HOW TO - Test ballast resistors

[glennr]

I have tried to find out what a ballast resistor actually does. There must be a few of us on here who are not electrically astute. Anyway, here is a good explanation Basically it reduces the current to the component which in turn doesn't wear the component out with an excessive charge that is not required. I hope I got that right gentlemen

[Warwick]

Not quite I'm afraid Glenn, but a valuable bit of testing information to post here, nonetheless.
The video clip shows clearly what the ballast resistor wire does; but not why. You're spot on with the observation that it protects the coil from voltage that is too high - but the reason it can be too high is because they deliberately use a coil whose voltage rating is low.

Glenn, perhaps you could edit the subject of your post and change it to HOW TO - Test ballast resistors
Phil or Admin could then make it a sticky.

A car battery is nominally a 12 volt battery. It can be as high as 14.4 volts while charging, but the voltage drops away when charging stops. When the battery is delivering power to a device, the battery voltage drops. The more power the device consumes, the more the voltage drops. The smaller the battery, the greater the drain on it so the more the voltage drops. A big battery suffers less from voltage drop when large loads are placed on it.

Unless you have a massive doof-doof sound system or your P5 is festooned with searchlights, your starter motor will be the biggest electricity-drawing component in the car; by a wide margin. When you turn the key and the starter begins cranking, the battery voltage drops.

The problem is, all the car's electical equipment is designed to run on 12 volts and they don't work very well on 9 or 10 volts, which is all that may be available when the starter motor starts sucking current. If you start a car with the headlights on, you will notice that they dim as the starter motor causes the battery voltage to drop. (Headlights are also a large consumer that cause voltage drop - but nowhere near as much as the starter).

The ignition system was originally designed to run on 12 volts, for a 12 volt car. It doesn't work as well on a lower voltage so if the battery voltage drops, the spark is weakened and the engine is harder to start.

You can install a much larger battery and overcome the problem. Bigger battery - reduced voltage drop when cranking - and the ignition system still get approximately 12 volts.

Or you can design the ignition system to operate on say 9 volts so that when you turn the key and operate the starter, and the battery voltage drops, the coil still gets around 9 volts. Perfect! Not quite.

Unfortunately, when the starter isn't cranking, your 9 volt coil is going to get 12 volts or up to 14 volts when the battery is being charged. This will cook the coil.

The answer is to fit a resistor between the coil and the battery. This, as its name suggests, introduces some resistance to the flow of electricity and causes a voltage drop. Choose the right resistor and 12 volts is reduced to 9 volts.

So now all you need to do is to by-pass the resistor when the starter is cranking, so you get battery voltage (lower than usual) through to the coil, and then switch over to include the resistor when battery voltage returns to normal when you release the key from the start position.

This was all nice and simple and easy to see until some clever d i c k thought it was a good idea to use a wire with a certain resistance built-in over its length, instead of a resistor attached to the coil terminal. Then this new "resistor" disappeared from sight into the wiring loom so as to confuse everyone.

They probably chose the word "ballast" because of the similarity (if you stretch your imagination) with adding ballast to say a ship, in order to keep it steady when something else is removed.


Electricity generally ...

Those of you who have trouble understanding electricity, think of it as water in a header tank on a stand beside your house. Imagine your water comes from this tank and not from the town mains. As if you were on a farm perhaps.

The tank is the car's battery. The house is the car. The pipes are the wires.
To fill the bath takes a lot of water. If you have larger pipes the bath will fill faster, because the pipe allows more flow or current.
If you want to fill it very quickly you will need a very large pipe, but if you try to fill a glass of water at the same time, at the kitchen sink, you may find that it takes forever because the bath is taking most of the flow and dropping the pressure (voltage) in the system of pipes.

If you also run a big pipe to the kitchen tap, then filling the glass while the bath is filling will be easy again. But when the bath isn't filling, the flow from the kitchen tap will make it impossible to fill your glass without having water everywhere.

Now let's pretend that it isn't possible to adjust the tap. It has to be either on or off. We can solve the glass filling problem by running 2 big pipes and having 2 taps. One tap has a disc jammed in the outlet and the disc has a small hole in it. Now when the bath is filling, you fill your glass from the tap with no restriction and you get good flow despite the lower pressure (voltage) in the pipe. When the bath isn't filling, you fill the glass from the tap with the resistor attached.

Now if you start to have problems with the kitchen plumbing, you can inspect the 2 taps. You can see which one has the resistor. You can even remove it and inspect it. You can check that it still has the right size hole. Nice and simple.

But then along comes clever D i c k (let's call him Joseph from Lucas Plumbing), and he decides to eliminate the need for the restricting disc in the second tap. He replaces one of the big pipes with a small one. Same effect - it restricts the flow. But now you can't tell which tap is which, just by looking at it because the pipes are hidden in the walls.

And while we're using this water analogy, if you need a bigger battery so that it doesn't go flat so easily, you install a bigger water tank.
If you need to increase the voltage (pressure) in the system, you increase the height of the tank stand.

And while on the subject of batteries -
A battery is a collection of cells, just as you can have a battery of guns or a battery of hens.

A car battery consists of a collection of six 2-volt cells. 2 x 6 = 12.
On the older type unsealed batteries, each screw cap services one cell. They are completely separate from each other - separate compartments. They are only connected electrically.

Therefore, technically speaking, a dry cell is not a battery. If your torch (flashlight) takes two D-cells, it contains one 3 volt battery comprising of two 1.5 volt cells.

Lead-acid car batteries have cells that are really 2.1 volts.
An alkaline dry cell is 1.5 volts.
A Ni-Cad (nickel cadmium) cell is 1.2 volts.

If you've ever wondered why they don't standardize on voltage, it's because they can't. The voltage is entirely dependent on the particular chemical reaction that is going on and cannot be changed. The chemicals and materials involved are chosen for their other properties such as electrical storage capacity, current draw-off ability, how long it takes to recharge, how many times it can be recharged, how heavy it is, how much space it occupies, etc. After trying to optimize all of these, the voltage that results just has to be accepted.

[johnwp5bcoupe]

Warwick

[Warwick]

Thanks John.
Please feel free to add your 2 bobs worth. I'm just an amateur - you're the retired electrical chap.


I was probably a bit unkind to Mr. Lucas. After all, he was well and truly dead before his company had anything to do with this sort of stuff - and secondly, Lucas probably weren't the first to use a resistance wire in the loom instead of a small component (resistor) attached to the coil's terminal.

[Paul - P5B Coupe]

Thanks for making time to explain this in simple terms Warwick.
Iit is very helpful for people with little or no knowlecge of electrickery such as myself.
Regards,
Paul.

[glennr]

Yep, cheers Warwick. Very interesting. I changed the title btw

[johnwp5bcoupe]

Just to add there is a shunt from the starter motor which bypasses the balast so under cranking conditions it gets whatever voltage is applied to the starter this is still on my Dec 69 coupe and the connections for the balast are joined together this has been like this since I bought the car in 73 so when it was done? but it looks like a factory job may be one of the changeover models?

[Phil Nottingham]

It would be the changeover period as no shunt is required for the loom wire - it supplies 12v until it has warmed up. Not a good idea long term as the PVC covering (pink) wlil harden over time though which can cause shorts and fires as this circuit is unfused!

I like old fashioned electrics systems with simple things - solid state stuff is far too complicated although I can just about get around and cope the modern resistors/diodes etc that you can pick up and handle individually!

In the UK at least a ballasted coil is not required for good V8 starting - I removed mine years ago with no problems. 3 Litres never had this complication anyway and the later theif proof setup was unballasted too

[eightofthem (Andy)]

Thats the kind of explenation most of us hard heads prefer, clear and simple, easy to understand. Vote warick for president

[Warwick]

There's nothing to fear from electicity - unless of course you have an enquiring finger and you're standing in a puddle.

[adrianp]

Found this searching for starting problems and have to admit, I laughed out loud ! Great explanation of electrics though thankyou

[enigmas]

Nicely explained Warwick. I think you missed your calling.

Warning: Never do any electrical wiring (of the 240 volt kind) when your spouse is trying to engage you in conversation.

[Warwick]

Gawd, this is old. I must've written it back when I was young and knew stuff.

[Deleted]

Vince, someone did in fact call him?

[Warwick]

Sept 1, 2014 13:43:28 GMT enigmas said:

Warning: Never do any electrical wiring (of the 240 volt kind) when your spouse is trying to engage you in conversation.

Vince, I think that rule extends well beyond electrical work.

[bigandy]

ok so im a numbskull
my white lead from ignition to coil fried a few weeks back and since no spark can be found --i chopped the armoured cable off --and the burnt end of the wire and re-connected it to a new coil there is power to coil but nowt from new coil to anywhere else car turns well but no spark i know i may need to look at new lumenition as brain may be fried if so where from ?(new coil supplied by JRW for my car for electronic ignition.)
what makes this lead ballast resisted ?
what does a ballast resister look like ?
why can't i get no spark ?
sob sob sob
a very annoyed
Big andy

[Phil Nottingham]

What year is your car and why did the white wire "Fry".  The wiring does not appear to be standard of which there are 3 versions ie early, mid and late

[bigandy]

Hi Phil 1972
big a

[enigmas]

Hi Andy, very simply put, a ballast resistor drops the voltage down by resisting the flow of current. Think of an electric heater radiator; the wires glow resisting current flow. This causes the radiant heat. Resistors do get hot as a consequence of their task. To check a resistor or wire that may have a resistor in circuit you use a multimeter on the 20v setting and note the reading.

If your car has an electronic module fitted it may be cooked? If the white wire is an ignition wire check it for 12 volts or if it has a ballast, for whatever voltage it's putting out.

If it's all mumbo jumbo to you, you're better off getting an electrically competent mate or auto electrician to sort your issue, especially if electronic ignition is fitted.

[bigandy]

I'm unsure why it fried i think the wire corroded and shorted on the armoured cable but thats a guess ?

[bigandy]

thanks enigmas

i give up and will contact someone --anyone recommend a good notable electrician in north london ?

[Phil Nottingham]

The cable in the armour should be YELLOW.  The armoured cable/ignition wiring is not typical so you must consult the correct wiring diagram as the switch contacts may have burnt out as the armoured feed is a separate terminal.  NO Ballast should be present either for that model year so its likely the wiring has been messed around.  

As suggested an auto electrician needs to look at it