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#1
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No 12v to Fuel Pump
My 1972 350sl won't start. I check to wires to fuel pump, not hot. If I run a set of jumpers back to the fuel pump it will start, and the wires that I disconnected are now hot. There is something stopping the motor from starting and allowing the fuel pump to start pumping. In a perfect world the car will start and 2 or 3 seconds later the fuel pump gets 12v to keep it running. Any help would be great.
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#2
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There is a device called "fuel pump relay". Its operation is to come on for a couple of seconds when the key is turned on then go off until it senses an rpm signal from the tach indicating that the engine is turning over. This is to prevent the fuel pump from continuing to pump fuel in the event of an accident where a fuel line is ruptured but the engine is stopped with the key still on - a very good design. It's likely that your tach is inop or the fuel pump relay has gone bad or you've got a blown fuse...check the fuse first, it's the easiest and cheapest to fix.
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Current Stable
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#3
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Don't just visually check the fuse. If it is original it is aluminum and they are famous for corroding at the contacts. Replace with a brass/copper? ceramic fuse.
The fuel pump relay is top left in the relay panel above the fuses. It is the same as all the others. You can swap another from the right side to test. Just don't try the one below it as it is the ECU relay. |
#4
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I answered this in the Tech forum, but this is a better place for it.
BTW Mach4, are you sure the engine rpm is sensed from the tach? From memory (which is not always good!), I seem to recall that the djet ECU gets that from the trigger points. Quote:
If after testing you still don't have power to pump, then ... There are two items that control power to fuel pump. A fuel pump fuse. A fuel pump relay. Fuse is just one of those in fuse box. 8amp, I believe. Forget which one. Relay is one in pic below (located above fuse box) You can switch it with one of the others (they are all the same) and see if you then get power to your pump.
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Graham 85 300D,72 350SL, 98 E320, Outback 2.5 |
#5
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Quote:
Guess that's what I get for making assumptions - my bad and I apologize. Accordingly, please disregard my post above.
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Current Stable
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#6
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Quote:
In the case of K-jets, the TD signal comes from the EZL, and simultaneously to the fuel pump relay and the tach. |
#7
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Frank, you lost me with D-jets and K-jets. Will test the relay tomorrow with my spare.
Thanks Les |
#8
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Quote:
Quote:
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Graham 85 300D,72 350SL, 98 E320, Outback 2.5 |
#9
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By "hot," do you mean warm to the touch or do you mean that they are electrically charged? If the former, you have a problem; the latter would be expected from the point of jumper attachment back to the relay.
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#10
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By Hot I mean 12 Volts, not hot to touch. When I remove both wires from the fuel pump and run jumper cables from the battery to the fuel pump the car will start and the two wires I took off the pump now have 12 volts to them. If I remove the jumper cables the car stops running and the two wires are not hot.
I tested all the fuses and the fuel pump relay, all good. |
#11
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Quote:
When the potential on both sides of a motor is the same the motor will not run. |
#12
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Well I changed all the fuse's, the fuel pump relay, the Pump & Filter and stood on my head on the passenger side and IT RUNS. My thanks to all for the help.
Les |
#13
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Glad to hear it
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Graham 85 300D,72 350SL, 98 E320, Outback 2.5 |
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