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I have a nice 1980 mercedes 450slc that I bought about a year ago. I have been driving it since august 2011 and it has been running fine until one day I was sitting in traffic and it died and wouldnt start back up! I had it towed back home and put a full tank in it and it started and ran for about a minuet and then died.every now and then we can get it to start for a couple of seconds and then it will just die.Could it be the fuel pump or the fuel pump relay?I put in a new fuel filter but that didnt seem to do anything but im thinking its a fuel pump issue can anyone help?
 

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I have a nice 1980 Mercedes 450slc that I bought about a year ago. I have been driving it since august 2011 and it has been running fine until one day I was sitting in traffic and it died and wouldn't start back up! I had it towed back home and put a full tank in it and it started and ran for about a minute and then died.every now and then we can get it to start for a couple of seconds and then it will just die.Could it be the fuel pump or the fuel pump relay?I put in a new fuel filter but that didn't seem to do anything but I'm thinking its a fuel pump issue can anyone help?
Test's.


1. Remove the fuel pump relay behind the glove box, or sometimes mounted above fuse panel. Test inputs on the relay socket using a Digital Multimeter (DMM).

2. Terminal 2 on the socket (30 on relay) should be Battery Voltage (B+) at all times. Terminal 3 on the socket (15 on relay) should be B+ Key on Engine Off (KOEO) or Key On Engine Running (KOER). Terminal 6 on the socket (50 on relay) should be B+ Key On Engine Cranking (KOEC). Terminal 5 on the socket (31 on relay) should be ground at all times. Terminal 4 on the socket (TD on relay) should be a pulsed voltage KOEC or KOER, approximately 6-11v. Terminal 1 on the socket (87 on relay) is the output to the fuel pump, approximately 9-12v KOER when relay is connected and active.


And.


1. Tee in a fuel pressure gauge on the line which is connected to the warm-up regulator from the top center of the fuel distributor. The valve should have a shutoff valve in the tee which should be located on the warm-up regulator side of the gauge. System pressure will be indicated with the shutoff valve closed which should be 77-80 PSI. After the key is shut off, residual pressure should not drop below 22 PSI for 45 minutes.


And..


The fuel pump relay is on a panel above the fuses in the passenger's side kick panel. It is identified by a code tag with the number 21 on the wire harness. Inputs to the relay are: terminal 30, constant battery voltage; terminal 15, ignition key switched voltage in run or start; terminal 31, ground, terminal 50, battery voltage during start only, and terminal TD, tachometer (HZ) signal from ignition. Terminal 87 is relay output to the pump. If all inputs are good, apply power to the socket for terminal 87 and see if the fuel pump runs. If it does, replace the fuel pump relay.


And...


1. With ignition on, test for voltage at the first resistor. This resistor is blue and should have battery voltage on the front side and less than that on the back. Resistance spec. 0.4 ohm.

2. At the next resistor that is metallic in color you should have the same voltage on the front side as you had at the back side of the blue resistor. The back side of the metallic resistor should have less voltage on it than the front side did. Resistance spec. 0.6 ohm.

3. Verify that the voltage from the metallic resistor is found at the positive side of the ignition coil. Test the voltage on the ground side of the coil. The voltage spec is between 0.5 and 2.0 volts. If out of range, make sure to test powers and grounds at the switching unit and verify that the coil was not damaged before replacement of ignition module switching unit or coil.

4. Primary resistance of the coil is 0.33 to 0.46 ohms. Secondary resistance is 7-12K ohms.

5. Test for ignition dwell value of 5 to 23 degrees key on engine cranking. If no reading, test for faulty distributor pickup coil. (If not equipped with points) Test resistance at connection plug of ignition module between the two terminals disconnected from the ignition module, read 600 +/- 100 ohms while still connected to distributor.


And....


1. Test fuel pressure, at the fuel distributor, with the gauge teed in the line with the inlet fitting. System pressure specification is 72.5-81.2PSI/5.0-5.6Bar. When the engine is shut off, residual fuel pressure should be maintained above 20 PSI/1.4Bar for at least 20 minutes.

2. To test the fuel pump check valve for leakage, pinch the fuel inlet hose to the pump and test the residual pressure for loss. If there is no change, pinch the vent hose on the fuel accumulator which returns to the fuel pump feed hose from tank.


Actually spray some starting fluid into the intake and see if it will run longer then a few seconds, if it does, its in the fuel system. I would check under the cap and look at the center electrode first. Then do the test's.
 

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