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Voltage low; fuel economy down

 
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ScottMPala
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PostPosted: Tue Nov 06, 2012 2:52 am    Post subject:  Voltage low; fuel economy down Reply with quote

I have put off posting about this for a long time so I could research and test and have gotten nowhere so far. Issue started with low voltage from original alternator. I noticed voltage reading and bought a used alternator to tide me over til I bought a 180 amp I'd had my eye on. No improvement with the used alternator. Installed 180 amp alt and removed voltage booster with new alternator. Alternator seemed to have decent voltage by itself.(14.2-14.6). Reinstalled voltage booster after voltage dropped significantly. At the time when issue was first noted there had been no recent changes. All voltage tests were initially noted on my Scangauge II, but confirmed with a volt meter at battery post, jump start post, and alternator bolt.

Background info:
Added stereo, 2 amps, subs, red top, alt rewire, voltage booster, 4 ga battery and ground in stock locations, 4 ga ground battery to alt case, added ebay h.i.d.s, added underdrive crank pulley. Getting 14.6-15.0, fused alt rewire with upgraded alternator. All issues started long after any mods. Normal voltage with booster with stock alternator is14.8-15.2

Symptoms
* Low voltage with voltage booster, old alternator, cruising (13.2-14.0) norm 15.0
* Low voltage in park, neutral (14.2-14.6) normally around 15.0
* Lower voltage with brake pedal engaged, stopped in gear (10.9-12.8)
* Lower voltage with brake pedal engaged, slowing from 65 mph (12.9-13.2)
* No improvement with used alternator
* Some improvement with 180 amp alt. Effect lasted 2 or 3 weeks
* No improvement with 180 amp alt. *** except one night when I got 15.6v and my charging light illuminated. Power cycled car and issue stopped. Never repeated.
* Low voltage with load (in gear). Neutral or park has good voltage (14.2-14.6)
* Removed voltage booster, voltage low (12.8-13.2)
* Significant drop in fuel economy, driving same highway, (340 miles per tank to 280, 22mpg to 18 mpg)

Tests performed
* Replaced w/ another red top
* Bench tested alternator at part store, 15.0 v
* Tested alternator amperage, 40+ no load, 120+ under load from test equiptment, although results not the same when tested with stereo running
* Removed quad tail lights; no improvement
* Removed underdrive pulley; no improvement
* Removed H.I.D. lights; no improvement
* Replaced brake light switch; no improvement
* Removed capacitor/amps from circuit; no improvement
* Tested abs circuit, unplugged abs sensors; no improvement
* Tested all fuse box circuits for overload; normal
* Tested voltage regulator reference voltage and output; normal
* Paid electrician to diagnose, no issues except high parasitic drain from alarm. Check with installed, drainage normal.
* Changed pcm to stock. Seemed to correct voltage drain when depressing brake pedal, issue returned following week with stock pcm installed. Reinstalled DHP pcm.
* Replaced pcm with Overkill w/ slightly raised voltage. Slight improvement in idling voltage. No improvement on voltage with brake pedal depressed.

No one I have spoken to locally has any ideas. I don't anticipate any results here, although there are members here that have forgotten more about this car than I have learned! So hopefully someone can point me in a direction I hadn't thought of. Thanks for any help

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98ss
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PostPosted: Tue Nov 06, 2012 2:52 pm    Post subject:   Reply with quote

Would it be possible to have a bad ground causing some of your issues?
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Lingenfelter
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PostPosted: Tue Nov 06, 2012 6:04 pm    Post subject:   Reply with quote

How fast did your mileage drop off?

It looks like you have some sort of electrical issue but the fuel economy may be separate (plugs, fuel filter, etc)

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ScottMPala
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PostPosted: Tue Nov 06, 2012 11:14 pm    Post subject:   Reply with quote

My grounds should be fantastic. I tested several and no voltage swings. All enine grounds were upraded to 4ga battery cable. I added a 4ga from top post to alternator case. Haven't changed fuel filter in almost 2 years. The issues seemed to happen at once tho, mileage drop was immediate, overnight basically

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PostPosted: Wed Nov 07, 2012 2:00 am    Post subject:   Reply with quote

So.... you mention that the voltage drops when depressing your brake pedal? Is that the only time it happens? If so, I'd try to troubleshoot your ABS system, brake light system, etc. Maybe there's something shorting out when you press the brakes. That's the only thing I can think of.

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ScottMPala
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PostPosted: Wed Nov 07, 2012 3:27 am    Post subject:   Reply with quote

jmakinen wrote:
So.... you mention that the voltage drops when depressing your brake pedal? Is that the only time it happens? If so, I'd try to troubleshoot your ABS system, brake light system, etc. Maybe there's something shorting out when you press the brakes. That's the only thing I can think of.

The voltage drops significantly when depressing brake pedal but is lower than normal for my setup. Normal was 14.8 to 15.0 with booster. Checked amp draw from abs fuse; no signs of issues. Unplugged front abs wires; no change. Didn't try rears. Unplugged quad tails, replaced brake light switch, unplugged all brake lights. Still drops under 13 in gear with brake depressed.

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PostPosted: Wed Nov 07, 2012 10:14 am    Post subject:  voltage Reply with quote

You say ' Low voltage with load (in gear). Neutral or park has good voltage (14.2-14.6)'.
If in gear with load and you raise RPM's a tad, say to 1000rpm, what is voltage then?. Cool

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ScottMPala
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PostPosted: Wed Nov 07, 2012 3:49 pm    Post subject:  Re: voltage Reply with quote

topgun wrote:
You say ' Low voltage with load (in gear). Neutral or park has good voltage (14.2-14.6)'.
If in gear with load and you raise RPM's a tad, say to 1000rpm, what is voltage then?. Cool


Lol, I do that when i am sitting at lights. The voltage is raised to normal if i bump the idle up. At long lights i usually throw the parking brake on and idle up to 900 or so

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PostPosted: Wed Nov 07, 2012 5:35 pm    Post subject:  voltage Reply with quote

so u expect to have 13ish volts at idle with a load on the electrical system? Cool

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PostPosted: Wed Nov 07, 2012 6:36 pm    Post subject:  Re: voltage Reply with quote

topgun wrote:
so u expect to have 13ish volts at idle with a load on the electrical system? Cool


Always did before Wink Not saying with loads though, just idling, no stereo, no lights I have always had 14.5 to 15 volts at idle with the voltage booster with whether my foot is on the brake or not. I noticed the issue when I started getting lower than normal voltage at stop lights. I bought a new alternator thinking it was crapping out. Now my voltage drops under battery voltage (as low as 11.6) when depressing the brake pedal at stop lights. Those are two separate but related issues i am trying to fix in the electrical system.

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PostPosted: Sun Nov 18, 2012 10:53 pm    Post subject:   Reply with quote

New symptom. There has been a couple times when the voltage light would come on, when the voltage was really low. Today the light came on with 13.4 volts while cruising. Stayed on for several minutes. I have no idea at this point what it could be. Any ideas would certainly help.

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PostPosted: Wed Nov 21, 2012 9:28 pm    Post subject:   Reply with quote

I had a 1994 Buick Regal that had pretty much the same issues. One time I thought I fixed it it was corrosion/loose connection in the main electrical box. After cleaning / treating / tightening the voltage was back to normal, but it didnt last too long.

One thing you could try is to start the car and then start pulling fuses until the voltage goes back to normal. Could help pinpoint the issue.

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