1986 MB 420 SEL Start Problem - Mercedes-Benz Forum

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Old 02-18-2007, 07:27 PM   #1 (permalink)
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Date registered: Feb 2007
Vehicle: '86 MB 420 SEL, '87 MB 300E, '91 MB 190E, 92 MB 190E
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1986 MB 420 SEL Start Problem

I recently bought this '86 420 SEL with 174K miles. It wouldn't start, which is one main reason the previous owner sold it. I had a feeling it would not be a big problem...
Turning the ignition key, you could hear something making some noise in the engine compatment. The fan blade would roll, but car wouldn't start. Going through several forums, I had a feeling the problem was related to the fuel delivery system...
I put some gas in the carburetor and the car started briefly and then died...
Taking out the fuel pump relay and smelling it, you can smell that it got burnt.
From the forums, I decided to jumper ports 30 and 87 on the fuel pump relay. The car started right away. I left the relay on and the following morning it appears the battery went dead. Put in a replacement battery and the car started again. Right now, I have the defective fuel pump relay out of the car so it wouldn't run down the battery again. Will get replacement from a MB dealer anytime soon.
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Old 02-19-2007, 12:53 AM   #2 (permalink)
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Fuel pump relay problems are a common problem. Take the cover off and resolder all the connections especially the larger ones. Vibration causes the joints to break. Save money, $0.00 in cost to you.
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Old 02-19-2007, 09:17 PM   #3 (permalink)
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Quote:
Originally Posted by wbain
Fuel pump relay problems are a common problem. Take the cover off and resolder all the connections especially the larger ones. Vibration causes the joints to break. Save money, $0.00 in cost to you.
Thanks. I might try just that.
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Old 02-20-2007, 02:43 PM   #4 (permalink)
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I have the same problem. Can you please tell me how to take the cover off?

Last edited by Uli73; 02-20-2007 at 02:51 PM.
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Old 02-20-2007, 02:50 PM   #5 (permalink)
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In the fuse box on pre 86 cars and in front of the brake booster on post 85 cars.

It has the label 'kickdown' on it.

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Old 04-12-2008, 08:06 PM   #6 (permalink)
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i've got a 1986 420 sel that will run sometimes but recently will not. when it doesnt run its not geting fuel cuz it starts if i pour gas in the intake for a few secs. i was wondering what it could be and how to tell what it is. im debating between the fuel pumps or the fuel pump relay is what i think also how did you jumper the relay?
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Old 04-12-2008, 08:59 PM   #7 (permalink)
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You just pull the relay out of the socket and place a jumper wire between terminals 30 and 87(see above) in that socket.
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Old 04-13-2008, 11:09 AM   #8 (permalink)
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okay that worked to get it started but if the throttle is put to the floor it acts like its staving for fuel because it only revs up to 4000 rpms and will not go any higher with very little power. i was wonder since it has two fuel pumps could one of them be bad only giving enough fuel to start the car? how do the fuel pumps run? one at low gas need and both at high need or do they both always run? thanks
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Old 04-13-2008, 07:30 PM   #9 (permalink)
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Carburetor?


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