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Old 04/29/2017, 11:38 AM   #1
TexAg's Avatar

Location: Austin, TX
Joined: Aug 12
Posts: 111

My Garage
Charging issues. (Follow up to headlight harness post)

This started as the Headlight Harness thread because I thought I knew where the problem was. After a few more tests, I'm not so sure.

Ok, a little more information. I built a separate harness to bypass the headlight harness.

From the VR:
I - Not used on my car
A - Directly to battery +
S - 12v power source to simulate ignition switch
F - Directly to Field on Alternator
Grounded VR to frame

From alternator
B+ post - Directly to battery +
Ground - To engine ground and battery -
Field - Directly to F on VR

I then took the following readings:
12.58v at battery posts
A and F on VR read 13v+, sometimes up to 15 or 16v.

The strange thing is that no voltage is getting to the battery. This is the same condition with the original harness?

Why would I have 13v to 16v at the A post of the VR, but then a reading on the same wire, 18 inches away only have 12.58v? I used 12 gauge wire for this test, which looks to be the same size as the yellow wire on the VR regulator plug.

When I put it under load, the results where the same, except the reading at the battery was much lower, about 12.3v.

The last strange part is that when I first started the car, the voltage at the battery climbed to about 13.2v, which i would expect, but then stopped and dropped off back to 12.58.

Could this be an alternator issue after all? I've had it tested, and it is showing proper voltage at the VR. Could it be too small? Or maybe it is intermittently faulting?

'68 Highland Green GT/CS J-code
67 Resto-Mod Shelby fiberglass with EFI 5.0
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Old 04/30/2017, 09:23 PM   #2

Joined: Mar 17
Posts: 10

1. Wire it all back to factory. Ensure it is wired correctly. [Check all harnesses in general to make sure nobody in the past has thought they were better automotive electrical engineers than the factory Ford engineers and did some "custom" wiring. If not, it could be instantly frying new voltage regulators]. Study the charging system wiring diagram that applies to your vehicle year make and model and make sure everything pertaining to the charging system is connected properly

2. Test the alternator on a proper machine (if possible), to ensure it is operating correctly.

3. Replace the voltage regulator with a standard OEM replacement.

3. Test 1. Start and test for proper voltage at alternator and + solenoid post. Test 2. Turn electrical accessories on (such as headlights) and take another reading, the alternator should be compensating for the increased demand.

If it is still low, locate the large charging wire connected to the large post on the back of the alternator. Follow this wire, it should bolt to the positive side of the starter solenoid that is attached to the passenger side inner skirt near the battery - the same post that the positive cable coming from the battery is connected to. There should only be a very minimal voltage drop measured between the alternator charging post and the positive side solenoid post. If you have ~14 at the alternator and ~12 at the solenoid post the charging wire between the alternator and solenoid post is bad.

For a factory appearing replacement purchase a reproduction alternator harness or replace the charging wire with the same or one size thicker gauge cable. To do this, strip the tape from the alternator harness, slice the rubber boot on the alternator wiring block to remove the large gauge cable. Replace it with the new cable, put a new lug on each end, glue the boot back together, tape up the harness and you should be good to go.

There is not much complexity to the charging system on older Fords, it is quite simple as noted above.
[8R01S; Ford Certified Technician].
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Old 05/01/2017, 04:35 PM   #3
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Location: Austin, TX
Joined: Aug 12
Posts: 111

My Garage
Thanks, 8R01S.

After a lot more research, I have determined that I want to keep it original, so I guess I'll start with #1. I had the alternator tested, but with it still in the car. I'll pull it out and take it to the local alternator place to be sure.

Simple, yes, but I've never seen a condition like this. It's rather frustrating.

Thanks again!

'68 Highland Green GT/CS J-code
67 Resto-Mod Shelby fiberglass with EFI 5.0
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