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Discussion Starter #1
I replaced the fuel relay on my 380sl probably 3k miles ago with an aftermarket unit. Now I'm having trouble again. Is the problem with the aftermarket unit or a common problem with connection?
 

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1987 560SL (L.Tonk) [92,700 miles]
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263 Posts
For me, the engine will occasionally stutter when accelerating gently from a dead stop. I let off the gas and It always recovers. Seems more prevalent when it’s warm out (rarely gets above 80 where I live). Don’t know for sure it’s the relay (and It likely isn’t, now that I know it’s already been replaced).


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1985 380sl , 90,000 miles
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13 Posts
Have you tested the relay? I just learned and prevents the blind replacement solution I previously used.

I am working my way though a different fuel issue and have learned that there are more possible reasons for fuel delivery issues than solutions! I am no expert but I haven’t seen anything that suggests a relay can fail at higher temperatures but work at lower? My thoughts on your problem are fuel pump or gas tank obstructions. A good kjetronic fuel pressure tester seems required to fix any fuel issue in these cars without doing the blind replacement technique.
 

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560SL '88 Suzuki GS1000E '78
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65 Posts
Hi, the easiest way to check the relay is to connect a 12V-bulb with 2 cables on the terminals of the fuelpump. If the cables are long enought so that you can see the ligth (or no light) out from the drivers seat (...), it's easy to check if the circuit works proper.
How old is the fuelfilter?
@oncebitter: you may check the KE-Jetronik regarding duty cycle and the potentiometer of the airflow-meter as well. There is a lot information available on Youtube ("mercedes air flow sensor potentiometer "). But first make sure, that you distributor ist perfect aligned and the engine doesn't suffer from poor seals in the intake-manifold.
Regards Martin
 

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w108 & w107
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63 Posts
Otherwise you can take out the relay and jump the terminals with a short piece of electrical wire to provide continuous power to the fuel pump. Can't remember the pin numbers to jump, but a quick search will show this if this is an option.
 

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560SL '88 Suzuki GS1000E '78
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65 Posts
You only need to connect terminal 7 and 9 (resp. these cables) on the connector-side, see pic.
Regards Martin
 

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1987 560SL (L.Tonk) [92,700 miles]
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263 Posts
Hi, the easiest way to check the relay is to connect a 12V-bulb with 2 cables on the terminals of the fuelpump. If the cables are long enought so that you can see the ligth (or no light) out from the drivers seat (...), it's easy to check if the circuit works proper.
How old is the fuelfilter?
@oncebitter: you may check the KE-Jetronik regarding duty cycle and the potentiometer of the airflow-meter as well. There is a lot information available on Youtube ("mercedes air flow sensor potentiometer "). But first make sure, that you distributor ist perfect aligned and the engine doesn't suffer from poor seals in the intake-manifold.
Regards Martin
Thanks Martin. The car is hibernating for another few months but I’ll add that to my list for when the warm weather returns. I verified the distributed alignment when I replaced the chain guides and it was spot on.

What’s odd is the car runs near perfectly otherwise, and the stutter is not a frequent occurrence. I only fear it will finally stall out and strand me somewhere.


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Outstanding Contributor , Bob's Your Uncle!
-----'83 280 SL----- 5 speed....The PIG
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29,196 Posts
If your relay has suffered an early death....you might want to look downstream.

Clogged fuel strainer in the tank or clogged fuel filter might be making the fuel pump work harder which in turn overheats the relay.

I know an old lady
She swallowed a fly
But I don't know why
She swallow the fly
- I guess she'll die
I know an old lady
Who swallow a spider
It wiggled and jiggled
And tickled inside her
She swallowed a spider
To catch the fly
But I don't know why
She swallowed the fly
- I guess she'll die
I know and old lady
Who swallowed a bird
Isn't that ubsurd
To swallow a bird?
She swallowed the bird
To catch the spider
Who wiggled and jiggled
And tickled inside her
She swallowed the spider
To catch the fly
But I don't know why
She swallowed a fly
- I guess she'll die
I know and old lady
Who swallowed a cat
Just imagine that
She swallowed a cat
She swallowed the cat
To catch the bird
She swallowed the bird
To catch the spider
Who wiggled and jiggled
And tickled inside her
And she swallowed the spider
To catch the fly
But I don't know why
She swallowed the fly
- I guess she'll die!


You get the idea?:)
 
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