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Old 10-31-2011, 11:16 AM   #1
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Default Voltage takes a big dump all over your chest

Under boost (up to and over 30psi), my system voltage takes a big **** and drops from 14v down to 12v. I swapped out the alternator and it still does it. The only electrical thing I have changed is adding a second Bosch 044 fuel pump.

Note that I have an NB alternator which is ECU controlled. I'm using the plain ol' alternator control settings from FM.

Why it do this?
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Old 10-31-2011, 11:20 AM   #2
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Faeflora the Fecal Fornicator
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Old 10-31-2011, 12:53 PM   #3
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Possibly swap in an NA 1.8 alternator and see if you still have the problems.
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Old 10-31-2011, 01:02 PM   #4
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Install a 2nd alternator.
Do it.
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Old 10-31-2011, 01:19 PM   #5
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Instead of a coldside superchalternator, you need a hotside turbochalternator. Then you can stop running shitty band-aids like a 5th spark plug to provide extra current.

But for cereal, is the voltage drop rpm dependent?
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Old 10-31-2011, 02:19 PM   #6
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Quote:
Originally Posted by ScottFW View Post
Instead of a coldside superchalternator, you need a hotside turbochalternator. Then you can stop running shitty band-aids like a 5th spark plug to provide extra current.

But for cereal, is the voltage drop rpm dependent?
Yes RPM dependent. Let me check if it is kpa dependent too.
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Old 10-31-2011, 02:28 PM   #7
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If I had to guess I would say that the alternator is having trouble with the twin fuel pumps at high fuel pressures.
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Old 10-31-2011, 02:41 PM   #8
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Quote:
Originally Posted by Savington View Post
If I had to guess I would say that the alternator is having trouble with the twin fuel pumps at high fuel pressures.
What the hell kind of pumps is he running?!

To pull a spec at random, a Walbro GSL392 (aka 255 HP) maxes out at 20A, delivering 85 gallons per hour at 140 PSI. Two of them in parallel would draw an absolute maximum of 40A, while emptying the entire contents of your fuel tank in just under 4 minutes, against a restriction of ten atmospheres.

(That kind of fuel flow, incidentally, would support a little over three thousand horsepower.)



I have a really hard time imagining this to be the case.
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Old 10-31-2011, 02:50 PM   #9
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Take it into an auto parts store and have them test the alternator on the car. If its under 50 amps at 2000 rpm, replace it.
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Old 10-31-2011, 02:55 PM   #10
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What ECU are you using. If it controls the alternator, then that is the first suspect that must be eliminated. For example, from what I have read on other forums, Hydra 2.7 has 'less than pretty' code in the alternator control.
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Old 10-31-2011, 03:06 PM   #11
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Quote:
Originally Posted by Savington View Post
If I had to guess I would say that the alternator is having trouble with the twin fuel pumps at high fuel pressures.
Yeah, that's what I am thinking. Now how should I deal with it?

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Originally Posted by Joe Perez View Post
What the hell kind of pumps is he running?!
Two Bosch 044 pumps wired in parallel using stock relay and wiring.
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Old 10-31-2011, 03:13 PM   #12
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Quote:
Originally Posted by chpmnsws6 View Post
Take it into an auto parts store and have them test the alternator on the car. If its under 50 amps at 2000 rpm, replace it.
Um you mean drive it to Advance Auto and say test my **** bitch? Is that it?

Quote:
Originally Posted by exexx View Post
What ECU are you using. If it controls the alternator, then that is the first suspect that must be eliminated. For example, from what I have read on other forums, Hydra 2.7 has 'less than pretty' code in the alternator control.
Hydra 2.6 EMS.
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Old 10-31-2011, 03:13 PM   #13
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Oh BTW I have an AFPR now. I set the AFPR to be 70psi at idle.
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Old 10-31-2011, 03:21 PM   #14
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Quote:
Originally Posted by Faeflora View Post
Um you mean drive it to Advance Auto and say test my **** bitch? Is that it?
Yes that is it.
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Old 10-31-2011, 03:31 PM   #15
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Check your grounds. High RPM vibration can make a loose connection them lose contact when they would otherwise be okay.
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Old 10-31-2011, 03:32 PM   #16
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Quote:
Originally Posted by Joe Perez View Post
What the hell kind of pumps is he running?!

To pull a spec at random, a Walbro GSL392 (aka 255 HP) maxes out at 20A, delivering 85 gallons per hour at 140 PSI. Two of them in parallel would draw an absolute maximum of 40A, while emptying the entire contents of your fuel tank in just under 4 minutes, against a restriction of ten atmospheres.
Well, not quite. It delivers 85gph at 0psi of restriction, but only 5gph at 140psi of restriction (~10atm). If the pumps were in series then that would change, but faefauna is running them in parallel, so the flow capacity goes up but not the pressure capacity.

70psi base + 30psi boost = ~100psi of fuel pressure, so the pumps alone are dragging 15A each (more if the 044s draw more power, which IIRC they do). I'll just assume you have each one on a dedicated relay with 14awg wire straight to a dedicated fuse?
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Old 10-31-2011, 03:45 PM   #17
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You say you swapped the alternator -- what did you swap it WITH? New? Rebuilt? Or "known good out of another car"?

This is exactly the failure mode my alternator died with, at high revs the current output (and thus voltage) would sag. Not enough to kill the car, but enough to notice the headlights dim and (once it got worse) enough to shut down the boost control solenoid.

JasonC tested it out on my car, IIRC we jumpered the alternator control to push it to full power and watched the current/voltage output on his scope. Above 4000 RPM, it topped out at 30 amps and about 11.5 volts.

So if it was "known good from another car", maybe it wasn't actually known good after all.

Another thing to check is the voltmeter built into the Hydra. Mine is off by almost a volt, as compared to what I see with a couple of external DVMs.

--Ian
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Old 10-31-2011, 03:47 PM   #18
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Quote:
Originally Posted by Savington View Post
Well, not quite. It delivers 85gph at 0psi of restriction, but only 5gph at 140psi of restriction (~10atm).
Yeah, I rushed through that and didn't think.

Key point: even if both pumps were operating at full-tilt-boogie, they're not drawing enough power to pull down a reasonably healthy alternator.

Quote:
Originally Posted by Faeflora View Post
Two Bosch 044 pumps wired in parallel using stock relay and wiring.
And that pretty much guarantees that they're not operating at full-tilt-boogie. The stock fuel pump feed comes from the main relay, which is fused at 30A, and that fuse is also powering the injectors, ECU, etc. Not to mention that the wiring itself would be getting pretty toasty at 40A or anywhere near that.

This just seems odd to me.
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Old 10-31-2011, 05:20 PM   #19
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Don't charge the ***** while stepping on the gas?


Perhaps you can try tossing FM a pm and confirming your alternator outputs are good?
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Old 10-31-2011, 05:29 PM   #20
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Quote:
Originally Posted by shuiend View Post
Yes that is it.
What he said. Can you guys switch to another bigger internally regulated alternator? RX7?

Also, rewire your fuel pumps. The fuel pump wiring was designed for a tiny maybe 10 amp pump. Your setup is asking for trouble. In the domestic world, it would have fried your harness long ago.
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