Fuel Pump Relay Jumper...Why?
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Fuel Pump Relay Jumper...Why?
In trying to diagnose my starting problems. I have read many threads where they advise one to remove the fuel pump relay and jumper it to test if the pump is working, i.e. the engine is not running and the noise of the pump is very obvious.
Why would you just not turn the key, but not start the car. The pump comes on in this key position. Clearly one would not want a lot of random potential for a spark, but if there were no leaks evident to begin with and the goal was to just see if you can hear the pump run, why jumper the relay?
Why would you just not turn the key, but not start the car. The pump comes on in this key position. Clearly one would not want a lot of random potential for a spark, but if there were no leaks evident to begin with and the goal was to just see if you can hear the pump run, why jumper the relay?
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Sometimes the relay is the culprit. On my car when I turn the key to the on position the relay just clicks rapidly and doesn't engage the pump. Just a way to eliminate a potential problem source. And a spark in the cabin should not cause any concern. And I have not seen a spark from jumping the relay there when I have done so.
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In trying to diagnose my starting problems. I have read many threads where they advise one to remove the fuel pump relay and jumper it to test if the pump is working, i.e. the engine is not running and the noise of the pump is very obvious.
Why would you just not turn the key, but not start the car. The pump comes on in this key position.....
Why would you just not turn the key, but not start the car. The pump comes on in this key position.....
Alan
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Perhaps a matter of definition...
"As designed" operation is for the pump to run for a couple of seconds each time that the ignition is turned on.
The usual recommendation for the earlier cars is to make a proper relay jumper and keep it in the glove box. The fuel pump relay for these cars is a more complex, hard to replace electronic device, whereas the late cars use a standard "53" relay, with several "spares" installed in the panel. The relay jumper will get you home when the early relay fails. It is also much easier to do the testing by yourself with the jumper - you need to carefully listen at the rear bumper to make sure whether the pump is running or not.
"As designed" operation is for the pump to run for a couple of seconds each time that the ignition is turned on.
The usual recommendation for the earlier cars is to make a proper relay jumper and keep it in the glove box. The fuel pump relay for these cars is a more complex, hard to replace electronic device, whereas the late cars use a standard "53" relay, with several "spares" installed in the panel. The relay jumper will get you home when the early relay fails. It is also much easier to do the testing by yourself with the jumper - you need to carefully listen at the rear bumper to make sure whether the pump is running or not.
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Yes true it primes for maybe 2 seconds on first turning on the ignition - hardly useful in this usage scenario - and suggests the OP's car does not work as intended by Porsche.
Many fuel pump relays seem to fail to do the priming pulse - leads to slightly longer cranking - otherwise no ill effects.
If your relay turns on and stays on without crank rotation - its both unusual and a safety bypass.
Some manufacturers use impact sensing fuel cutoffs switches - Porsche detects engine rotation - if the engine isn't rotating the fuel pump turns off - a rather better system I think.
Many fuel pump relays seem to fail to do the priming pulse - leads to slightly longer cranking - otherwise no ill effects.
If your relay turns on and stays on without crank rotation - its both unusual and a safety bypass.
Some manufacturers use impact sensing fuel cutoffs switches - Porsche detects engine rotation - if the engine isn't rotating the fuel pump turns off - a rather better system I think.
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From: Ball Ground, GA
Porsche used a very expensive special relay - Chevrolet used a $2 oil pressure switch as the fuel pump ground. Even slower starting fuel pressure, but a lot cheaper...
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Yes - but in the later cars it became a cheap relay (53B) with the safety feature integrated into the LH...
Strangely without the priming pulse though (which it clearly could have implemented...).
Anyway - I think we can agree Porsche probably wasn't so concerned about absolute lowest possible cost in volume.
Alan
Strangely without the priming pulse though (which it clearly could have implemented...).
Anyway - I think we can agree Porsche probably wasn't so concerned about absolute lowest possible cost in volume.
Alan
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If you do make up a jumper for this (or for anything else) use an inline fusholder with 1/4" male blade terminals on the ends. Use a 30A 10 AWG ATO/ATC type holder - matches the fuses you already have for the later cars (no such option really for early cars).
By having a fuse - you can easily pull the fuse when needed without needing to plug in/out of the relay socket repeatedly, also better to be protected it you accidentally plug it in the wrong place. You should always disconnect the battery first anyway - but I know many of you won't - once you have plugged one end in the other end could be live so its safer with a fuse - use the lowest the circuit you need will allow.
Alan
By having a fuse - you can easily pull the fuse when needed without needing to plug in/out of the relay socket repeatedly, also better to be protected it you accidentally plug it in the wrong place. You should always disconnect the battery first anyway - but I know many of you won't - once you have plugged one end in the other end could be live so its safer with a fuse - use the lowest the circuit you need will allow.
Alan
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Amazing the detail one never knows about.
When I turned the key you could clearly hear the pump run and then shut off. I just assumed there was pressure switch in the pump itself so that once it reached the required 2 bars it shut off. So with the jumper in place, the pump will continue to pump. Ah ha!
This helps clarify a discussion I had with a Chrysler electrical systems engineer/928 owner. I showed him the relay with the circuit diagram on the side and asked why what I thought should be a simple on/off circuit had a IC chip in it.
When I turned the key you could clearly hear the pump run and then shut off. I just assumed there was pressure switch in the pump itself so that once it reached the required 2 bars it shut off. So with the jumper in place, the pump will continue to pump. Ah ha!
This helps clarify a discussion I had with a Chrysler electrical systems engineer/928 owner. I showed him the relay with the circuit diagram on the side and asked why what I thought should be a simple on/off circuit had a IC chip in it.
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Exactly !! so if you crash it stall the engine and it catches on fire the pump does not keep pumping.... Or if you are working on the sunroof , swapping radios, fixing windows and leave on the key the pump is not running which is good if you have a leaking injector...keeps you from filling a cylinder with liquid fuel. Trying to compress a liquid is a BAD THING
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Amazing the detail one never knows about.
When I turned the key you could clearly hear the pump run and then shut off. I just assumed there was pressure switch in the pump itself so that once it reached the required 2 bars it shut off. So with the jumper in place, the pump will continue to pump. Ah ha!
This helps clarify a discussion I had with a Chrysler electrical systems engineer/928 owner. I showed him the relay with the circuit diagram on the side and asked why what I thought should be a simple on/off circuit had a IC chip in it.
When I turned the key you could clearly hear the pump run and then shut off. I just assumed there was pressure switch in the pump itself so that once it reached the required 2 bars it shut off. So with the jumper in place, the pump will continue to pump. Ah ha!
This helps clarify a discussion I had with a Chrysler electrical systems engineer/928 owner. I showed him the relay with the circuit diagram on the side and asked why what I thought should be a simple on/off circuit had a IC chip in it.
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To answer Dan's question about the symptom is the car can be driving along at 60 and the engine just quits. Roll to a stop and it will not restart. Initially it would restart after a 1/2 hour. Now it takes overnight. Restart requires the throttle full open and the smell of fuel. There is no apparent fuel leak and when the engine finally starts there is a large cloud of black smoke which suggests the engine is flooded.
I have installed a fuel pressure gauge and fuel pressure seems fine (30+psi). I have changed fuel pump relay and the injector relay. Also tried swapping out the L-jet brain with a used one that had been tested and found to be ok. The green wire is 3 yrs old.
Any thoughts would be welcome..
I have installed a fuel pressure gauge and fuel pressure seems fine (30+psi). I have changed fuel pump relay and the injector relay. Also tried swapping out the L-jet brain with a used one that had been tested and found to be ok. The green wire is 3 yrs old.
Any thoughts would be welcome..
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Well, there are 2 very obvious questions:
1 - When it won't start, are you getting spark? An induction timing light will tell you.
2 - Are you getting an injector pulse? A noid light will tell you.
1 - When it won't start, are you getting spark? An induction timing light will tell you.
2 - Are you getting an injector pulse? A noid light will tell you.
