I have been trying to resolve a driveability issue on the 78 V12. It has been running smooth at idle and low speeds but cutting out and seeming to drop half the cylinders at higher speeds when accelerating. I have been running with the original magnetic reed switch trigger plate. Because this is a known weak part, and because a failure would be likely to result in half the cylinders dropping out, ?I decided to upgrade to the hall effect switch trigger board. As I hear it, few of the original reed switch boards are still operating. Of course, it is difficult to properly diagnose because it does run good in the garage, only acts up on the road. I thought I made a pretty good score, I got an NOS Lucas hall effect trigger board off eBay for $60. Just the board, no rotor, no mounting parts. I put it in, transferred the nasty old mounting blocks from my old board and attached it with nylon screws I bought off eBay. I believe I have 100 of them, if anyone is in need. I go to start the car (it did idle fine on the old trigger board) and I get a no start. A little investigation shows me the wires broke at the terminating resistor connector as I was installing the new board. A couple new terminals and now I have a consistent start-stall. I quickly convince myself it is running off the cold start injectors, and the new board is not working. A look at the XJS help book indicates I need a second design rotor to go with the new trigger plate. OK, $45 later I have a second design rotor, and still have the start-stall. Today I decided to get to the bottom of it. I pulled the trigger board, verified it had power and ground at the solder joints on the board, and verified it would , trigger the injectors if I tagged the control circuits with the test light, connected to ground. So, no problem yet, why doesn't it run? With the board in one hand I take the rotor in the other hand and rotate it over the board, and the injectors fire and the fuel pump turns on. What the heck?! After a bit more testing, I realize I only have voltage to the coil when in the start position, it goes away in the run position. It seems that unplugging the ignition resistor and plugging it back in was too much for the resistor. My problem had nothing to do with the new trigger board, my ignition resistor died in the process of installing it. I jumpered past the resistor and the engine is running again. I have not had it on the road yet, I certainly hope my running problem is resolved. I still have to install the new ignition coil resistor, $6 at my local car quest store. I guess the moral of this story, is, as my first year gasoline engines instructor told me back in 1972, "Test, don't guess!"

 

Bugga.

Those reed switch boards were flaky at best, so the hall board is a more reliable item.

The resistor going AWOL at the same time is just plain scary. Ours killed the multi resistor block a very long time ago. Nothing available (pre internet days), so Crane XR700 was duly fited. Still sweet.

The 3 wire trigger board was swapped out at the same time, and I fitted an ignition relay to supply the 12v to that 4th board wire, just to be sure.

Good find though at the end of the day.