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Do I need a Ballast Resistor in my ignition system?


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Following the problems with "Jessie's" electronic ignition, I now have a replacement unit to fit, so should be back on the road shortly. That system is 24 volt, and was supplied with a ballast resistor, and the electronics are inside a wartime Auto-lite distributor. Previously it had the sealed Hotchkiss unit, and I don't know what went on in that big metal case!

 

Now I have just got the Ford GPW on the road, and that is a traditional 12 volt points/condenser ignition system. I don't currently have a resistor fitted. I've read the reasoning behind them; to protect the ignition system once the starter load has ceased, and you get a surge of power into the ignition. Am I more likely to burn out points and condensers without one, or is it not necessary?

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My electronic ignition failure has no connection to my question about my points ignition Jeep requiring a ballast resistor or not. It was only mentioned because that system is fitted with a ballast resistor.

 

The 24v electronic ignition unit failed recently after only 1700 miles running, and the unit being only 3 years 9 months old. Despite being out of warranty, the manufacturer offered a replacement unit.

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  • 2 weeks later...

A slight aside but I recently managed to blow the optronic module in my CVRT by bypassing the ballast resistor on the starting circuit (ie putting 24 volts through it when the starter button is pushed). This was the original set up pre the leccy ignition and was modded when the optronic set up was fitted so that the feed always goes through the ballast resistor. I had used a 1973 wiring diagram - doh!

 

A very experienced cvrt person told me this was the first module failure he'd seen in 20 years...

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Thanks. I could see why it would be more important for electronic ignition, where the components may be susceptible to damage from a surge, but points seems so basic I didn't think it would need it.

 

Well, presumably, dealing with surges is the job of the condensor. The condensor shouldn't care where the surge comes from, coil or starter. Saying that, you might need some kind of ballast resistor if you notice any interference with people's radios.

 

trevor

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Now I have just got the Ford GPW on the road, and that is a traditional 12 volt points/condenser ignition system. I don't currently have a resistor fitted. I've read the reasoning behind them; to protect the ignition system once the starter load has ceased, and you get a surge of power into the ignition. Am I more likely to burn out points and condensers without one, or is it not necessary?

 

Usually the resistance of the ballast resistor is equal to the resistance of the coil. In a 12v system the total resistance of the two in series is about 3 ohms. This allows about 4 amps to be switched at the contact breaker, this is the practical limit of reliability for a mechanical switch like this.

 

(A switching transistor in an electronic ignition system can switch much higher currents & this allows a coil to have a lower resistance, carry more current & designed to give more HT from the additional magnetic field produced)

 

 

A ballast resistor has two functions.

 

1. During start up the ballast resistor is short circuit & applies the full battery volts to the coil, although this may be a bit lower due to the drain of the starter motor. Depends how good the battery is. Prolonged hanging on the starter button (more than 10 secs) can overheat the coil. (A feature not actually utilised on Land Rovers with a ballast resistor)

 

2. To boost the HT output. This can be in high performance vehicles but in MVs particularly in screened ignition systems operating on 24v. Adding resistance to the primary circuit offsets some of the effect of inductance of the coil. It has the effect of reducing the time constant of the circuit. The result is that at high revs the engine can perform better by a more rapid collapse of the magnetic field in the coil when the contact breaker opens. This is used in screened systems to try to compensate for the HT that is lost by trying to 'charge up' the capacitance in the screened cables.

 

So in summary if you vehicle is 12v & the resistance of the coil is about 3 ohms you don't need a ballast resistor.

 

Capacitors (condensers)

As these have been brought into the discussion it is important to differentiate between the two types that may be found around an ignition system.

 

There will often be a capacitor in the feed to the coil (SW) this might be rated a bit above the normal vehicle voltage say 25v. Its purpose is to try to dampen down some of the interference from the contact breaker from entering the vehicle wiring system. It is of limited effect & in FFR vehicles a more sophisticated filter is often built into the ballast resistor enclosure.

 

The important capacitor is the one in or connected to the distributor this is rated at about 500v, as the back EMF in the primary winding when the contact breaker opens is around 300v. The small spark at the contact breaker is the effect of this voltage. Without it the points will wear more rapidly because the greater spark occurring when the coil is switched off will be less abrupt there will be a reduced HT output. The capacitor can have the effect of improving HT output by 30% compared with no capacitor.

 

Capacitors rarely fail outright but the insulation will deteriorate with time even if unused. The only way to get a reliable assessment of capacitor performance is to test it at its rated voltage. Trying test with a multimeter might only weed out a few absolutely awful capacitors, it will not distinguish poorly performing capacitors that are contributing to poor engine performance.

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