Been chasing an ignition gremlin on my Roadrunner. Anyway, as part of my diagnostics, I checked the voltage at the battery. On fast idle, I got 16.5 volts on a fully charged battery. Seemed excessive. I'd expect about 14.5 tops.

I've also recently experienced an occasional electrical hiccup were everything shuts. Thus far, it's only happened a couple of times at initial start-up, never on the road.

I'm thinking the issues are related.

Currently have the stock alternator on there, and a stock-type regulator. It's a Borg-Warner with "Made in the USA" stamped in it. That there tells me it's probably pretty old.

Anyway, I went ahead and ordered a new VR. It'll be one the the dated coded reproduction ones.

Am I on the right track? Need to nip this in the bud before I burn something up.

excellent place to start and it will probably end right there!

If it's a '70 or later try disconnecting the green wire from the alternator, if it still charges high the alternator has in internal short.

If it drops to battery voltage with the green wire disconnected the VR is bad or the green wire is shorted to ground.

regulator/regulator ground/bad batt/wiring dead short that's full fielding it. Are you getting an electronic relacenent reg (highly recommended) or electromechanical (they have alot of probs)

Make sure that the regulator case has a good connection to ground.

I used to have the same issue on my Duster. The sheet metal under the regulator was rusty. Cleaned it off and replaced the regulator and all was good again.

I swapped in the MP restoration VR this weekend. On 1800 rpm fast idle, I'm getting about 15 volts. Flipping on the high beams drops the voltage down to 13 or so (I run relays off the alt to the HLs). Still seems a bit high.

Disconnecting the green field wire lets the voltage pull down to 13 or so.

Going to check for a short to ground, I'll also run a jumper to ground on the VR, but I've got a hunch there is an alternator issue.

Update: No short on the green wire. I've got about 500 ohms to ground on the blue wire though with the VR, Alt field, and ballast resister disconnected. That wire splits off in several directions under the dash. Have to dig through the wiring diagrams to figure out if that should have any continuity to ground.

If battery potential is much over 12.6V, with any kind of load, and the green wire disco'd, that's prima facie evidence, as was mentioned above, that the alt. field has an internal short.

Rick

I'll test it again to be certain.

Unfortunately with the car in an attached garage, and the children sleeping, it's going to have to wait until tomorrow.

Your latest test voltages are within reason. Nominal float charging voltage on a lead acid battery is 2.4 Volts/cell or 14.4 Volts. I have seen in the high 14's on some cars as there is a wide variation in parts tolerances in the regulator. I trust you are measuring right across the battery terminals with a good digital meter. As was mentioned a good ground, especially on the regulator, is important as well as good block to firewall and negative side of the battery.

Those reproduction VRs have TON of paint on them, front and back. No way I'd ever get a decent ground out of the box. I wire brushed the back side under one mounting hole and used a star washer underneath to ensure a good ground. A continuity check with a multimeter confirmed, so that won't be a problem.

The motor is grounded to the radiator support, but not to the firewall. I'll probably add a strap to the back of the motor for good measure.

I'll recheck the voltage tonight.

with the ign in run,check the voltage at the ballast(i think the blue wire feeds the reg)
if it is low(10 v?)it will call for the alt to charge even though the bat is fine.
prob need to clean all connections with a scotchbright pad...especially the bulkhead

Tested it again tonight. Charging around 15 at fast idle. Voltage drops below 13 when I disconnect the green field wire.

It should stop charging completely when you disconnect the green wire, so it should go back to the battery voltage.

What I'd test now is the voltage on the blue wire at the alt, compared to the voltage at the battery. I bet you'll see ~0.5v of difference there. That's the sum of the voltage drops from the resistance of the several plugs it goes through to get to that point.

If you jump battery voltage to the blue wire, the voltage at the battery should drop back to ~14.5v. It will also keep running after you turn the key off ()

I ended up installing a relay to power the coil/VR/alt, as I was losing over 1v in the wiring, and hitting almost 16v at the battery

The weirdness continues.

Drove the car to work today. All seemed well. Tonight when it was time to go home, I noticed my radio presets had disappeared. I had used them in the morning, so I know I reset them after the last battery disconnect.

Cranked the car over once and the (mini-type) starter stopped dead after a revolution. Hmmm. Tried again and it fired right up. Nothing unusual on the amp gage. Showed slight charge going down the road as you'd expect. Dim headlights at idle, as you'd expect.

Got home and pulled in the garage. Heard a couple back-fire pops under the hood. Shut the car off thinking I might have an ignition issue. I've heard them before but thus far have been unable to track it down. Sounds like it's coming through the exhaust.

So I pop the hood and grab my DMM to check voltage. About 13 V.

I try to restart the car in the dark to see if I've got some spark scatter. Hit the key and get nothing but the starter solenoid rattling.

Do it again again while watching the DMM and the voltage drops below 5 V.

Dead battery!?!

I've now got the battery on trickle. I'll take it in tomorrow to have it tested. Thinking I might have a couple plates shorted.

UPDATE: Snuck over to O'Reilly's tonight to have the battery tested. It's toast.
I'll test it with the new one tomorrow.

Started the car this morning. On fast idle it's charging at 14+ volts and climbing. Little worried that it might still be trying to over charge. I'll be keeping a very close eye on it.

Might be worth pulling the alternator to have it checked.

I'd hate to have to change it. It's a single pulley round back. Correct replacements are $$$$!

Measure the voltage difference between the alternator output and the blue ignition power wire (either at the alternator field connection or ballast resistor.) The voltage regulator only knows the voltage on the blue wire, so if there is a bad connection somewhere the regulator will sense a lower voltage than what is being output by the alternator. A bad ground reference at the regulator could also cause similar problems.

A quick test might be putting a jumper from the alternator output to the blue wire mentioned above and see if the output voltage drops.
If the voltage drops, it shows there is resistance in the ignition circuit wiring, maybe a bad connection or partly burned fuseable link?

..i had a that problem with about 15.2 volts at the battery.
on long trips the battery would start to boil and i eventually noticed battery acid in the tray and were it dripped on the inner fender and down on the suspension
..what a @^#@>>***mess.
..cleaned all terminals including bulkhead..blue wire voltage went from 10.5 to 11.6.(key on,engine not running)
Battery down to 13.8 - 14.0 when running.

Seems to be under control now. It's charging at a steady 14.5 max.