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Old 12-29-2012, 11:58 AM
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BMR_LVR BMR_LVR is offline
Humble E34 lover
Location: Asheboro, NC
Join Date: Dec 2008
Posts: 5,464
Mein Auto: 1992 525i
Originally Posted by robertobaggio20 View Post
If the fuel pump only runs when cranking at key3, how could the engine run after it starts up and you are back at key2 ? Can you remember how you conducted the test on your E36 ?

I have checked it out on my car and there is current at the fp's connector in the trunk when at key2. The current disappears when the relay is pulled, but reappears when the relay sockets are jumped, which means that the pump keeps running whether the key is in the car or not. I had to drive for around 10 miles once, many moons ago, with a jumped fuel relay socket, and each time I shut off the engine I had to remove the jumped relay wires to prevent the battery from being drained down, and replace them when I wanted to drive again. Gosh that was not a nice 2 days.

Okay. Let me clarify. When the engine is not running, the fp is only energized for a short duration by turning the key to position 2 (for priming and pressurizing the fuel system) and then it is de-energized once the pressure builds up. In a properly functioning system, this usually takes less than 1 second. The fuel pump is energized when the key is put to position 3 so that it can crank the engine and keep supplying fuel to the engine. It then remains energized when the key is released to position 2 as long as the engine is running.

I tested this by removing my rear seat on the E36. The pump was easily heard when it was running. With the engine off, I put the key to position 2 and the pump only ran for just a second. It then stopped. Once the key was turned to position 3, the car cranked and the pump continued to run as expected with the key in position 2.

Just to be sure that the E34 operates the same way, I repeated the experiment just moments ago with an assistant to manipulate the key. However, the fp on my E34 is unbelievably quiet so I actually had to remove the carpet flooring in the trunk, remove the fp cover and then place my ear over the fp in order to hear it running. Upon repeating the experiment, I got the exact same results: key to position 2 and the pump ran for less than one second and turned off. Turned it to position 3 and the engine cranked and ran as expected. I tried to repeat the process and capture it on video, but the fuel system was now pressurized and therefore the pump did not energize in key position 2 so I could not capture it on video.

If you have 12v at the fuel pump all the time (not the fuel level sender you must make sure you are checking the correct terminals) in key position 2 without the engine running, then you have a problem in the system somewhere. Feel free to repeat the experiment on your car. Nothing has to be disconnected and it does not even require a multi meter, just the human ear and an assistant to manipulate the key (unless of course you have a very noisy pump). In fact, because of the possibility of checking for voltage the wrong point (fuel level sender), my method is probably better because it eliminates the possibility of the error.

This is the very reason that the fp checks can't be done by simply putting the key in position 2. That's why terminals 30 and 87 have to be jumped. This supplies constant voltage to the fp regardless of the ignition key presence or position thereby allowing one to test the fp.

And yes, you can run your car with the fp relay removed and terminals 30 and 87 jumped. However, that will cause a fault code to be generated and stored in the DME. Once the new fp relay is put in, then you can simply erase the fault code.

I hope this clarifies it and sorry if I caused any confusion.
Most problems are usually something simple !

Originally Posted by noego View Post
It's Deja Poo - as in, I've heard this **** before.

Calypso Red 1992 525i with 225K miles

1991 735i - Sold
1992 525i - Sold
1995 325is - Sold
2000 528i - Sold

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