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8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc

Can anybody confirm what this relay does in a 408, please?

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Old 27-02-21, 11:00 PM
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Default Can anybody confirm what this relay does in a 408, please?

My 408 has, amongst other changes, an alternator in place of the original dynamo/generator. From the wiring diagram, it seems there were originally 3 relays in the electrical system - horn relay, cooling fan relay and something called 'ballast relay'. My car also has an electronic distributor. Does that render the 'ballast relay' unnecessary and is the 'ballast relay' the one in the righthand wing bay with the electrical distribution board and fuses. You can see there is one Lucas relay in there in this photo. The modern plastic relay and associated wiring on the silver backing board are replacements for the original cooling fan relay which has been removed. Also, is the empty space above the Lucas relay and fuse box the location of the control box/regulator that was needed for the dynamo? Thanks for any help.

David
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Last edited by dwomby; 27-02-21 at 11:23 PM.
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Old 28-02-21, 03:18 PM
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I can't answer all of your questions, but I can tell you what the ballast relay is/was for in older cars that I am familiar with. In older cars with an ignition coil and a distributor with points it is necessary to have 12 volts to the coil for starting and 9 volts to the coil for running. There are therefore 2 circuits to the coil from the battery, one of which incorporates a resistor (the ballast resistor) to reduce the voltage to 9 volts. The ballast relay switches between these 2 circuits. The relay is sometimes incorporated into the starter motor solenoid circuits when these are part of the starter.

If the coil is allowed to run on 12 volts it will sometimes overheat and the ignition system will not function and you have to sit for about 45 minutes before the car will run again (my experience).

I'm not familiar with the modern technology and so can't tell you if these circuits are still required.
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Old 28-02-21, 03:24 PM
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Oh, I looked at your picture last after posting. Right below and in parallel with the terminal strips is a tan ceramic looking block which appears to be a ballast resistor (it looks similar to ones I am familiar with).
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Old 28-02-21, 03:36 PM
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Quote:
Originally Posted by John Simmons View Post
I can't answer all of your questions, but I can tell you what the ballast relay is/was for in older cars that I am familiar with. In older cars with an ignition coil and a distributor with points it is necessary to have 12 volts to the coil for starting and 9 volts to the coil for running. There are therefore 2 circuits to the coil from the battery, one of which incorporates a resistor (the ballast resistor) to reduce the voltage to 9 volts. The ballast relay switches between these 2 circuits. The relay is sometimes incorporated into the starter motor solenoid circuits when these are part of the starter.
>
>
Quote:
Originally Posted by John Simmons View Post
Oh, I looked at your picture last after posting. Right below and in parallel with the terminal strips is a tan ceramic looking block which appears to be a ballast resistor (it looks similar to ones I am familiar with).
John, thank you for the very helpful replies. I never knew about older cars using different voltages across the coil but that makes sense. Much appreciated.

David
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Old 28-02-21, 06:23 PM
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I recently went from points to an electronic distributor and the one I bought https://www.summitracing.com/int/par.../make/chrysler
does require a ballast resistor.
Hope this is of some help.
Andrew
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Old 28-02-21, 07:46 PM
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Conventional uk practise amongst better class cars of the period, including Bristols, was to use a ballast resistor in series with the primary circuit of the coil. So the coil was normally running at less than 12 volts. The ballast relay was short circuited by the ballast relay when the starter solenoid was activated to deliver all volts available in the interests of a bigger and better spark to get the lump turning over. I say all volts available as starter motors of the period placed a tremendous load on the battery and the cabling to the starter, resulting in a significant voltage drop to the coil whilst the starter motor is activated. (If you look carefully at your Bristol you will note that the main feed to all circuits from the battery is actually wired off the starter solenoid - why they did that the lord only knows)

The ballast relay is indeed the tan ceramic looking block that John Simmons points out.

Not that this applies to Bristols but I noted a Chrysler maintenance film on Utube not that many months ago that indicated that Chrysler practise was to have a ballast resistor in series with the coil at all times, so no ballast resistor or relay. This in the interests of reducing arcing across the points and prolonging point life. I donít claim to understand the principles involved, but thereís another reason for having one, for those who stick with points.

Another plus for a ballast resistor is that if running on points the coil is much less likely to overheat if you inadvertently leave the ignition on whilst the engine isnít running. I have noticed that the 410s coil barely gets warm in this circumstance whilst the Morris vanís (a great fun car, but not one that one could call better class) gets quite uncomfortable to touch.

I canít actually swear that the original regulator box for the dynamo on a 408 was fitted above the fuse box but thatís where the alternator control box was fitted by Bristols on the 410 so it seems more than likely.

Roger
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Old 28-02-21, 08:06 PM
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Thanks, Roger. I will be investigating to see if, in fact, the ballast resistor is still in use on my car. More poking around with the multimeter and circuit tester!

David
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