240 General: Ballast resistor: 1984 GLT

[davidcheshire]

Anyone have thoughts about replacing a 33-year old ignition ballast resistor?

It is showing a higher resistance than it should, apparently, but whether a replacement is crucial is the question.

Genuine Bosch spares are available - between £50 and £60 - and alternatives are about the same.

The VehiclePartsDatabase has a picture of it here:
http://www.vehiclepartsdatabase.com/...sch/0227900010

Thanks in anticipation.

[Stephen Edwin]

I have no idea .. but ... to help more knowledgeable people?

What is the context? e.g. why was the resistor tested? What is the reading? Is there a problem with the car?


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[rtbcomp]

If you know what resistance it should be and how much current it needs to handle than you can calculate the wattage it has to dissipate. W = (I^2)*R

From that get a ceramic or wire-wound resistor from an electronics supply company (RS, Farnells, CPC, eBay &c&c).

You may need to make some sort of bracket/heatsink.

[Michael J]

How far off spec. is it? What should correct spec. actually be? Inquiring minds, 'n'all... Higher resistance is better than lower, it will help to prevent the coil from burning out even more than correct spec. would. As long as the ballast hasn't failed and you still have good sparks then you're golden, imo.

[davidcheshire]

Well, as almost everything else electrical under the bonnet of my 33-year old car has had to be replaced this year, I wondered whether the ballast resistor might also.

Having done a little more research, it may be OK. The Volvo Manual requires an ambient temperature of 20 degrees C to check the resistance, which should be in the range 0.8-1.0 Ohm. I've had a measurement of 1.45 in a warm engine compartment, but have also read that the 'resting' resistance of the multimeter should be deducted, which was 0.45 and would make the resistor seem just right. The 'resting' resistance is the measurement shown when the two probes of the multimeter are placed one on the other.

How accurate the 'resting' resistance was on this particular occasion is difficult to say. It varies. Only another test next summer will tell, if we can manage 20C here in Aberdeen - it is possible, because it happened about four times in 2016!

My conclusion at the moment is that the resistor is probably doing its job without impairment, and next summer may confirm it.

Thanks for the input from those who have.

[rtbcomp]

Doesn't your multimeter have a zero knob? Short the probes and adjust the knob so the needle reads zero ohms.

Come to think of it, if you have a digital meter then it may not have such a knob.

[davidcheshire]

Thanks RTB.

It's digital, and no zero facility.

[B00TS]

The precise value of the resistor isn't crucial really so long as it's around the 1 ohm mark and you are getting strong sparks at the plugs under all conditions. (No sign of misfire under heavy load or in the wet.)

The resistance element itself does age extremely slowly but will easily outlast the engine.

Much more important from a reliability point of view is the physical condition of the ceramic case and the terminals. If the case is cracked or the terminals severely corroded, it might be worth considering replacing.

If the case and terminals look ok, I'd let sleeping dogs lie!

One way to have peace of mind would be to add some short lengths of wire and insulated crocodile clips to this: http://uk.farnell.com/cgs-te-connect...0-5/dp/1259460 and keep it in the car. If the ballast resistor were to fail open circuit (meaning no sparks) you could clip the temporary one across the terminals until you got a replacement.

The symptoms when the ballast resistor has failed are that the engine will fire while the starter motor is engaged but die as soon as you release the key.