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Fuel Pump current supply issue


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Dear All,

This post may not be related to tuning but more towards wiring problem but I am hoping that I could get some basic guidelines as I consider myself still new in the arena and still got a lot to learn

I am currently using Link G4+ Monsoon on a 3SGTE ST246 engine on my Toyota MR2. Car was running great for the past few months until recently I could not get my car to start. After I did the basic check-up I found that the fuel pressure is '0'. Therefore my focus would definitely be the fuel pump. For your information, the wiring was not done by me but I am now learning to do such wiring myself.

My fuel pump is activated from a relay that uses AUX 6 of the Link G4+ Monsoon. I try to run the test using PC Link and can hear the sound of the relay clicking but I don't hear the priming sound from the fuel pump. Then I try to directly run the fuel pump from the battery (12v current) to its connector and I can hear the fuel pump priming. For that, I concur that my fuel pump is working and is not the fuel pump issue itself.

Next from the connector that I have disconnected from the fuel pump, I tested it with a multimeter. What I found that there is a current supply of 8v-9v to the fuel pump when the key is ON (after finish the 2-second priming that I have set at Link G4+). When I set at PC Link Software 'test ON' for AUX 6 which is the pump, the current supply becomes 2.8v-3v along with the clicking sound. Picture attached is as per my explanation.

Picture #1 8v-9v before TEST ON fuel pump at PC Link

Picture #2 2.8v-3v during TEST ON fuel pump at PC Link

Next, I try to test all the fuses that I can have my hands on but all the fuse seem okay. As my logic and understanding of wiring, I don't feel its the fuse as there are still current flows to the pump but it just seems that the current is low for the pump to operate. As I mentioned above the wiring was not done by me, therefore, I try my best to detect the wiring with the limited knowledge that I have.

I seek the advice here to guide me on how could i further diagnose and solve this issue myself. Your advice and guidance is very much appreciated.

Picture #2.jpg

Picture #1.jpg

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Im not sure if you are testing correctly, but you should not have 8-9V going to the fuel pump when the relay is off (after prime has finished).  It should be close to zero volt as one wire should be ground and the other should be the open circuit side of the relay?

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9 hours ago, Solace said:

y fuel pump is activated from a relay that uses AUX 6 of the Link G4+ Monsoon. I try to run the test using PC Link and can hear the sound of the relay clicking but I don't hear the priming sound from the fuel pump. Then I try to directly run the fuel pump from the battery (12v current) to its connector and I can hear the fuel pump priming. For that, I concur that my fuel pump is working and is not the fuel pump issue itself.

Relays, coils and backfeeding can send you down a wormhole of trying to diagnose things with a volt meter.  When there's no load on these outputs, when they're open circuit, there are many things that can make for odd voltage readings.

Let's forget about your voltages for a second.  Simple test:  Pull the wire going to Aux 6 (pin 26) out of the Monsoon ECU connector, then with everything powered up as normal, ground that wire to the chassis.  Does the fuel pump operate as normal?

Then disconnect that wire at the relay (in Bosch speak, terminal 86) and reconnect it at Pin 26.  Put your volt meter on the wire from Aux 6 (no longer connected to the relay) and the other side at the relay input 12V (in Bosch Speak, terminal 30) then power cycle the ECU.  Does the voltage go to VBATT for the duration that the prime is on?  If so, it would seem to me that the ECU is working properly and the problem is somewhere with your relay or wiring.

-Neel Vasavada

apex speed technology

@apexspeedtech

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Missing load is the key thing here. Its going to make all the tests suspect. Have you got a test light handy? How about an old fog light or head light bulb - these draw about half the load of a fuel pump so are completely safe to substitute.

Unplug the 2 pin plug in front of the shifter that feeds the fuel pump. Connect you headlight bulb instead of the fuel pump. If you want additional numbers, connect your multimeter in parallel. With a ~5amp load sitting there, the voltage should be 0.

turn the aux output on, the bulb should light up brightly and you should still see 11+ volt on the meter. If you see 0 and the bulb doesnt light, its likely a broken wire or faulty relay. If you see something like 3 -8v and the light is on very dimly, then you probbaly have a correoded contact or a slightly damaged wire somewhere.

Repeat this test but conencted to the outputs on the back of the fuel pump relay in the fuse box. This will tell you if its the wiring from relay->pump thats the problem, or the relay/something before it. Keep checking "easy to get to" places along the path from battery to pump.

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