Why Do Regulators Fail? COLUMN BY BOB THOMAS For as long as they have been around, voltage reg- ulators have taken most of the blame for charging system failures from back- yard mechanics, service technicians, parts counter personnel and even electrical rebuilders. How many times have you heard or even said yourself, “It must be the voltage #%&* regulator again.” Have you ever wondered why? The regulator is but one part of the charging system, and many factors besides its own vulnerability affect its reliability. If we want to rebuild a better, more profitable unit and save ourselves some headaches, we better begin won- dering why more often. The regulator does its job by moni- toring the system voltage and adjusting the field current to control output and maintain a predetermined setting. Early regulators used vibrating contacts to do the job. These mechanical regulators were limited by the life of the contacts and the amount of field current they could handle. Just as electrical systems became hungry for more amperage in the early 70s, the electronic regulators came along to save the day. First, they had no contacts to wear out. Second, by running the transistors in pairs, these new regulators could handle field draw in excess of 3 amps, making the 60 amp passenger car alter- nator a possibility. Anyone who was rebuilding in the early 70s remembers that although there were no contacts to wear out, these pioneer regulators did have problems. Vibration, heat and contamination can all cause electronic failure and the automotive alternator has an abundance of all three. In spite of this, reliability was eventually achieved. As the decades passed, automotive engineers called for more and more amperage output, which in turn meant higher field current. Regulator manu- facturers struggled at times but managed to keep up by utilizing the latest tech- nology available. Now we commonly see rotors with 5-plus amps field draw (a few even approaching 10 amps), internal alter- nator temperatures in excess of 400 degrees Fahrenheit, and electrical loads that are constantly changing. It is a wonder to me how voltage regulators survive at all, yet they are engineered to withstand this environment and do a pretty good job of it. Why then do we all seem to have so many regulator failures? There can be many reasons, but to understand them you have to realize that the voltage reg- ulator is part of the field circuit. We are accustomed to thinking of the rotor and brushes as the field, but the regulator, as the switch, is also part of the complete circuit. The power or field transistor must carry the full load of the rotor coil and switch several hundreds of times every second. Any problem in this cir- cuit can stress the regulator and cause premature failure. The first and most obvious culprit is the rotor. Yes, field current draw can be high, but it should not exceed the speci- fication. We need to know exactly what each rotor should read, and ensure the one we are using does not exceed that. When you are rebuilding a group of like units, you can make comparisons. However, the proliferation of alternator numbers and applications makes it diffi- cult for the small shop to build most units in batches. Therefore, it helps to keep records. A simple notebook next to the rotor tester is a good substitute to our ever decreasing memory capacity and will provide guidelines when you sus- pect field amperage may be too high for a particular unit. Some of us have come to think of rotors with high readings as being better, equating higher field draw with higher output. This might be true, but not if the coil is internally shorted. Even if there is no short in the windings, the extra field current will cause the reg- ulator to generate more internal heat, surely to shorten the regulator’s life. It is also possible for rotors to develop problems that only show up during operating speeds. Centrifugal forces on the coil can cause shorts, either coil to coil or coil to ground. Both can be fatal to the regulator, and any unit that “eats” several regulators for no apparent rea- son could contain a rotor with a flying short. Probable shorts to ground can be located using a high voltage test, but flying internal shorts are practically impossible to check, unless you can monitor the field draw on your test bench at very high RPM. Since this test is not always possible, I would replace the rotor if no other cause of regulator failure can be found. Another often neglected source of premature regulator failure is poor ground of the regulator. This is espe- cially true with the three most common Japanese manufacturers, Denso, Hitachi and Mitsubishi. All of them use a reg- ulator mounting screw to hold a tinned brass ground strap to the aluminum SRE housing. This ground point must carry the full amperage of the field cir- cuit (with A circuit fields), often forgot- ten by too many of us. You have a bimetal electrical connection that is held together by