I have a 72 Cutlass stock 350. While I was driving today I kept smelling rotten eggs. When I got home I popped the hood and could hear the battery sizzling. I immediately removed it. Upon removal I did check the voltage and it was charged at 13.7v. I let it cool down a few hours, tested it and it was at 13.4v and installed it back in so I could check to see if it was overcharging. In fact it was and upon inspection I received a 17.8v reading. Shut down the car and once again disconnected the battery. My silly question for everyone is am I correct in thinking this is a voltage regulator issue and not the alternator itself since the regulator is external. Sorry for the question but I must say this is the first time I've ever run into this. Thanks, Mike.

 

Bad VR or faulting VR wiring. The ALT is producing more than adequate voltage (on demand) to recharge the battery. The VR is failing to regulate the amount of voltage. Voltage should never go above 15VDC at idle.

 

As stated above, your voltage regulator is bad.

 

Thank you guys. This was my thought but sometimes a little verification from seasoned pros is helpful!

 

You could check at Fusick for a new VR.

https://www.fusickautomotiveproducts.com/

 

Or just go to your local auto parts store.

 

if you have a Delco Remy VR sometimes the cover fits the replacement to keep the OE look.

 

Luckily I am very close to a Summit store so I was able to pick one up today. I'll be replacing it tomorrow and will look into the swapping the cover to keep the OE look. Thanks for the information.

 

I’m no electrical pro, but I wonder if polishing the contacts in your voltage regulator and checking the distance between open & closed points would be a good step. It’s a quick step to remove the regulator from the firewall, unbend the 4 tabs that hold the top on, and check out the guts inside. It could well be that your points are burned, or they’re stuck open, or stuck shut and cooking your battery.

If the regulator points are burned, or carbon-ed up, polish them up with a bit of 600 grit sandpaper to restore clean metal-to-metal points contact. I just fold sandpaper so that both edges have grit outward-facing to polish both points at the same time. Usually burnt points need just 5-20 back&forth scrubs to get the carbon cr*p off. If the points are too close together, as measured against the Chassis Service Manual, bend the too-close arm (or arms) back up to the correct gap specification. I could be very wrong, but the number .025” sticks in my head as the proper gap…

I went to internally regulated alternators a long time ago, but I remember that the older systems basically shorted out the output from the alternator when the battery was full. Except that the regulator was mechanical with points that could burn and springs & what not.

Just suggesting a quick-fix that might get you back on the road with minimal pain. The longer term fix, of course, is a new voltage regulator and maybe an alternator too.

Hope that helps.
Chris

 

Close, but not quite. The mechanical regulators controlled the alternator output by controllling the field current going into the alternator (which generates the spinning magnetic field that is used to excite the 3 stator coils whos output is rectified and fed back to the battery). The stronger the field current, the stronger the stator output current. The battery voltage is connected to a coil inside the regulator that pulls on a 3-position set of contacts (closed to one contact, open to both, closed to the other contact). If the battery voltage is too low, one set of contacts are connected which provides maximum field current. As the battery charges up, the contacts move to the open to both postion which un-shorts a resistor that now reduces the field current and in turn the stator output current. If the battery voltage gets too high, the contacts move to the closed-to-other-contact position which then shorts the field current input to the alternator to ground, reducing the field current to zero and in turn reduces the stator output to zero. As the battery then discharges, the contacts will then move back to the open-to-both postion and now the field current increases to the middle level again. As loads on the system change, the duty-cycle of the contacts moving between the zero and middle field current position changes as required to maintain the battery voltage. If electrical loads are high, the regulator contacts may also reach the max field current position.

In the OP's case, either the contacts are stuck in the max field current position, or the contacts for the zero field current position are fried and not making contact when they should. So yes, cleaning the contacts and ensuring they work properly is solid advice. It may also be possible that the coil inside the regulator that moves the 3-position contacts has opened up which would result in constant max field current and overcharging of the battery.

 

Test the reg and alt.
follow this...

 

Many thanks for schooling us up on the internals of the voltage regulator. Very useful to know. I switched from the external voltage regulator so long ago, that I never really knew how they worked. I never knew there were 2 sets of point. Cool. Most of the GM relays back in those days only worked 1 way.

Somewhere along the line I decided that if the 70’s-era internally regulated voltage regulators were solid state & kept the lights brighter at idle, that was an advance worth upgrading to. Plus the cleanliness/simplicity of just 2 output (vs. 4) wires was really attractive. I wired mine using instructions found here, or nearby…

The electrical upgrade was kind of based on my experience with quadrajets. Electrical systems and carburetors advanced a lot between 1966 & 1978 or so, I just took advantage of the later designs since GM had all those years to learn how to make the systems more reliable. Disc brakes are the same kind of thing. The later designs were better, so why not upgrade since I’m not a purist? I was lucky to have cars from a year where the later stuff retrofits well.

Apologies to those of you who are purists. Carry on. I just use my cars as drivers, not show cars, so I’d like them to be just a little closer to safe & modern In my mind, my 1966 cars are about as safe as 1975 cars. It’s a good step forward over 1966 technology, but doesn’t hold a candle to 2026 cars for safety. That’s o.k. by me.

Since 5 or so years ago, my alternator “Sense” wire reads voltage at the horn really as a proxy for system voltage which is less optimistic than jumpering it at the alternator itself. I figured the horn relay junction would be a good point to measure system voltage and that has worked well. I’m not a big fan of the “simple’ 1-wire deals since they mechanically assume that voltage at the alternator = system voltage.

Anyway, thanks again for the information on the guts of the external voltage regulators. Glad I wasn’t off-base about suggesting cleaning the points. Over the decades I’ve found a little steel wool or other abrasive can very easily restore their function to something like factory.

Cheers
Chris