Greetings, i was bored today because it was 15F outside and in the garage but I was curious to check the health of my Ballast Resistor. See photos. It appears with wires removed and multimeter set to 200 on the Ohms dial it was reading 2.2. This is a reading at 15F. Opinions appreciated.
View attachment 1987587
View attachment 1987588
As mentioned, test lead resistance or poor probe connection at the terminal is adding to the ballast resistor reading. The "830" based Cen-Tech meter (part number 90899, or similar) in the picture, has adequate measuring ability for a ballast resistor measurement, but the test leads, and their connection to the meter jacks can be problematic high resistance points.
Temperature has an effect on the resistance, and should be measured at 70-80 degrees Fahrenheit (21-27 degrees Celsius). The resistance is lower at lower temperatures than the nominal rating, and increases as the resistor heats up.
The following might be helpful:
Measuring a Ballast Resistor
The following procedures show the basic method of measuring a low resistance value ballast resistor with a digital multimeter (DMM) or analog volt-ohm-milliameter (VOM) using a two wire measurement method.
The ballast resistor being measured is a Chrysler part number 2095501, which was used on 1960 - 1972 point ignition systems. The specification nominal value of this resistor is 0.55 ohm, with an acceptable resistance range of 0.5 - 0.6 ohm at 70-80 degrees Fahrenheit (21-27 degrees Celsius). This resistor is usually referenced as 0.5 ohm, and original Chrysler resistors in good condition typically measure closer to 0.5 ohm than the nominal value.
When measuring low resistances, including ballast resistors, account for the resistance of the test leads connected to the measuring device. This can be done by shorting the leads together, noting the resistance, and subtracting it from the measurement readings, or use the meter's relative setting (if equipped) similarly with the lead shorting, or use a meter that measures conductance and convert accordingly, or use a four wire ohms measurement setup with the appropriate device. An analog meter can also be used but resolution might be lacking on the lower resistance. The same test lead considerations hold true. Also because the analog meter has to drive the needle, a weak battery can hamper the measurement.
The following images show a Fluke 27 DMM and a Simpson 260 Series 7PM VOM being used to measure the Chrysler 2095501 resistor.
Test conditions: Temperature = 72 degrees Fahrenheit, Relative Humidity = 30%.
Resistor to be measured:
Checking with a Digital Meter
Checking test lead resistance:
Ensure that all connections are clean and tight, and the test probe tips are making good contact.
NOTE: If lead resistance is over 1 ohm, the test leads should be replaced with better leads.
Set the meter on the lowest resistance scale or allow autoranging if the meter supports it.
Any number measured here is to be subtracted from subsequent measurements, or the meter zeroed to remove it. This meter has a relative function, “REL”, which can be used to “zero” the test leads so subtraction is not required. In this case, my lead resistance is lower than the resolution of the meter (< 0.1 ohm), so no further zero or tracking of the lead resistance is needed.
Measuring the resistance:
Ensure that all connections are clean and tight, and the test probe tips are making good contact.
In this case, the resistor is measuring 0.5 ohm, and there is no test lead resistance for which to account.
Checking with an Analog Meter
Checking or Setting Meter Zero:
Analog meters must have the mechanical zero checked and set depending on the meter and its position.
Setting mechanical zero:
Position the meter as it will be used for measurements. Test leads are not used for this check/setting.
Ensure the meter needle is on the zero mark at the bottom of the scale. If needed, use the mechanical zero adjustment to center the needle at the bottom of the scale.
Checking and zeroing test lead resistance:
Ensure the VOM has a good battery. The meter has to drive the test current through the resistor being tested as well as drive the needle. A weak battery might not allow the meter to electrically zero, leading to erroneous readings.
Ensure that all connections are clean and tight, and the test probe tips are making good contact.
NOTE: If lead resistance cannot be zeroed and the battery is good, the test leads should be replaced with better leads.
Set the meter on the lowest resistance range. In this case, “Rx1”.
Use the “ohms adjust” or “zero adjust” to electrically zero the meter needle at zero with the leads connected together.
Measuring the resistance:
Ensure that all connections are clean and tight, and the test probe tips are making good contact.
With the meter still on the lowest resistance range as previously set, measure the resistance.
In this case, the resistor is measuring 0.5 ohm, and there is no test lead resistance for which to account due to the previous zeroing of the meter.
NOTE: The resolution at the far end of the scale is not as good as the digital meter in this case with this meter, but is usable. If using a milliohmmeter, with extended range, a better measurement is possible.
Optional Checking of the Meter
Checking the Meter with a Higher Standard:
If there is a question of meter accuracy, and if the equipment is available, check the meter against a known standard. In this case, I am double checking my meter at 1.00 ohm as verification, using my General Radio 1433M Decade Resistance:
In the absence of a higher standard, the meter can still be cross checked measuring another known resistor or comparison to another known good meter.
With more resolution, the more precise value of the ballast resistor measured here is 0.5277 ohms as measured in my lab using a four-wire method, as shown in the following image:
Excerpted from “Measuring Ballast Resistors”, Gary Lewallen, aka. Vaanth
For more information about Chrysler’s electronic ignition, including ballast resistors, and their use with other components, refer to
Chrysler Electronic Ignition
