[dave-r]

BALLAST RESISTORS
A ballast resistor is used in series between the battery and the coil primary winding in order to reduce the current in the coil during normal running. They have a low resistance when cold and a higher resistance when hot. This means that the coil current is high for running when cold and then falls off to a level where it does not overload the points. A motor that runs very rough with about 1000 miles on the points, especially when hot, but then runs OK for 1000 miles on new points, could be missing the ballast resistor. Without the resistor the coil will also run hotter and its life will be shortened as a result.

There is usually a by-pass system built into the starter circuit which shorts out the resistor for initial start-up.

G.M. use a by-pass circuit in the solenoid on the starter motor. Ford use a separate starter relay.

Chrysler use a circuit in the ignition switch.

Which ever system is used, they all momentarily connect full battery voltage to the wire between the resistor and the positive terminal of the coil. This connection can be at the resistor or the coil.

Since the early '60s, G.M. has used a stainless steel wire that looks like a normal wire, instead of the ceramic encased wire wound resistor used by Ford and Chrysler. If this steel wire has been cut or spliced into at any time, then it is ruined and must be replaced with a complete steel wire or a normal wire with a universal ceramic resistor spliced in. Wires connecting the resistor should be at least 16-gauge.

The exception to all this is the General Motors HEI units which have a special current limiter circuit inside their amplifier module. So in the HEI system, there is no ballast resistor.

By MoparKev (Kev) on Thursday, March 01, 2001 - 02:15 pm:


Dave, Could you give us some Ohmic values and their applications, IE Standard electronic ig standard coil, hi power coil, MSD Etc.

By David Robson (Dave) on Thursday, March 01, 2001 - 02:32 pm:


I think I have something somewhere. I will look them up tonight.

I am not an autoelectrician. I am just pretending.

By MoparKev (Kev) on Thursday, March 01, 2001 - 02:45 pm:


You are doing a fine job!

By David Robson (Dave) on Friday, March 02, 2001 - 11:12 am:


Kev!
I have decided to put in a full ignition fault finding bit when I have finished doing background stuff on the components. This will include expected voltages and ohm readings at various points. Can you wait for that or do you need something now?

I will cover everything including plugs over the next week or so.

By David Robson (Dave) on Friday, March 02, 2001 - 02:44 pm:


Changed my mind. I am adding fault finding as I go. Can't lay my hands on ballast resistance values just yet but will post them here when I find out more. I might have to add a bit about dual resistors etc if I can work out what that is all about!

I do know that the primary of the coil can run higher currents with an electronic ignition because you have no worries about causing the contact points to burn or pit. So the ballast resistor is usualy smaller in electronic ignition systems. I think. More later when I have scratched my head.

[dave-r]

UPDATE.

From a book I found;

Dual Ballast Resistor.

Compensating resistance = 0.5-0.6 ohms @ 70-80 deg F.

Auxiliary Ballast = 4.75-5.75 ohms @ 70-80 deg F.

[Tim Ellison (Tim)]

Got me Dodge back from the local autoelectricians on Monday night. Amongst other things, they sorted out the ignition wiring, which was suffering from an accumulation of years of 'fixes'. Whilst sorting through the routing, they found that the ballast resistor had been disconnected. After checking with me, they re- connected it.

When I collected the car, it fired first time and ran at smooth idle. However, as soon as I bought the RPM up above 1800, it started missing very badly, and got worse as the RPM went up. By the time I got home (very slowly)and the engine was hot, it ran smooth and strong through the RPM range. I left it about two hours to cool off, fired it up, and it started missing again above 1800 RPM.

The car's back in on Saturday to try and correct the problem, but I'm trying to narrow the fault down, to make sure it gets fixed.

Does anyone think re- connecting the ballast resistor could be causing the problem? The car is fitted with a Pertronix Flamethrower coil, which had been running fine since I fitted it about two months ago.

Any ideas?

[Dave-R (Roppa440)]

Almost without exception most ignition coils are designed to run at less than 12 volts which is why you have to have a resistor in series with the primary winding. You should only have full 12 volts on the coil during cranking. You can do without the resistor but the coil will overheat and heat makes the output votlage DROP. It can also lead to the coil failing.

So you need the resistor and unless there is something wrong with it your problem is elsewhere.

[Tim Ellison (Tim)]

Dave,

Disconnected the ballast resistor, and you're right, it didn't make any difference. The car's still displaying the same symptoms (runs like sh~* when cold, then smooths out as it warms up). I got the Company in question to re- wire me Tach/ shiftlite, so that the wiring matched the diagram that came with it. It's now running from the coil, which it wasn't before. Before the re- wire, it worked fine. Now, the shiftlite doesn't operate at the pre- set limit. I'm wondering if it's causing feedback into the ignition system.

The other possibility is that they've been revving the snot out of the engine, and knackered something mechanical (maybe something in the carb). I'm trying to ignore that though at the moment.

[Henry (Henry)]

Normally a coil would break down as it got hotter. But knowing my luck this one will prove me wrong.

I wonder if some damp has got into the distributor, that could cause a miss when it's cold. I would try this first as it's an easy thing to deal with. I tend to replace dist caps if they look a bit suss, as it is a good preventative measure.

[Tim Ellison (Tim)]

Henry,

I did wonder if it was damp in the dizzy/ plugs, as the car had been out in several good showers before I picked it up. I dismissed it at the time, 'cos there was no sign of water ingress on my nice shiny air cleaner.

On second thoughts though, I'm going to check it tonight. The symptoms do fit.

Thanks for the tip.

[Hans (72challenger)]

Talking about ballast resistors, right now I still have the dual resistor in it (I gave up my chrome ignition box troubles) but also have a single resistor laying around. Could I install the single one in a stock ignition system with a orange box and MSD blaster 2 coil without problems. I know it will run OK, but what in the longer time...

[Dave_R (Roppa440)]

Probably OK. You don't want a full 12 volts over the coil primary when the engine is warmed up otherwise it will get hot and performance will drop off (hot wires do not conduct well). But you don't want too small a voltage either. I am sure the coil must be designed to run with a particular rated resistor. Try contacting MSD and ask them.