I have a weird problem that you might be able to help me with. I have an Olds F-85 that I upgraded to a later model internally-regulated alternator. This alternator uses the flat two-wire connector. The car runs and starts fine, but some strange things are happening on the electrical side.

I have one of those Dakota Digital dashes that appear analog but display some digital information. At odd times the display will go blank at the same time that the speakers in my stereo start making a weird popping sound. At the same time, the blower fan in my Vintage Air system will suddenly surge and I can hear it begin to spin up and then die again. I’ve even disconnected the amplifier power and there’s still weird popping and scratching noises coming from my speakers. It is so annoying that the car is now no fun to drive. I really need to fix this but all my local repair sources just scratch their heads. Do you have any suggestions?


a close up shot of the front fgrille of an oldmsobile f85 parked at summit racing near akron ohio
(Image/Summit Racing)

Those weird noises could be generated through several different paths but the most likely source is the alternator. All alternators are actually alternating current (AC) generators. This AC current is then directed through a series of diodes, which are electrical one-way switches that eliminate half of the AC wave, creating direct current (DC) at the appropriate amperage and voltage.

All of this works extremely well until one of the diode packages fails. When this happens, AC voltage is allowed to surge through your electrical system and this could be what is causing the weird noises and fan operation. This AC power can produce odd results, some of which may be what you are experiencing. As an example, during our research a few years ago on radio frequency interference (RFI), we discovered an online video from an automotive repair shop where a car came in with speedometer problems.

The car was sitting in the shop with the engine running and as you revved the engine, the speedometer would indicate 20 to 30 mph and higher speeds as rpm increased. The issue was an alternator where one diode pack had failed allowing AC voltage into the vehicle. Most late model speedometers are now driven by an AC electrical frequency created by the vehicle speed sensor (VSS) in the transmission. The failed diode created an AC voltage in the frequency range used by the VSS to indicate speed – which was picked up by the speedometer and displayed.

It would appear your alternator is still capable of outputting sufficient current and voltage to power your car, but at least one diode (there are always six diodes in a three-phase alternator) has failed. When this occurs, the AC voltage will move through the electrical system and could be causing the issues you describe. We shared this information with the technicians at Powermaster and they said they’ve seen this happen before and it is usually traced to a bad output diode in the alternator.

Some may question this because the charging system is still functioning but one bad diode will not dramatically reduce the output of the alternator, especially if it is a higher output unit. However, a bad diode can also allow current to leak through the alternator to ground and cause the battery to drain if the car is stored for an extended period of time.

One way to test if the alternator is bad would be to put it on an alternator tester perhaps at your local auto parts store. Short of that, you can perform your own test by starting the engine and engaging as many high amperage devices as possible. This would include the headlights, heater blower motor on high, turn signals, the stereo, and certainly engage any electrical cooling fans. Then test the voltage at the back of the alternator. It should be capable of 13.9 to 14.1 volts or higher. If the voltage is closer to 12.5 to 13.0 volts, then it’s possible the alternator has a bad diode or two.

Other possible explanations include poor grounds between the body and the chassis or a poor ground between the alternator and the engine. This is especially a problem if the alternator is painted or powder coated—this can lead to poor alternator performance. Make sure your stereo is properly grounded to the body and the body is grounded to the battery. Most old cars have long since lost those slick little copper braided ground straps that were included between the engine and the body to reduce the voltage drop caused by poor grounds.

But our money is on the alternator as the source of your problems.

All automotive alternators are AC generators that use diodes to convert to 12-volt DC power. A bad diode will allow AC voltage into the system and could be the cause of your problems. Here, we’re adding a separate ground to this powdercoated Tuff Stuff alternator to ensure a proper ground to the battery. (Image/Jeff Smith)

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.