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overcharging problem on 1972 mustang

4K views 6 replies 4 participants last post by  DaSouthWon 
#1 ·
I originally posted on allfordmustang.com and haven't really gotten much help so I thought I'd post here to see if anyone had any ideas or could help.

Original post on AFM

I hope some one can help me with diagnosing an overcharging problem on a friends 72 Mach 1. The car is complete with a fresh 351, the new charging system consists of all new regulator, battery, and a 3 post alternator setup for a serpentine belt. The first issue was a non-charging one, w/ no alternator dummy light illuminated. The battery and regulator were replaced and a new problem presented itself. When the car is cranked the alternator dummy light comes on and the voltmeter slowly climbs to 16+ volts. Checked it with a secondary handheld voltmeter and sure enough 16 volts. Checked the wiring against a schematic and from what can be seen, all appears to be correct. By searching through other threads I read that the only way to get 16 volts on the voltmeter would be a bad regulator, replaced the regulator again and the problem is still present. This is driving us nuts. My next step is to pull the wiring harness apart and start over. Any ideas as to what specific wire either being shorted or spliced could create this problem? Also, whats the square harness plug (approx .5"x.5") with the fuseable link tab on it for?

replies:

#1 Most common reason is faulty or missing ground strap from engine block to body.

My reply:

The ground strap is good. I don't believe its a ground issue. I'm leaning more towards something being wired into the harness somewhere causing an improper load, but I'm not sure what exactly would create the issue I've got. I did find the radio hum capacitor wired straight from the neg terminal on the battery to the fender to ground it, rather than from the "A" terminal on the battery.

My new problem:

ok this is really starting to drive me nuts. HELP. I took the alternator off and had it bench tested, it checked ok and reinstalled it on the car. I installed a new wiring harness and a new voltage regulator. and heres what happened.

Start the car and it shows between 12.3-12.7v across the battery. If I turn the lights on it shows around 12.1 volts. There is no change in voltage with rpm speed, and if I cut the lights off the voltage returns to between 12.3-12.7v. I also now have the alternator indicator on my dash on. It seems to be charging but seems a little low to me, and whats the deal with the indicator lamp? Could it be wired backwards?
 
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#3 ·
DaSouthWon said:
.......My new problem:

ok this is really starting to drive me nuts. HELP. I took the alternator off and had it bench tested, it checked ok and reinstalled it on the car. I installed a new wiring harness and a new voltage regulator. and heres what happened.

Start the car and it shows between 12.3-12.7v across the battery. If I turn the lights on it shows around 12.1 volts. There is no change in voltage with rpm speed, and if I cut the lights off the voltage returns to between 12.3-12.7v. I also now have the alternator indicator on my dash on. It seems to be charging but seems a little low to me, and whats the deal with the indicator lamp? Could it be wired backwards?
DaSouth, this statement indicates there's no charging going on, but everything else is normal. I think you have the wiring wrong between the alternator and regulator......?
 
#4 ·
It's gotta be right, I've been through it 100 times. the only thing not exactly like the schematic is the radio hum suppressor. It broke, can't find another one. I don't think this could be the issue unless it makes the voltage that the regulator sees incorrect.
 
#5 ·
I don't think the suppressor is the problem, but remove it anyway for now.

I'd take a voltage reading with the engine off (at the battery). It should be around 12.2v or more. Then start the engine, and take another voltage reading. This reading with engine idleing should be about 13.5v, or in that range, if the system is indeed charging.

This will tell if the problem's in the actual charging or the charging indicators. You have a new (tested good) alternator, right? And a new regulator, right?

And this is a three pole regulator, with terminals for BATT, GROUND, and FIELD?

Does the warning light glow the same brightness all the time, when the engine's revved?
 
#6 ·
DaSouthWon said:
It's gotta be right, I've been through it 100 times. the only thing not exactly like the schematic is the radio hum suppressor. It broke, can't find another one. I don't think this could be the issue unless it makes the voltage that the regulator sees incorrect.
Well if all you changed was the harness and the problem went from over charging to not charging then most likely that (the harness) is the root of the issue. If you are absolutly sure the regualtor is working and the alternator is good, the problem is most likely a wiring issue or a grounding issue. You can run a ground wire right from the alternator mount to the regulator ground connecton and the case of the regulator. See if the charging goes back to normal. If so you know you have a ground issue.

Check the field terminal on the alternator with a meter, this is what controls the charging rate. If you apply battery voltage to the field terminal the alternator should put out 16-18 volts or so. I think this was your original issue, full field all the time. Again if it puts out then you have a regulator wiring issue.

Chet
 
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