What causes system voltage to exceed 17 volts?
 

My ammeter started running at about +30 to +45 Amps while driving. At idle, it settles at zero but when the rpm's increase the amps increase. The battery will not hold a charge overnight, and when freshly charged it's good for about a day. The alternator is making a high-pitched whine.

I had replaced the alternator about six months ago. Standard early Ford 3-wire alternator.

I first suspected the voltage regulator. Replaced the voltage regulator and still had a high reading on my dash ammeter.

Battery was going dead, so I put my DMM in current mode and put it in line with the battery to look for a draining current. 0 A. Also, 0 mA. Nothing draining the battery while the car is sitting.

I figured the battery had a bad cell, so I replaced it today. I was surprised to find I still had 23 months of warranty on the old battery. Fired it up and took a short ride, and as the engine rpm's increase so do the amps on my ammeter.

Oh, by the way, I know: ammeters are pretty useless; get a voltmeter. On the list.

So, I put my DMM on the battery to test voltage. 12.5 V with the key off, drops when cranking, but with the engine running the voltage climbs all the way up to about 18V :eek:

Shut it down and don't plan to drive it until I resolve this.

I plan to take the alternator down to get it tested, as this is the only remaining item in the charging system. The only other thing I can think of is checking the grounds for continuity.

Anything else I'm missing? :JEKYLHYDE

DD

Faulty voltage regulator or wiring fault towards your external regulator.

Either way, the regulator is not working.

The system is acting as if the alternator field terminal has a constant 12 volts. That voltage should normally be reduced by the regulator so that the alternator output is 13.8-14.5V. Not what you have...

Also check that the V-R is properly grounded. If it can't tell what's zero potential, it might do some strange things.

My car recently went through the 7 gates of electrical hell. I ended up replacing the alternator twice. Beware auto store "alternator testing", this gave false results and made the whole experience 10x as bad as it should have been. Good luck.

2nd what CowtownCobra said. Don't always trust the auto store alternator testing. You can check your regulator again and everything but the 18 volts is to high for sure. Your new alternator doesn't have an internal regulator in it does it?

Ron :confused:

took my one wire alternator down to test the other day on a new machine. showed a little over 16 volts, which was within specs.

Like if you have the wires crossed to your voltage regulator.
Check to see if your voltage regulator is wired correctly to your alternator. The voltage regulator is marked IASF

The most common method of wiring your voltage regulator is:

I= Indicator light on your dash
A=Alternator output
S=Stator on your alternator
F=Field to the alternator

Tom,

Our FIA (and our early 427s) didn't use an alternator charge light. See the charging troubleshooting section in this "manual":
"Wiring Primer"

To Bob's advise, I checked the ground for the voltage regulator and it ohmed-out fine (0 Ohms to block, firewall, and chassis ground). I also verified the wiring to the alternator, and the ERA VR wiring is an idiot-proof connector. Checked the Alt ground. All is well.

I read in a Mustang Monthly article that the resistance between the "I" and the "F" terminals of the voltage regulator should be less than 10 Ohms---and on both voltage regulators I have (one on the Cobra, the other one is/was my tester) my DMM read open circuit... Uh-oh!

Frankly, I am not sure if this test is valid (but I read it on the internet!). Anyone vouch for the I-F resistance test described above? 10 Ohms or less between I and F on the VR.

So, it's down to the VR and the Alt. I did get that open circuit reading between the I-F terminals on the VR---both the one currently in my Cobra AND my test one. So, if they were both fried by a faulty alternator (what if the Field signal is read incorrectly by the alternator in the first place?), then I should replace both at the same time.

Which also seems to be the common wisdom: if either the alternator or the voltage regulator fail, then replace both at the same time.

So, other than thinking this through and following the advise submitted here, and of course, procuring a new VR and Alt, I've totally goofed off today. As it should be, I took off with my 10-year old son today in my Jeep and explored the back roads up to Mt. Hamilton and all the way over to Patterson, and then looped around by the San Luis reservoir and back again. As much as I wanted to fix my Cobra, kick'n it with my kid is far more important.

So, in my garage sit a new alternator ($50) and a new voltage regulator (another $50) and I'll likely be out there at the crack of dawn tinkering away.

Stay tuned, and thanks for the help!
DD

Success!

Replaced the alternator and the voltage regulator and the ammeter no longer runs up to 45 Amps, and the voltage is rock-solid at about 14.5V.

There was a big sticker on the alternator warning that the voltage regulator should be changed at the same time. The new alternator is a NAPA/Motorcraft unit, made in the USA.

In regard to the resistance between the I and the F terminals of the voltage regulator needing to be between 0 and 10 Ohms: I'm not buying it. I read it in the article on the web, but when I tested my brand new voltage regulator, it wasn't an open circuit but it was about 10 MOhms. However, the VR and Alt work fine.

I think I'll go for a test drive!
Cheers,
DD

Good to hear you fixed it Doug.

10Mohm would be classed as o/c in the automotive world.

So, I guess I possibly have two test voltage regulators, now.

After all this dust settled, I recalled that when I first installed the failed alternator, I noticed the F terminal was a little loose and I tightened it. It was months and months later when the over-charging came up. Perhaps related. Not exactly sure how long the over-charging was going on.

I examined the battery I had been using prior to the alternator replacement. The sides were bulging a bit. That battery could have blown up and this whole thing could have been disasterous.

So, I've effectively replaced the whole charging system: battery (bulged), alternator (the presumed suspect), and the voltage regulator (a good practise to replace with the alternator).

Well, at least I can now focus on the fun stuff: back to picking out a carburetor for my new 331!

DD

After breakfast with a couple of CC pals, I hop into my Cobra for the jaunt over to Canepa and notice that my ammeter current is getting pulled down by my turn signals. I try my headlights, and they suck it down a tad further. After Canepa & Coffee, I check it out. Battery voltage when off is 12.29. Battery voltage when running is 12.19, and gets pulled down by lights, turn signals, fans.

Sigh...

Once home from the beach I pop the hood and start checking things out. I move the field wire to take it loose and the eyelet breaks off. An eyelet and some heat shrink later I fire it up. Healthy 13.6 Volts.

It would appear all the bump and grind of trouble shooting my charging system left the eyelet worn and weak. Good thing to keep in mind.

DD