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I'm installing Autometer 5" Monster Tach in my 240Z. As some of you may know, the stock Tach runs off of positive pulses from the coil. Whereas the Autometer runs off negative pulses. OK, that's easy enough. Problem is that the stock tach servesa dual purpose. It not only reads the pulses on the coil, it also is a necessary part of the circuit. Removing the tach prevents the engine from running. So, in my quest for a solution, and after a few minutes of staring at the wiring diagram, I have a few questions. First, it seems quite easy to bypass the stock tach and just run a wire directly from the ballast resistor to the positive coil terminal. Then, while I'm at it, why not just remove the ballast resistor altogether? But, before I attempt this (I haven't had the time yet), I wanted to get some opinions as to whether or not this is a good/bad idea and why. What's the purpose of the ballast resistor anyway?
 

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I believe you need the ballast resistor for a distributor with points. I don't think you need one with an electronic distributor.
Mike
 

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Ignoring all the tacho text I can answer the basic question of what the ballast resistor does...

The coil actually contains a coil of wire that sets up a magnetic field, opening points collapse the field inducing a high energy output on the HT (high tension) lead which gets routed to the relevant spark plug by the rotor/distributor-cap combination... (something like that anyway).

What's important is that (intermittently) a current flows through the coil between the positive and negative terminals. The more current that flows, the greater the spark produced when the field collapses. But additionally, the more current that flows, the greater the heat produced, the greater the battery drain etc. up to a point where the coil will "melt". From Ohms' law:

Current = Voltage/Resistance

We can equate current to "spark":

Spark = Voltage/Resistance

Thus a greater spark can be produced by decreasing the resistance of the coil, or increasing the voltage provided.

Coils come in two types called either "internal resistor" or "external resistor". Internal resistor coils are run without an external ballast resistor and are optimised to product a decent spark at the typical 13.8 volts of the average (running) car.

However, when starting a car the starter motor is a huge drain on the battery and the available voltage from the battery drops a few volts (notice how the headlights get dimmer when you engage the starter motor?). Less voltage equals less spark, and just when its' needed most to fire up a cold engine.

To compensate for this, external resistor models have lower internal resistance and use an external ballast resistor in series with the coil when running normally. However, when the starter motor is engaged, a seperate positive wire (usually coming straight from the starter motor or ignition switch) bypasses the ballast resistor and directly feeds the positive coil terminal. Thus a normal, or even stronger spark can be produced when cranking the engine because the resistance is reduced to match the lower voltage (specifically, the ballast resistor is bypassed when cranking the motor).

If you ever find that your motor will fire when your turning it over, but dies as soon as you release the starter then you'll now know about the two alternate sources of power into the coil.

Brendan.
 

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RE: Ballast Resistor..is necesary

Yup, you'll burn up something without it and I've never heard of a tach that replaces it. It does need to dissapate a bit of heat. I can't imagine that in the tach, but I don't know everythin'
 
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