erratic voltage

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erratic voltage

Postby jeffs73v8vega » Sun Aug 16, 2015 8:02 pm

I'm having an erratic voltage issue when my Taurus fan is engaged/powered up. I'm running flexalite fan controller to power the fan. The car holds steady 13.9-14.1 volts on the hand held without the fan powered up. When the fan is powered up the voltage jumped from 13-14 volts and is NEVER steady. It almost seems like the fan is drawing too much power and isn't leaving enough to power the ecu. The car started bucking and backfiring several times today at random times and vac/rpm/ air fuel seem to be fine and correct.

The car is a 350 with edelbrock efi, 100 amp alternator
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1973 Chevrolet Vega GT


Re: erratic voltage

Postby spencerforhire » Mon Aug 17, 2015 8:52 pm

Sounds like it could possibly be a charging issue, but maybe not. is the 100a alternator of the "1 wire" type? It may not be making anywhere near it's rated output at lower RPM. A pair of Taurus fans should only be drawing 40 or 50 amps. You really shouldn't have any problems with the ECU until the voltage gets down in the 11.5 v range. Where do the fan relays get their power from? Aftermarket EFI systems are often quite fussy about where they get power and ground from, and also where the ECU is mounted. It has to be as far away from electrically "noisy" devices like coils, ignition boxes and even electric motors(like fans).
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.45@146.19)(6.01@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

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Re: erratic voltage

Postby TimMcCabe » Tue Aug 18, 2015 9:16 am

Hi Jeff,

If the fan controller has variable speed, it is using PWM (pulse width modulation). This can introduce a lot of electrical noise into the system and may be the root cause of the issue.
The controller should be wired directly to the battery and you should also make sure that you have a good ground connection for the controller as well for the ECM.
As the electrical load goes up, you start getting more voltage drop through the system, and voltage drop across the ground is often overlooked.
The other thing to look for could be a bad diode in the alternator, it's not noticable until you load it up at idle when the fans starts drawing 50+ amps, then you have an AC ripple going through they whole 12V system.

Good luck,
Tim
1972 TPI Vega T56 6 speed

Contact me for all of your Tachometer needs
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Re: erratic voltage

Postby spencerforhire » Tue Aug 18, 2015 6:27 pm

Good tips from Tim; these modern fuel injection systems are very susceptible to electrical "noise". I've had to re-wire a couple of F.A.S.T. systems that were improperly installed. One was on an older C1 Corvette. Had an MSD 6AL under the hood and the ECU under the dash. Both mounted to the fiberglass firewall, back to back. It would start and idle, but went crazy when you tried to rev it up. The installation instructions recommend 2ft of separation, and these two boxes were about 1/4" apart.
The "fleet"-
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(9.45@146.19)(6.01@117.28 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....

Visit http://www.spencerforhire.ca
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1972 Chevrolet Vega Hatchback

Re: erratic voltage

Postby chevyart » Wed Aug 19, 2015 9:57 am

spence is this problem an offshoot of RFI(radio frequency interference) i have always heard of that term in racing but never really knew what it was art
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