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  #1  
Old 08-05-2003, 08:21 AM
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Checking 380SL Fuel Pump Relay

On Sunday, the Wayward Woman ('81 380SL) turned over but refused to start. Very politely, in the garage. No noise from fuel pump. I jumped pins 1 & 2, and the pump ran, which would indicate a bad relay, but I decided to run through the tests in the manual to be sure. I thought I would write them up, since they are fairly straighforward, and there are conditions that could lead to replacing a relay when a wire or the TDI (ignition unit) is bad.

Note that this test is for cars with the older six-pin relay. The tests for the newer twelve-pin are different.

Remove relay.

1. Ignition off. Pin 2 - ground: 12V
2. Ignition on. Pin 3 - ground: 12V
3. Ignition off. Pin 2 - Pin 5 (ground): 12V
4. Check for RPM signal at Pin 4 with starter operating. I ran this test with my twenty-year-old Sears analog dwell/RPM tester, connecting the lead that would normally go to "-" on the coil to Pin 4.

Insert relay partially.

5. Check for voltage on the partially-inserted Pin 6 with the starter operating.

Corrective action for steps 1-3 is fix open circuit. Step 4 is to check for a bad connection from the TDI, and if none, replace the TDI (gulp!). Step 5 is to replace the relay.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #2  
Old 01-20-2008, 06:33 PM
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1985 380sl

Chuck, I have a 1985 380SL. I'm not getting a 12V from the fuel pump terminal and reading this forum it was recommended to check/replace the fuel pump relay. I did replaced the fuel pump relay last year and now I'm getting the same problem (won't start and no 12V on the pump terminals). Could it be the fuel pump relay again? Do you have any suggestion or how to test a 7 pin relay. Any help will be much appreciated.
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  #3  
Old 01-21-2008, 11:13 AM
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How are you testing for voltage at the pump? You will only get voltage when you crank the engine. No voltage with just key ON.

Here are the basic tests if the engine is not firing at all when cranking:


0. Pull the relay out.

1. Test pin 11 for ground.

2. With key ON, test pin 8 for 12V

3. Jump pins 7 and 8 on the socket with a piece of #12 wire. Pump should run. If it doesn't then it's most likely a wiring problem between the relay and the pump. Be sure you have ground at the pump.

4. Put a meter or test lamp between 11 and 12, and see if you have current when you crank the starter. If no current, fix the wiring problem - I believe 12 gets current from the purple wire that runs from the big connector on the right fender well (where the batter is connected).

If these four tests are OK, you need a new relay.

Good luck.

If the
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #4  
Old 01-21-2008, 10:07 PM
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Thumbs up 1985 380sl

Chuck, I would like to THANK YOU for all the help you are giving us in this forum. I will try the test procedures you have given and will update the results in this forum.
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  #5  
Old 01-22-2008, 07:39 PM
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1985 380sl

Greetings Chuck. I tried to work on my car today, I did pull the fuel pump relay and the Pin Numbers you gave me are not on the relay. I got 7 pins total (3 on one side and 4 on the other side. The 3 pins are 31, 15 and 87. The other 4 pins are 50, TD, 30 and 87K. Can you please advise me what output I'm suppose to get and on what pin number. Regards and thanks for any help you can provide.
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  #6  
Old 01-22-2008, 07:56 PM
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Take a look at the socket, it should have numbers next to the holes.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #7  
Old 01-23-2008, 08:16 AM
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1985 380sl

Thanks again Chuck. Will check the socket output this weekend. Regards.
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  #8  
Old 01-25-2008, 04:06 PM
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Just in case ...

Here is the layout for the FP relay socket. Pin 8 will have 12V at all times.
Attached Thumbnails
Checking 380SL Fuel Pump Relay-1985-380-fuel-pump-socket.gif  
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #9  
Old 01-25-2008, 10:13 PM
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1985 380SL Fuel Pump Relay

Chuck, it was so nice of you to send a layout of the socket relay. To be honest with you I'm really having hard time locating the socket numbers due to the confined space of the glove compartment where the fuel pump relay was located. With this layout it will be easier for now to verify all the output you have given me. Thanks and Regards.
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  #10  
Old 01-26-2008, 07:42 AM
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Uhh, you did see the 8mm nut that holds the socket? If you remove and pull the socket down, it's a lot easier to get to the pins.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #11  
Old 01-26-2008, 02:20 PM
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1985 380SL Fuel Pump Relay

Chuck, Thank You so much for the advice . I will do that and will publish the results here in this forum for reference in case somebody will have the same issues with Fuel Pump relay troubleshooting. Regards.
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  #12  
Old 01-26-2008, 06:58 PM
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1985 380SL Fuel Pump Relay

Chuck, I did the following test you have given me and here are the results:

0. Pull the relay out.

1. Test pin 11 for ground. - GOOD ground

2. With key ON, test pin 8 for 12V - GOOD, 12V Present

3. Jump pins 7 and 8 on the socket with a piece of #12 wire. Pump should run. If it doesn't then it's most likely a wiring problem between the relay and the pump. Be sure you have ground at the pump. - GOOD, Pump is running

4. Put a meter or test lamp between 11 and 12, and see if you have current when you crank the starter. If no current, fix the wiring problem - I believe 12 gets current from the purple wire that runs from the big connector on the right fender well (where the batter is connected). - GOOD, lamp lights up when starter is turning


Seem its a relay problem. Chuck, I also have a 1980 450SLC. My question is, can I use the relay of my 1980 450SLC and put it in my 1985 380SL just to test if it is really a fuel pump relay issue? Is there any other thing I can do to run the car without a relay with the use of a jumper? Please advise if not I will just order the relay. Regards.
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  #13  
Old 01-26-2008, 09:57 PM
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No, the relay from the 450 won't work.

You have validated that all the inputs to the relay are there, that the pump runs, and that the pump wiring is OK. Nothing else to do but replace the relay.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #14  
Old 01-26-2008, 10:22 PM
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Checking 380SL Fuel Pump Relay

Greetings Chuck, I'm willing to replace the Fuel Pump Relay, I just want to be 100% sure . Can I jump pin 7 and 8 on the relay socket and start the car (just to make sure that it is a relay problem) will the car start?. Chuck, what if its the ECU that is the culprit. Will I have the same problem (won't start)? Regards and thanks a lot for your support.
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  #15  
Old 01-27-2008, 07:45 AM
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No problem to start the car with 7 and 8 jumped - that's the connection that the relay supplies when it's working. Not a good idea to drive it that way, except in and emergency.

There are any number of things that can cause a 380 not to start, but the ECU is down the list. It's function is to make minor tweaks to the mixture when the car is running. It's also quite rare for one of them to fail.
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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