Tackling the TR8 Ignition

   My TR8's previous owner apparently had a problem with the original Lucas distributor and replaced its output transistor. Then replaced the circuit board assembly wth a new one. Then replaced the entire distributor with a Mallory Unilite unit. A month later, the Unilite blew up. Hah! Take that! It's brutal under the hood of a Wedge.

   Like the Lucas distributor in the TR7, the TR8 distributor uses OPUS magnetic triggering technology that is not Hall-effect or variable reluctance. It's closer to what a treasure-hunting metal detector uses. Eight small trigger pins are embedded in a plastic wheel mounted below the distributor rotor. The circuit detects them as they sweep past and unbalance a sensor assembly and momentarily interrupts the current through the ignition coil, firing the spark plugs.

   Unlike a TR7, the electronics are housed completely inside the circular body of the distributor and potted with a soft, green compound that is easily removed and doesn't appear to cause problems.

    I ran some tests and noted the voltages on the oscillator schematic (below). At left is a picture of the circuit's 500 KHz output as it appeared on my oscilloscope. Anyone using this information for troubleshooting should keep in mind - touching a probe to pin 2 of the sensor will disturb it enough to turn off the coil, so the differences between the "target" and "no target" waveforms are deceptive. A small screwdriver blade worked well as a trigger - the unit usually won't trigger with non-ferrous targets.

   There is an interference suppression capacitor (it measured 180 pF) bolted to the outside of the distributor housing that isn't shown on the schematic. The red/black power wire exits the distributor body and is attached to the capacitor, then runs to the rest of the wiring harness.

   If the distributor is dead, check for:

   1. A bad output transistor. First, turn the ignition OFF. Take a 12 volt test light, apply 12V power to one lead and touch the other lead to the black striped white wire coming from the distributor. If the bulb lights, either the big suppression diode or the transistor are shorted. If not, the transistor could still be bad (open), but that's harder to verify. I think the original output transistor is operated pretty close to the edge of what it can tolerate - that's why the ballast resistor is only partially bypassed while the engine is cranking. If you combine an inappropriate aftermarket coil or ballast resistor with the OEM distributor, you're playing with fire.

   If you are bound and determined to replace the output transistor, use something at least as good as an NTE98 or a BU931. Spec-wise, they look pretty good, but if your repair fails in traffic, someone could get hurt. A Crane or Luminition conversion is a much better idea.

   2. Bad ferrite. If the sensor is broken, you're toast. 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 ferrite is glued to something that expands and contracts at a different rate, it will crack eventually. I found a posting in the long-gone mailing list archive that described a problem with cracked ferrite pins in the trigger wheel, and another where some had fallen out!

   Use a feeler gauge to carefully measure the gap between the sensor and the trigger wheel. It should be .014 to .016 inches. If the gap is too large, the ignition may cut out at certain RPMs or stop working when things get hot. Make sure things don't wobble.

   You can test it 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 trigger wheel 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 trigger wheel 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 above the pc board, to be sure it doesn't get too hot.

   3. Bad internal wiring. I've heard that one common Rover SD1 problem is caused by the sensor cable inside the distributor. It fatigues, then breaks its connection as soon as the the vacuum advance moves the base plate. The engine starts OK, but dies as soon as the throttle is opened.

   4. Bad solder joints. Any number of symptoms can be caused by a bad solder joint on the circuit board. The compound coating the board is soft and can be removed with care and patience. After resoldering the joints, clean them thoroughly with a solvent recommended by the solder manufacturer - you don't want them to corrode later. Spray the board with a silicone conformal coating such as Chemtronics Konform SR2000. It's carried by lots of electronic supply houses and mail order outfits such as Newark Electronics. Don't slather regular silicone RTV all over the board, as it releases acetic acid as it cures.

    Here are the centrifugal advance curves for the distributors in carburetted and injected TR8s, as listed in the Repair Operation Manual. You'll notice that the EFI distributor has more centrifugal advance than even the non-emission controlled UK unit. After taking into account the differences in static timing, the EFI and UK distributors wind up with the same advance above 1400 RPM, with the EFI cars' ignition timing retarded at idle.

   If you're feeling a little ambitious, it's possible to fit a points-type or even an HEI distributor from certain Buicks with a relatively small amount of whacking, cutting and banging. The HEI disributor is a bit of a squeeze, though.

   Here is an edited exchange from the TR7/TR8 mailing list:

Click on pictures to enlarge: