime" and when I activate the starter,
it takes longer for the engine to fire-up.

Studying the wiring diagram has not given me any solutions. In desperation I bit the bullet and did the wrong thing. I got a new fuel pump relay without proving it was faulty. It did not fix the problem. Rather than continuing the "throwing parts at it" syndrome, I am hoping to get some help. Any suggestions out there? I would greatly appreciate it.


'84 380SL
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Old 04-17-2004, 12:31 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I can't say as I have ever noticed the one second activity on the model you speak. I am sure it doesn't matter.

What is your basic problem? If it is slow starting the lack of the one second pumping feature should only be a one second delay or none at all if you immediately start the car. Does the pump run while the starter is running? That is what is important. The fuel pump relay should energize with a terminal 50 input or a engine speed signal of 300rpm.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 04-17-2004, 01:25 PM
Lurch's Avatar
Join Date: May 2003
Location: Toronto
Posts: 53
fuel pump...

Having had a similar problem twice (one of which cost me a starter ) check for the simplest thing and see if the electrical contact to the fuel pump is corroded..... a 25 part which could save considerably more down the road.

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