Not Great Voltage Drop - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 3 Old 09-08-2017, 10:45 PM Thread Starter
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Not Great Voltage Drop

I recently checked my '89 Fox for voltage drop across the engine and accessory grounds. I noticed that while driving for a while in city stop and go when it's really hot outside, my voltage gauge was showing abnormally low voltage (12V max), so I knew something was not right. Sure enough, I've got about 0.36V of voltage drop across the engine and accessory grounds, which is bad enough to require fixing. What is the best way to fix the problem? Add ground straps from the negative battery terminal to the engine and the chassis? 200A Powermaster 3G alternator feeding the main terminal on the starter solenoid via a 2/0 gauge run of welding cable (also goes through a 200A mega fuse). Primary engine ground is a 1/0 welding cable from the battery negative to the engine block. Another ground strap exists from the back of one of the cylinder heads to the firewall.

The visible accessory grounds I see include 2 grounds, one near each end of the factory rad support and another one on the inner fender on the driver side for the EEC (not sure if that's a stock location ground or not at this point). The accessory ground voltage graph I've attached shows how the voltage drop increases as I start flicking on the headlights, the stereo, open the door to turn on the courtesy lights, etc.





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File Type: png EngGnd2BatNegTermCranking.png (7.6 KB, 7 views)
File Type: png AccGndDrop.png (10.7 KB, 8 views)

'89 5.0L LX hatch: all 90mm 2.2L Blowzilla Dart Iron Eagle 306 @ 8.4:1, ported Vic Jr heads, TMoss ported GT40 lower, Deka 80# injectors, EDIS8, GUFB-A9L2-Moates tuned, Tremec 3550, 3.31 gears w/31-spline TruTrac
'66 GT 2+2 Fastback: carb'd 289, Canfield heads - EFI transformation pending

'71 Mach 1: totaled - R.I.P.
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post #2 of 3 Old 09-10-2017, 11:56 AM
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Measure amp draw of system, and amp production of the the alt

I bet your system draws more amps than your alt can put out at low engine speeds


89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #3 of 3 Old 09-10-2017, 02:28 PM Thread Starter
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So was reading about 0.4V accessory voltage drop (blue curve). Added a 4 AWG stranded copper wire ground strap between the chassis and the battery negative terminal. Dropped down to almost negligible voltage drop (red curve). Success!

The engine cranking voltage drop was equally bad, close to 0.4V (blue curve). After fixing the accessory grounds I checked the cranking drop again and it was still bad (red curve). So I added a 1/0 AWG ground cable between the engine block and the passenger frame rail in the engine compartment. The resulting voltage drop is the green curve, around 0.1V, but I'm going to say this is actually acceptable.

Went for a test drive and with all the accessories on, including the AC, the voltage never dropped below 14.2V. I'm pretty sure the electrical sytem hasn't been happier ever.
Attached Images
File Type: png accdrop.png (8.1 KB, 6 views)
File Type: png engdrop.png (7.8 KB, 7 views)

'89 5.0L LX hatch: all 90mm 2.2L Blowzilla Dart Iron Eagle 306 @ 8.4:1, ported Vic Jr heads, TMoss ported GT40 lower, Deka 80# injectors, EDIS8, GUFB-A9L2-Moates tuned, Tremec 3550, 3.31 gears w/31-spline TruTrac
'66 GT 2+2 Fastback: carb'd 289, Canfield heads - EFI transformation pending

'71 Mach 1: totaled - R.I.P.
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