1979 Triumph Bonneville T140E Wiring

Reader Contribution by Keith Fellenstein
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Bonneville wiring

Q: I have recently acquired a pristine original 1979 Triumph Bonneville T140E 750. It has been in storage since 1981. I am having difficulty getting fire to the plugs. This bike has a Lucas pointless system and I haven’t been able to find a manual that provides info for this particular system. I’m relatively certain the ignition module is shot as there is no fire at the spark plug. There is power to the module. Without specific info concerning the Lucas components, I cannot be certain what to replace or not. So far, I haven’t found the “E” designation on any of the manuals I’ve researched. It’s my understanding that is the emission compliant version for the model years 1978 through 1983, hence the electronic ignition. –  Dale Lawless/via email

A: Your T140E is indeed the emissions compliant version of the 750 twin. It would have the Lucas Rita electronic ignition, which has been out of production for many years. If it has failed you have a variety of options for replacement, including Sparx, Pazon, Boyer and Tri-Spark, to name a few. But before you replace the system, let’s run through a few tests to see if there’s a simple fix. We’ll start with the trigger, found in the cavity on the right side of the engine where the points would be on a standard ignition. You probably have the type used with the AB11 amplifier. Disconnect the leads to the trigger and using a volt ohmeter (VOM) set to read ohms, measure the resistance across the leads to the trigger. It should be in the range of 600 to 700 ohms. Reconnect the trigger. Next, remove a plug or attach a spare spark plug to one of the high tension leads. Lay the plug on the cylinder head. Turn the ignition key on and off, this should produce a spark at the plug. Next, check for voltage at the left coil by switching the volt ohmmeter to DC volts. Place the positive probe on the positive terminal of the left coil, and ground the negative lead of the VOM to frame ground. With the key on, you should read 12 volts on the VOM. Move the positive lead to the negative terminal of the left coil and check the VOM again. Your voltage will drop slightly as you move from coil to coil, but an abrupt drop to 0 will indicate a bad coil. Move to the positive terminal of the right coil and check the VOM. This and the last reading from the negative terminal of the left coil should be the same as the coils are connected in series, with the negative of the left and the positive of the right directly connected. Move the positive lead to the right coil negative terminal for the last of this test and again check the VOM. Since the coils are connected in series, a failure in one coil will bring the whole circuit down. Finally, switch the ignition off, set the VOM to ohms and check the resistance from the right negative terminal to ground. It should read 0 ohms if the wiring is good. If none of these tests show the fault, the amplifier is probably bad and you’ll have to buy a new electronic ignition. Finally, disconnect the black/white and black/yellow leads that come from the amplifier to the coils and with the VOM set to DC volts, test each lead to ground. If both read 12 volts, the amplifier is bad. MC

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