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The voltage at the coil through the resistor on my 62 340 SHP reads 7.75 volts. The spec is 5 to 7 volts. Should I be concerned about the high reading? When I try to measure resistance of the resistor it does not show any reading on my VOM using a 20 Ohm scale. How do you check a resistor to see if it is within spec?
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Re: Resistor volt ohm readings
7.75 volts is fine. To check the resistor, isolate it (disconnect any wires at both ends) and put your leads on the terminals with the VOM set on the 10-ohm scale. My digital is auto-ranging, I just set it to ohms. The correct resistor for a '62 should read about 0.3 ohms ('64-up should show about 1.8 ohms). -
Re: Resistor volt ohm readings
You need a good digital voltmeter to read the ballast on '62 engines, which is why the voltage at the coil is higher than "spec". The spec was for the 1.8 ohm ballast. Yours should read 0.3 ohms.
All early '63 engines used the 0.3 ohm ballast, but a running change swapped to the 1.8 ohm ballast for 250/300 HP only. SHP/FI continued to use the 0.3 ohm ballast through '64.
The above is documented in the AIM and a TSB. The JG may say otherwise.
The 0.3 ohm ballast offers greater ignition energy due to the higher primary current, but is tough on points, particularly in cold weather as the ballast has lower resistance at lower temperature. The 0.3 and 1.8 ohm specs are at room temperture.
The TSB suggested using a 1.8 ohm ballast in the winter only. I wonder how many owners swapped between a "winter" and "summer" ballast. Yeah, right!
DukeComment
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Re: Resistor volt ohm readings addendum
The 0.3 ohm ballast was paired with the 091 coil, and the 1.8 ohm ballast was paired with the 087 coil.
DukeComment
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Re: Resistor volt ohm readings
Measuring a high performance (0.3 Ohm) ballast can be pretty tricky unless you've got the right kind of ohm meter. You're working down near zero ohms to start with and the intrinsic resistance in meter's probe wires can generate SIGNIFICANT meter reading error(s)...
A better way to verify resistance is to connect the ballast to a LOW voltage power source (say 3 VDC) OR use 12 VDC but put another, higher resistance load in series with the ballast. Now, measure the branch circuit current that flows and switch the meter to voltage mode and measure the voltage drop across the ballast.
Using Kirchoff's Law (V=IR), calculate the ballast's resistance via R= V/I. That way you're not pushing the precision/resolution of your hand held ohm meter...Comment
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Re: Resistor volt ohm readings
If you have the 1.8 resistor what happens? Will it make the car start harder?
Thanks
TerryComment
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Re: Resistor volt ohm readings
You can't tell a difference. Regarding starting - during cranking the ballast is bypassed.
DukeComment
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Re: Resistor volt ohm readings
Duke
Thanks, then what is the advantage of having the right one?
TerryComment
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Re: Resistor volt ohm readings
Like I said the 0.3 ohm ballast provides more ignition energy due to higher primary current, but the higher primary current is tough on points. The higher ignition energy should theoretically allow the plug to fire with greater fouling, but with today's unleaded fuel, fouling is less, so if you're experiencing short point life, switching to the 1.8 ohm ballast should improve the situation.
If one set of points burns on a dual point system, the engine will still run. If it's a single point system the car is dead - happened on my SWC once. That's why Chevrolet decided to use the 1.8 ohm ballast on the lower performance engines as a running change in '63, but leave the 0.3 ohm on SHP/FI, which had more tendency to foul plugs. It was an engineering judgment call.
DukeComment
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Re: Resistor volt ohm readings
Duke
Thanks again for the easy to understand information.
TerryComment
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