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I recently purchased a 1986 560 SL that have been stored in a heated garage for the last 19 years. The paint and interior are nearly perfect and the wife is slobbering, wanting to get her hands on the car. The problem is reliability. I removed the fuel tank and cleaned it out blew out the fuel lines replaced fuel pumps and filter as well as the screen. New battery and tires and the car started right up and ran like a striped assed ape! For a couple weeks I took it on short trips 30 minutes or less and it ran perfect. My confidence built up and I took it on the longer trip and it suddenly died at 50 minutes. It re-started and ran perfect for 20 more minutes died suddenly again, restarted and ran the rest of the way home, about 20 more minutes. I did some reading on these forums, and replaced the fuel pump relay. On the next drive it died at 50 minutes again, started right back up and ran perfect the rest of the way home. I parked it in the garage and have been on a one-month bike trip, just got back yesterday, decided to drive the car in to town for groceries. 15 minutes to town. It made it to the store running perfectly, when I started the car to go back home it was missing badly, black smoke out the exhaust and the economy gauge would peg itself to the right side anytime I stepped on the throttle. Grossly over rich. Barely made it home. I have ordered new spark plugs and wires. The car does not have the OBD port. Just the 9 pin circular port for the oscilloscope. I believe I have 2 separate issues. I know there is a ton of knowledge on this site. Does this scenario sound familiar to anyone out there? Thanks in advance for any direction. Would love to let the wife drive this beauty!
 

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Discussion Starter #5
How did you address the low voltage to the ECU? Is there a relay that feeds it? Ignition switch?
Thanks for the hint!
 

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Thanks for the location. Must be right where I changed out the fuel pump relay, behind the glove box.
 

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How did you address the low voltage to the ECU? Is there a relay that feeds it? Ignition switch?
Thanks for the hint!
Eventually the car died and wouldn't restart. Finally found a working ECU that matched the 1 in the car. Replaced ECU. Car works fine now. There are a couple members here who might have one. But check everything else first. Also, you have to check the voltage to/from EACH terminal (pin) on the ECU. I don't think there's a relay that feeds it. Also, if the ignition has a fault, the ECU would work only when the ignition switch is fully functional. Ignition switch and circuits are easier to trace and less expensive to fix. If there is low voltage to 1 or more of the pins on the ECU, trace those circuits 1st and make any repairs. If none of the repairs help, or if there are no faults in the circuitry, it's the ECU.
 

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Discussion Starter #10
Thanks for the good advice. I have a newbie question. On my 560, if it fails to start on the first try, the starter will not engage again unless I turn the key to the off position first, then rotate to start again. Is yours like this? Or is this something wrong with mine?
 

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Thanks for the good advice. I have a newbie question. On my 560, if it fails to start on the first try, the starter will not engage again unless I turn the key to the off position first, then rotate to start again. Is yours like this? Or is this something wrong with mine?




It's a German thing, seen the same in VWs and Porsches of the era
 

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Thanks for the good advice. I have a newbie question. On my 560, if it fails to start on the first try, the starter will not engage again unless I turn the key to the off position first, then rotate to start again. Is yours like this? Or is this something wrong with mine?
That's not a bug, that's a feature. Older Mercedes have a mechanical interlock to prevent the key being turned to the start position a second time unless it's turned all the way off first. Later ones with Drive Authorization Systems (those fancy electronic keys) don't keep you from turning the key to the start position, but since the start is done entirely under computer control, it will simply refuse to attempt another start unless you turn the key all the way off.
 

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Eventually the car died and wouldn't restart. Finally found a working ECU that matched the 1 in the car. Replaced ECU. Car works fine now. There are a couple members here who might have one. But check everything else first. Also, you have to check the voltage to/from EACH terminal (pin) on the ECU. I don't think there's a relay that feeds it. Also, if the ignition has a fault, the ECU would work only when the ignition switch is fully functional. Ignition switch and circuits are easier to trace and less expensive to fix. If there is low voltage to 1 or more of the pins on the ECU, trace those circuits 1st and make any repairs. If none of the repairs help, or if there are no faults in the circuitry, it's the ECU.
The 560 has a totally different system. Yours is electronic injection. The 560 is not. There is a relay for your ECU. On the '73 t is the second one down on the right side of the relays just below the FP relay. Probably the same on yours. Your ECU is on the firewall, the 560 has a different type below the right side floor under the carpet.
 

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Discussion Starter #16
Hey thanks Rowdie! On the 560, the ECU is under the carpet? And is there an ECU relay feeding it? Thanks again!
 

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I should have said it is under a plastic cover under the carpet.
I don't know about a relay. Maybe it is in the online manual wiring diagram.
 

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It's a German thing, seen the same in VWs and Porsches of the era
The idea being that (with such a quiet engine) you don't try to start a running engine. Lesser cars would give you that unpleasant scraping sound of the stater gear trying to mesh with a turning flywheel.

I am trying to teach my teenage kids that they need to LISTEN to the engine starting before then releasing the key, unlike our modern cars where it is all done by the computer, you can flick the key (my son) or just hold hit there for 30 sec (my daughter). Old cars are just sooooo complicated - whatever..
 

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The 560 has a totally different system. Yours is electronic injection. The 560 is not. There is a relay for your ECU. On the '73 t is the second one down on the right side of the relays just below the FP relay. Probably the same on yours. Your ECU is on the firewall, the 560 has a different type below the right side floor under the carpet.
Thanks for the info Rowdie. That was my experience.
 

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You mentioned that you replaced the old tires but did you replace the other old rubber items such as hoses and belts? For a car that old (I owned one) rubber does not get better with age and blowing a hose of belt at speed is not recommended and will downgrade the reliability factor. I'd replace all!
 
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