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.

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.

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

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.:)

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.

Take a look at the socket, it should have numbers next to the holes.

1985 380sl
 

Thanks again Chuck. Will check the socket output this weekend. Regards.

Just in case ...
 

Here is the layout for the FP relay socket. Pin 8 will have 12V at all times.

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.:)

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.

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. ;)

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.

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.

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.

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.

1985 380SL Fuel Pump Relay
 

Greetings Chuck, No, I won't drive it with the jumper, I just want to make sure that it was the fuel pump relay because the part was a little bit expensive and I just replace this last year. If its the relay, do you have any idea what is causing this to break or its just a manufacturers defect and luck on my side :(. Chuck, you mentioned that jumping 7 and 8 is not a good idea to drive it this way and just for emergency. Can you please tell me what can go wrong if I do this (just curious but won't do it). Thanks again and Regards.

There are three issues, none of them technical, with driving the car with the relay jumpered.

- you may forget you did it and walk away leaving it running and end up with a dead battery

- your jumper might keep the pump running if the car were in an accident, a fire hazard

- your jumper might fall out an an inopportune time and kill the engine

The relays just seem to fail randomly. The only cause I can think of is excessive current draw by the pump. Alldata says it should be "approximately 7.5 amps." So you might check that.

You would do that with a meter on "amp" function, in line between the + side of the pump and the power wire.

Checking 380SL Fuel Pump Relay
 

Greetings Chuck, I did not work on my car today, I'm from Northern California and as you know we are having a big storm here since yesterday and it was bad. I will get back to you if I completed the test that you have suggested. I really appreciate all your help in solving the mystery in the car that I love. Regards and Thank you Chuck :vbac47679

Checking 380SL Fuel Pump Relay
 

Greetings Chuck, jumped relay socket 7 and 8 and the car runs. Confirmed it was the FUEL PUMP RELAY. Ordered the relay yesterday and will be in tomorrow. I cannot do this with out your help and just want you to know how grateful I am for helping me. Regards and Thank You very much.

Checking 380SL Fuel Pump Relay
 

Greetings Chuck, Replaced FUEL PUMP RELAY today and the car runs like a champ. Thanks and Regards. :D

1981 mercedes 380 sl
 

I have a 1981 380 sl that will not start.The fuel pump is not working. It is about 2 years old. I removed the relay. It is a 6 pin. I jumpered pins 1 & 2. Now I have no power at all to anything. What did I do wrong?

jumping in here..you likely removed the wrong relay or disurbed the main power supply relay which is the OVP relay on my car..Not familar with your car..Good luck..

Unclear what relay you removed and what connections you jumped. You may have blown a fuse or even fried a wire. But if this happened, though, there would have been a spark or some smoke.

To make sure you have the right relay, look for a large red-white-black wire connecting to the socket where the pin marked 87 would go. To be sure you are jumping the correct pins, look at the relay and jump the sockets where the pins marked 30 and 87 go.

The relay is a 6 pin relay with 31, 15, 87 on one row and 50, TD and 30 on the other row of pins. I jump 1 and 2 on the socket and every thing is completely dead now.