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Thread: ECU only outputing 4.5V to fuel pump relay, not switching pump on.

  1. #1

    Default ECU only outputing 4.5V to fuel pump relay, not switching pump on.

    So I was driving the other night doing some spirited pulls and I may have hit the rev limiter. Not sure, but that bogging sensation kicked in, but didn't go away and RPM dropped to 2500 RPM and I could not get them to climb, even in neutral for 2-3 seconds. Then the car died and would not restart. After getting home, here's what I can see:
    Everything, including all ECU grounds appear in tact.
    ECU syncs with the laptop and both it and the 3.5" touchscreen are synced and displaying sensor info accurately.
    TPS Autoset performed successfully. I typically perform this everytime I have some sort of glitch. Battery shows 12.75V. Green ECU fuel pump wire shows only 4.5V during "prime" with ignition in run, and does NOT trigger the relay.
    Direct jumper from battery DOES trigger pump and pressure builds normally. I have NO IDEA what to check next. Thoughts?

  2. #2
    Join Date
    Dec 2009
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    Connecticut
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    22,699

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    http://forums.holley.com/showthread....9623#post69623 (System Relay & Injector Wiring Troubleshooting)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3

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    Further developments: The 20 amp fuse on the main wiring harness, adjacent to the actual ECU is blown. Before I replace this, and see what happens, any thoughts on what caused this?

    Thanks Danny, I actually found that link elsewhere and it lead me to the fuse. I guess I just need to give it a go and see what happens, LOL!

    IT RUNS!!! Although I'm still a little concerned as to why that fuse popped.
    Last edited by Abubaca; 05-25-2019 at 07:53 PM.

  4. #4

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    UPDATE: So as I feared, the fuse blew again. Replicated this twice. Car starts for about 10 seconds, then it pops. Any thoughts on where to start?

  5. #5
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    Connecticut
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    http://forums.holley.com/showthread....4352#post44352 (ECU/Main Power Harness Troubleshooting Post)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #6

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    The problem is only happening when the car is running. I can toggle the key on & off, leave it in run, crank the car, no issue. But once it's running, I have about 5 to 10 seconds before it pops. Could a short somewhere else in the car cause this? I ask because I had an issue where the header melted through the chassis wiring loom recently. I didn't see any visible wiring damage, but I moved a lot of the chassis wiring around while trying to get a good look at the melted spot. I'm currently going through that area and checking for any loose or damaged wiring. Again, this is mostly chassis wiring, not Holley wiring. Only the WBO2 wiring is over in that area, and it checks out just fine. I'm not electrical expert of course, but I imagine any short in the car could cause an issue with any other system, right? I took a break to come in here and check the thread, so thank you for your response.

  7. #7
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    Connecticut
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    Yes, I'd definitely look into any other wiring that's connected to the battery. The damaged wiring is certainly a clue.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  8. #8

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    Well, I had all the wiring all torn apart and found nothing. All wires appeared in good condition, and no loose wires that I could find. And now the car runs beautifully! Didn't fix a thing, which has me worried. But such is hot-rodding. Good to be lucky on occasion I suppose.

  9. #9
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    I have a suspicion that you tampered with the damaged area and it's currently just not making contact. Perhaps a harness "wiggle test" (while the engine is running) is in order.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  10. #10
    Join Date
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    Brisbane, Australia
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    You could be looking for a pinhole in a wire. I've found many shorts to ground, some you need a magnifying glass to see. Gary
    Regards, Gary

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