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#1
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1978 280SL Fuel Pump Relay & Wiring Info
just purchased the car not running. 159,000 miles. PO ran the car last 3 or 4 summers. this spring filled tank and brought it to car wash where it would not start. Car would fire and run for several seconds...almost sounding like it would start and run but wouldnt. I have dissassembled and reassembled wur...system pressure 70 psi and control pressure is 20psi. Did not help. Flow check showed 18 ozs in 30 seconds. Also checked timing chain 6 degrees stretch. Distributor cap had vaseline on all the contacts. cleaned contacts and rotor and an improvement car starts and runs at a very low idle for 10-20 seconds and dies. Would a frozen auxillary air valve cause the car not to run? Have statically checked distributor position looks ok. Have not turned car over with starter and check timing with timing gun. What areas should I look at next?
Last edited by rfitts46; 08-13-2012 at 09:12 AM. Reason: clarification |
#2
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Sounds to me like a fuel supply problem. Can you jumper 30 to 87 in the fuel pump relay socket and see if it will run?
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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 |
#3
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Chuck....good to be connected. does jumpering 30 to 87 operate fuel pump?
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#4
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Chuck, I have been using the safety switch on the air horn to get the fuel pump to run. That is how I have been able to make my pressure measurements and perform fuel flow capacity via the fuel return line at the fuel distributor. Does jumpering 30 to 87 on the fuel relay do something different than supplying 12vdc to the fuel pump? I have been using the safety switch so I didnt have to look for the infamous fuel relay.....
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#5
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Chuck, I am somewhat onboard now....just because the pump operates for a 30 second fuel capacity check by pulling the safety switch doesnt necessarily mean that the fuel pump stays on when the car starts and tries to run. It is all about the current draw of the fuel pump.....hmmm. Will try jumpering the fpr and post results.
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#6
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chuck as stated previosuly i read early on about the relay but thought I was ruling that out by using the ignition switch and safety switch. thanks for chiming in and giving me a new direction to head in.
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#7
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Jumping the relay is the best way to make sure the pump is running.
__________________
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 |
#8
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Chuck, Jumpered 30 to 87......no start. Checked for 12V at relay socket and found no 12V. The 12V is fed to contact 30 from the engine harness. What are the chances that you know where the 12V is supplied from? Bob
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#9
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Odd ...
I am traveling so no wiring diagram. I believe that 30 comes unfused from the connection on the right inner fender where the main battery cable terminates. But it may be different on a 280.
But the mystery is how the car would run at all with no power to 30.
__________________
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 |
#10
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I acquired a fuel pressure relay troublshooting guide. Just to be sure the 12V should be there with the key in the park position? or in other words the 12V is always present on contact 30. This is the only 12V feed to the fuel pump. It is interesting that the fuel pump does operate when the key is in the first position and the safety switch is disconnected.
I can see a small terminal strip inbetween the battery and coolant reservoir. The wire color for the 30 wire is supposed to be red. A couple large gauge red wires are on the first couple connections and then some smaller gauge reds. |
#11
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also....I pulled the glovebox liner out and found no relays. The only relay I found was directly above the fuse box. The fusebox is located behind a small 6" X 4" plastic panel which pops in to the plastic kick panel. The relay was accessible by pushing a piece of cardboard out of the way directly above the plastic kick panel. This is a metal covered relay......It looks identical to the relays used in my federalized 280SE electric windows?
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#12
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I have read through these posts a couple of times; the pattern that I think I hear is that the fuel pump will run with the key in the "start" position, but not in the "run" position. Does 12V appear at the pump relay term 30 with key in "start"? The "start" position will bypass the relay timer functions, allowing the fuel accumulator to fill and providing enough fuel to run for a few seconds without a valid ignition pulse signal (TD) to the relay. Various MB wiring diagrams that I have show that the power to term 30 is fused and switched through the ign switch. Possibilities for an open circuit then include a fuse and the contacts in the ign switch. When turning the key alternately between off and "run", you should be able to hear the pump run for a couple of seconds each time you turn to "run"; the timer in the pump relay will then shut off the pump (no TD).
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#13
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Frank I am troubleshooting a european car not a federalized. The relay function also changes....I believe after 9/1981? The 1978 280SL only has a 5 pin relay. Everything I have read suggests a 12V hot line sits on one of the relay dry contacts....everyone says it is contact 30. This is the high amp (8 amps) supply line to power the fuel pump. The
12V ignition, I would describe as a control signal to the relay. This control signal is what energizes the magnetic relay latch and closes the dry contacts between contact 30 and 87. Relay contact 87 is the hot line (12V) out to the fuel pump. When the jumper is inserted in 30 and 87 the pump should operate without the ignition in the start or run position. There is actually more than this going on but these are the primary functions of the fuel pump relay. You are right, after the required 12V(8 amp) hot line is present then the 12V ignition control signal to the relay needs to be confirmed. If the 12V is present at contact 30 and the pump is known to be good, the 12V ignition control line is tested by putting the relay back in the socket and turning the ignition switch to start or run. There is also a tachometer line to some of the relays that confirm the starter has been engaged. Dont know if that function is part of a 1978 euro car. |
#14
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After looking at some other diagrams and notes, I agree with CTaylor that the 12v supply to pin 30 is un-fused. You have indicated that power is not present at pin 30 in the "off" and "run" key positions; is power present at pin 30 when you operate the safety switch? Is power present at pin 87 when you operate the safety switch? Will the safety switch operate the fuel pump with the relay removed?
Turning to other items, can you confirm ignition timing? Are valve clearances too tight? I daresay that you know this, however tight valves and retarded spark will make it very, very difficult to sort out any fuel system problems. |
#15
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Frank, thanks for getting involved. Tonight I put 12V directly to the pump....ran the pump backwards for a time (20s) until I realized i had not gotten the polarity correct. Put the relay back in, 12V to the pump, pump operating correctly and then tried to start the car. Not even trying to fire now. Checked spark at #1 cyl and it was firing. Now concerned with consequences of running pump backwards for a 20 seconds or so. I have not had a timing light on it, but did check chain stretch, and distributor rotor was pointing at #1 distributor contact. I should be able to get the car running by manually running pump.....as far as valve clearances the car was running good until it stopped running at the car wash....assuming valve clearances are okay. I am thinking about putting pressure gauge back into the fuel system to confirm syst and control fuel pressure and continue troubleshooting fuel pump relay. I just checked for any 12V at the relay socket and there was none. So i do have to find out where the 12V for contact 30 come from.
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