Hacking (up) the Opus Ignition
A while back, I decided to dig the secrets out of the Lucas Opus distributor in one of my TR7s. Since the Opus ignition has a reputation as one of the least reliable products ever made by Lucas (yikes!), I expected to find all sorts of bone-head design errors and shoddy components incorporated into the design. For the most part, I found exactly the opposite! The components are top quality, and the unit has several thoughtful design features that show that the designer knew his stuff. It's actually much better quality than the distributor in the TR8. But, of course, it couldn't carry the Lucas label without having manufacturing mistakes that would negate all of the care the original designers lavished on it - I'm sure Dilbert's boss had a hand in the production engineering.
If your Opus distributor is intermittent or dead, you may want to investigate the following items:
1. A bad internal ground connection. The circuit board electronics are grounded by a ring terminal screwed down to the distributor housing. A "star" washer is used to lock it down and provide good electrical contact. Unfortunately, the whole assembly is potted with a highly filled, very hard epoxy, and no provision was made to keep it away from the ground screw. The epoxy flows in and around the washer, destroying its ability to apply pressure between the screw and ring terminal, much like stiffening the car's ride by encasing the road springs in concrete. After a few thousand thermal cycles, I wouldn't hold out much hope for a good ground.
You can burrow down to the screw using a 30 to 60 watt pencil type soldering iron, or a small ice pick heated on the kitchen stove. You will have to re-tin the soldering iron after mistreating it like this, so you might want to have some non-corrosive soldering paste handy. Press the point of the iron gently into the epoxy over the screw. The epoxy will immediately disintegrate and become crumbly as it heats. Too much heat or pressure are counter productive. After removing enough epoxy, unscrew the screw and reach down onto the hole with a sharpened jeweler's screwdriver to scrape things clean. Clean the screw and washer and reinstall them snugly. Squirt some dielectric grease into the hole to seal things back up.
2. A shorted-out output transistor. This is sooooo dumb, it's funny. Instead of putting flat washers under the transistor mounting nuts, the folks at Lucas used small ring terminals that probably came with the transistor mounting kit. Tightening the nuts cause the ring terminals to rotate until they touch the distributor housing, shorting out the transistor. If the terminals only touch the housing gently, thermal expansion would cause the short to come and go. On my distributor, there was no short until I applied gentle pressure to the top of the transistor with my thumb. I doubt that all the Opus distributors have this problem, but it's worth checking.
Unplug the distributor and connect an ohmmeter between the gray striped white wire and ground. Press on the epoxy covering the transistor with your thumb, or heat it with a hair dryer. If the ohmmeter reading is constant and non zero (you might read the resistance of a diode that's in the circuit) you're OK. If the reading jumps around, dig into the epoxy with a soldering iron and take a look.
3. External Wiring. Even the wiring that is attached to the distributor is a little weird. The finned resistor screwed down near the coil is not the ballast resistor. It supplies current to the base terminal of the distributor's output transistor. Most other cars have the "base drive" resistor mounted inside the ignition module along with the rest of the electronics. If you upgrade your car with an unballasted "sport" coil and bypass that resistor, thinking it's the ballast, you'll instantly blow up the distributor. The TR7 ignition ballast is a long resistance wire inside the dash harness. It is white with a pink stripe and can be seen if the fake speaker grille on top of the dash is removed.
4. Bad ferrite. If the core of the sensor is cracked, it's time for a Luminition. The sensor consists of a coil wound on an "E" shaped core made from ferrite. Ferrite is the stuff speaker magnets are usually made from. It's very hard and brittle, like glass. If it receives a hard shock, it will crack - but unless you pulled your distributor and tossed it down onto the driveway, any cracking would most likely be caused by thermal cycling. If ferrite is glued to something that expands and contracts at a different rate, eventually it will crack. One posting to the TR7/TR8 mailing list described a problem with cracked ferrite pins in the rotor, and another where some had fallen out.
You might try checking things out like this - Unhook the coil and replace it in the circuit with a small light bulb. Turn on the ignition and rotate the engine by hand until one of the pins in the disk below the distributor rotor lines up with the small "nubs " sticking out of the sensor. The light should go out. Gently heat up the area around the sensor (not too much!) with a hair dryer and see if the light blinks on. If it does, the pin or the sensor may have a problem, or the gap between the sensor and the rotor is too great. Turn the engine some more to check each pin in turn. Don't overheat things, and you might want to monitor the temperature of the drive resistor mounted near the coil on the fire wall, to be sure it doesn't get too hot.
Also note that if your distributor has a vacuum unit, it is most likely for retarding the timing at idle rather than advancing it when the engine isn't loaded. If the vacuum capsule diaphragm splits, it will cause the engine to idle too fast. If you're trying to diagnose a suddenly too-fast idle and find the timing advanced, resetting it without checking the distributor's condition may result in a significant performance loss or overheating due to retarded timing at speed.
Click on pictures to enlarge.