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EJ22 Fuel Pump Kill Switch - No ground from ECU
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sdirghalli
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Location: Long Beach, CA
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PostPosted: Fri Aug 23, 2019 6:23 pm    Post subject: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

After someone tried to steal my van a few weeks ago (thankfully they were interrupted by a neighbor driving by) I decided I needed a simple kill switch. Having removed the rear heater core/blower a long time ago and reading somewhere here on The Samba that the switch that controls the blower could be used for my exact purpose, I did the following (n.b. the van functioned normally prior to this...):
1. Disconnected the ground on the main battery
2. Modified the rear blower control switch on the dash so that putting the fan in one particular position would close a circuit.
3. Cut the green-black ground wire supplying ground from the ECU to the fuel pump relay.
4. Connected the cut ends of the green-black ECU wire to the wires coming from the rear blower switch to create a circuit that would be closed when the switch was in the position mentioned above.
5. Reconnected the ground on the main battery

With the rear blower motor switch in the position needed to close the circuit I turned on the ignition but the fuel pump relay did not activate. I disconnected the green-black wire coming from the back of the ECU and instead connected a known, good ground. This time when the ignition was turned on the relay activated and I could hear the fuel pump operating. Essentially this gives me the results I want, but now the ECU is completely out of the loop (pun intended) in the control of the fuel pump relay. The other issue is that the wire that is supposed to provide the ground from the ECU to the fuel pump relay is not supplying said ground. The van seems to operate normally, so my two questions are:
1. Is there any harm in not having the ECU supply the ground to the fuel pump relay?
2. Any idea why the ECU is no longer supplying ground, and can I remedy this easily?

Thanks ahead of time for any help you can provide!!!

Stefan
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crazyvwvanman
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PostPosted: Fri Aug 23, 2019 6:35 pm    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

The ECU only supplies the FP relay ground when it thinks it should, as a safety feature. If it thinks the engine isn't turning it won't leave the ground turned on. The safety feature is so you and/or the vehicle don't go up in a ball of flames if there is a big fuel leak from a ruptured line such as from a crash.

The ground is switched on/off by a transistor inside the ECU. You could easily have fried that transistor if you didn't know to be careful to avoid doing so. Testing it improperly or touching a power to that ground wire while the ECU has it turned on is all it could take to kill the transistor.

Mark
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sdirghalli
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PostPosted: Fri Aug 23, 2019 6:39 pm    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

Mark,

Thanks for the reply and insight. I'm guessing that it's not possible to replace that transistor? Or in some other way correct this issue?

Thanks again.

Stefan
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crazyvwvanman
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PostPosted: Fri Aug 23, 2019 6:57 pm    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

I've never opened a Subaru ECU to see how serviceable it might be. I'd think any electronics tech should be able to trace the wire path from the harness connector pin to the transistor leg but who knows. That is pure speculation on my part and perhaps that transistor isn't replaceable anyway.

Mark
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Wildthings
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PostPosted: Sat Aug 24, 2019 10:32 am    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

A time delay relay which allows the engine to start but then kills power to the fuel pump after a minute or so will let the thief pull into traffic before the engine dies. He is not apt to spend a lot of time trying to figure out what is wrong once he becomes the center of attention.
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pinetreeporsche
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PostPosted: Sat Aug 24, 2019 2:42 pm    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

Wildthings wrote:
A time delay relay which allows the engine to start but then kills power to the fuel pump after a minute or so will let the thief pull into traffic before the engine dies. He is not apt to spend a lot of time trying to figure out what is wrong once he becomes the center of attention.

Do you have a name of that kind of switch, and a suggestion of where to get them?
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Wildthings
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PostPosted: Sat Aug 24, 2019 7:59 pm    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

pinetreeporsche wrote:
Wildthings wrote:
A time delay relay which allows the engine to start but then kills power to the fuel pump after a minute or so will let the thief pull into traffic before the engine dies. He is not apt to spend a lot of time trying to figure out what is wrong once he becomes the center of attention.

Do you have a name of that kind of switch, and a suggestion of where to get them?


Off Delay Timer
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Jake de Villiers
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PostPosted: Sun Aug 25, 2019 11:12 am    Post subject: Re: EJ22 Fuel Pump Kill Switch - No ground from ECU Reply with quote

Or your wife fires up the van and takes off for the gardening store - to have the van suddenly die on her.

No thanks!!
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