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erratic voltage

PostPosted: Sun Aug 16, 2015 8:02 pm
by jeffs73v8vega
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

Re: erratic voltage

PostPosted: Mon Aug 17, 2015 8:52 pm
by spencerforhire
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).

Re: erratic voltage

PostPosted: Tue Aug 18, 2015 9:16 am
by TimMcCabe
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,

Re: erratic voltage

PostPosted: Tue Aug 18, 2015 6:27 pm
by spencerforhire
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.

Re: erratic voltage

PostPosted: Wed Aug 19, 2015 9:57 am
by chevyart
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