1988 V12 XJS
Curious, when I turn on ignition and put test leads to + coil and - intake manifold ground, I get 12.5 V. But if I put it to + coil - coil, I only get 10.5V.
I’ve tested my ign amp module, and it’s ground. All ok.
Is this normal? If so, why wouldn’t the coil have 12.5V?
Car starts up fine.
Not sure I’m reading you right, Greg. Your first measurement shows that the coil + is connected to full battery voltage (battery negative connected to chassis, which includes intake manifold).
Your second measurement gets 10.5V across the coil itself. The reason: its - terminal is not grounded directly but via the ignition amplifier and or a series resistor. That (partial) grounding is interrupted 6X per crank revolution when the engine is running to fire the spark.
IMHO; I might be misunderstanding your question.
You answered my question.
I thought the coil needed 12+ volts to make a good spark. So the amp is controlling the ground, and sitting with ignition on but not cranking, it’s not allowing full voltage yet, but will.
I tried measuring voltage across coil while running, but my meter was all over the place. For a split sec, I did see 14 volts
Hahaha! Yes, it’s a dynamic voltage waveform…a transistor in the amp (or contact points back in the day) switches the coil on and off so that each plug gets a spark (on coil “off” once every two revolutions–that’s 6 on/off transitions per rev. To observe it you need an ignition oscilloscope.
The coil works fine with less than 12V when “on.” In fact most of them, particularly the new ones made for electronic ignitions, would overheat if they got that continuously.
If you had an old-fashioned analog voltmeter you would at least get an average reading (called a dwell meter in the day) but these new-fangled digital meters are confusing and often usless IMO.