Triumph Alternator Diagnosis and Repair
“Five bucks! That’s all for the core?”
A few months ago, when my alternator ceased charging my car’s battery, I took it to a local armature shop for exchange on a rebuilt unit. Considering what they offered me for my old unit, against the $100 for the rebuilt, I decided to keep it, take my time to diagnose the cause of failure, correct the defective component, and keep it for an immediate spare when I need it.
Diagnosing the problem was easy. Even the defective regulator was reasonably cheap (about $18 locally) and easy to replace. Explaining the process to someone else, perhaps with a slightly different wiring harness, regulator, or diode pack gets more complicated.
This article is aimed at those who enjoy diagnosing and repairing things, either to save a buck or just to be able to say they did it. For others it may provide a little more basic insight into how alternators work. Just for a disclaimer, there are some off-the-wall problems these testing procedures can miss (broken insulation on the field or stator winding comes to mind). I’ve tried to present simple testing that will catch the most likely problems.
As with any other auto component that’s been around for more than a year or two, there are bound to be variations as manufacturers try to consolidate model lines and improve performance. Lucas alternators are no exception.
I spoke with a very helpful tech rep, Ron Hellebuyek, at Lucas Aftermarket Operations (yes, I actually spoke to the Prince of Darkness and he was darn friendly). He was able to supply me with copies of pages from ancient tomes, describing and illustrating the differences in the alternators of our car’s vintage.
In an attempt to keep this article within proper perspective I’ll only attempt to cover the 15-18 ACR models. They were used on TR6s, 7s and 8s, Spitfires and GT6s (spanning the early ’70s through 1980). TR7s and 8s with air conditioning needed the much higher current output of the 20 or 25 ACR. They all feature similar internal regulators.
Even though the three wire harness connection(s) may be different, they’re basically the same in operation. The large brown wire (# 8 or 10) is the main output to charge the battery. The smaller brown wire (#14 or 16) comes from the harness and supplies battery sensing voltage (if used by the regulator). The small brown/yellow wire goes to the “IND”icator bulb on the dash.
Alternators, just like generators, produce alternating current. Generators convert, or “rectify”, the current to DC by means of the commutator and brushes. Semiconductor devices called diodes are used in alternators to keep the current flowing in one direction. Inside our alternators are six diodes connected together electrically into a single component known as a full-wave bridge rectifier. There are 3 more diodes in the pack that direct current back to the field windings once the alternator is turning at speed. Generators employ an external regulator that controls both voltage and current flow (amps) and, by using a cut-out, prevents any reverse flow. In alternators no cut- out is needed because a good diode pack only flows current in one direction. Due to the inherent electrical characteristics of the stator/field combination at high rpms, current level is self-limiting, eliminating the need for a current regulator. To keep from over charging the battery, Lucas does employ an internal, solid state voltage regulator that maintains alternator output between 13.6 and 14.4 volts. Output is controlled by regulating the on/off duration of the “excitation” current that’s fed back to the field windings once the alternator is up to speed.
Over the years, as voltage regulators have been superseded, there have been two, three, and four-wire hook-ups inside these alternators. Some are battery sensing, some are machine (alternator) sensing, some can do both. This is where some of the confusion starts.
The four-wire (Black, Red, Yellow, White) regulator has a dual sensing, “fail-safe” feature; it can monitor both the battery and, if that connection fails, the machine. The two-wire (Black, Yellow) unit monitors available field “excitation” voltage, while the three-wire (Black, Yellow, White or Red) can monitor either the machine or the battery, depending on its connection at the diode pack. (Have your eyes started glazing over yet?) In all units the “field” connection is now made via a small strap, or “link”, between the case of the regulator and the center field brush. This has replaced a green wire found on earlier units.
Unless you know your alternator came on your car, it may have been replaced with an aftermarket rebuild using non- Lucas regulators. Original Lucas regulators had a numerical code stamped into the case. Later units have a 3 letter and 3 digit code. Now the I.D. is something like UCB 104. Without those codes assume it’s non-Lucas aftermarket. As of mid-1994 Lucas was supplying two, three, and four-wire regulators.
There are a couple of other items that may or may not be cased inside your alternator. Earlier units housed a 3 microfarad capacitor, attached between ground and the main output terminal to suppress radio noise. This was discontinued in 1979 and the part is no longer available. On newer units there’s a surge protection diode wired between ground and the “IND” (indicator) conductor. As I’m finding out on some rebuilt units, this device may or may not be in place, probably depending on the vintage of the core. The alternator will work without it, but it does serve a purpose. This device is a zener diode, installed to protect the regulator’s main output transistor. It absorbs high transient voltage caused by a faulty connector or by removing a battery cable while the engine is running. This diode is still available from Lucas and, if you’re missing it, you may want to get one. (Check your parts catalogs for Lucas # UZB 106, cost is about $9)
Allow me to drive this point into your cranium; surge diode or no, NEVER, EVER, REMOVE EITHER BATTERY CABLE WHILE THE ALTERNATOR IS SUPPLYING CURRENT TO THE HARNESS. In less that a heartbeat an overvoltage spike could destroy a perfectly good regulator, or worse. ‘Nuff said?
OK, out of nowhere, your “IGN” light starts glowing, where do you start? That little glow indicates an electrical imbalance between the alternator and the battery.
The warning light circuit also supplies a small current, via the “IND” terminal, to the field coil when the ignition is switched “on”. That in turn sets up a magnetic field, or “flux”, within the field coil. Then, by spinning that magnetic field inside the stator windings, the alternator generates electricity. Once the car starts, the field becomes “self excited” and no longer needs current from the warning light circuit. If the alternator produces voltage that’s less than the battery (or none at all), current continues to flow from the battery through the bulb and it continues to glow. If, after the car starts, the output voltage of the alternator equals the voltage at the battery terminals no current can flow because both terminals of the bulb have the same “potential”.
If your battery goes flat and the bulb never comes on, you can check the operation of the bulb and it’s wiring. After recharging the battery, and with the engine off, disconnect the plug that attaches the brown/yellow wire to the back of the alternator and turn the ignition switch “on”. Using a jumper wire, ground the brown/yellow wire. The bulb should light. If it doesn’t, find out why. The bulb is either burned out or there’s a bad connection.
When your bulb starts glowing or the dashboard gauge shows a no- charge condition, see if charging current is reaching the battery. With the engine running at high idle (1500 rpm), measure the voltage at the battery terminals using a volt/ohm meter (VOM); digital is more accurate, but for this test an analog needle will work fine. On high idle, the voltage at the terminals should be between 13.6 and 14.4 volts. Outside that range indicates bad or dirty connections or defective alternator.
The shop manuals recommend test procedures, but they require a specialized set-up. Easier, take the alternator into a TRAK or one of the armature shops for a bench test. Chances are they’ll tell you it’s dead. They won’t tell you what’s wrong, they’ll just want to sell you another unit.
Barring physical damage (engine fire,