68 Alternator Whine

[Kurt B.]

Took the Corvette out today for the first time this year to get an inspection sticker.
Noticed that I get alternator whine through the radio.
I did not have that when I last drove it before winter.
Alternator was restored a few years ago to factory original specs by John Pirkle and has been perfect and now only has approx 1,000 miles on it.
What causes this and what is the fix?
Will it go away with a little use?
Annoying as hell,
Kurt #26406

[Rich G.]

Re: 68 Alternator Whine

Could be one bad diode. I bet it won't go away. Best to bench test it. Perhaps Jack H will jump in. He remembers more than me about this stuff.

Rich

[Kurt B.]

Re: 68 Alternator Whine

Rich,
How does a diode go bad? What causes it to go bad while it is just sitting there in a garage not being used?
I don't know anything about auto electronics and probably to old now to learn but I thought a new alternator would be quiet for a while.
Guess not.
Kurt

[Rich G.]

Re: 68 Alternator Whine

Semiconductors can fail in "infancy" due to poor handling causing a static charge. Alternator diodes should be pretty tough, but it does happen. My experience with this has been in airplanes where a lot of different things can cause a whine in the radios. Poor grounding is one, but that should not change. Also the heavier the load, the louder the whine. Now it is possible that your battery may be asking for a lot of current because the car has not run in a while, in which case you may have nothing wrong and the whine may go away when the battery is fully charged.

Do you notice a change with load, for example when you turn the lights and/or heater blower on?

Rich

[Kurt B.]

Re: 68 Alternator Whine

Rich,
No change with load except a change with acceleration, in other words as the revs rise the pitch of the whine increases also.
The battery is hooked up to a Battery Tender whenever the car is parked in the garage and the battery was new 2 years ago. It is a Delco 5 year battery.
Kurt

[Jack H.]

Re: 68 Alternator Whine

Alternator whine is 'normal'. That's why the 'kit' for the radio option included so many dang capacitors mounted here/there. They're 'hush up' fixes intended to give hi frequency electrical components a convenient path to ground at the source(s) of noise. On a '68, you still have an external voltage regulator and that's the purpose of the capacitor that's mounted to the side of the V-Reg can....

There are also general purpose capacitors strategically mounted on the heater blower motor and on instrument components in the dash. When one or more of these caps isn't present, isn't working properly, you can expect a 'whine' in the radio that progresses in lock-step to the RPM of the alternator.

The alternator is a 3-phase device with six diodes (two for each winding phase). Diodes can fail open circuit or short circuit. Excessive heat, over-voltage or over-current are the common causes of 'death'. Plus, it's possible to have a 'flakey' diode...one that 'heals' itself when power is removed, only to live for a SHORT period of time once power is restored and current flows through the diode again.

Depending on how a diode failed, you can either lose 1/3 of the alternator's output current OR wind up with a permanent current leakage path to ground causing the battery to drain when the engine isn't running. BUT, a bad diode isn't going to generate the whining noise you complain of....

How do you troubleshoot? You inspect the system with an oscilloscope looking at the cleanliness of the DC output of the alternator while it's running/charging and compare what you see on the scope to 'normal'. This takes some experience and most good auto electric shops have the tools and know how while few home mechanics do.

On RF capacitors, you pull them one by one and test them, but few have the necessary tools to ascertain a capacitor's health (you have to measure BOTH the part's raw storage profile AND it's stored charge leakage profile). Think of these as being to electrical flow what a bucket is to water.

If I sell you a 1-gallon bucket that has a jillion holes in the bottom/side, yep, it holds 1-gallon of water, as advertized! But, for how long? That's why simply measuring a capacitor's raw storage profile tells you only half the story...

The concept goes above the heads of most, so mechanics simply discarded/replaced capacitors when they did tune ups and changed ignition points to be on the 'safe' side. Lord knows how many good ignition condensors the world has thrown in the trash can over the years, but that's the way the world worked!

[Kurt B.]

Re: 68 Alternator Whine

Jack,
Excellent explanation as always.
Thank you
I assume that one thing a home (wanna be) mechanic like me could do is to replace the RF capacitors one by one until I eliminate the one that causes the whine.
Correct?
Kurt

[Kurt B.]

Re: 68 Alternator Whine

Jack,
I forgot to ask, where can I buy these RF capacitors and be sure that I am getting EXACTLY the same one as I am trying to replace since the car is a 68.
GM, Radio Shack, where ???
Since I don't know this stuff they could sell me anything and I might screw up.
Kurt

[Jack H.]

Re: 68 Alternator Whine

It doesn't do you any good to replace RF capacitors that were installed to hush up items other than the alternator (for example the cap on the TS flasher, the cap on the heater blower fan, Etc.). So, you're really at the cap on the voltage regulator (1964086), the radio power line cap (801249) and the ignition coil cap (1947452).

You're not going to find these at Radio Shack as they were typically Delco Remy components packaged to fit the application. Different catalogs have them in either factory original or service replacement form...

[Kurt B.]

Re: 68 Alternator Whine

Jack,
The numbers in parentesis, are they GM part number sor AC Delco numbers.
Will the local Chevy dealer know these numbers and can I assuem that these parts are still available from Chevrolet?
Sorry for all the questions but I know nothing about this stuff
Kurt

[Jack H.]

Re: 68 Alternator Whine

Yes, numbers mentioned are AC Delco/GM numbers. I don't know which of these are still available from Chevy, but expect those that are to be functional service replacements differing from factory production configuration in minor ways.

[Kurt B.]

Re: 68 Alternator Whine

Thank you Jack
I will try to buy them this week
Kurt