1946 GMC EF350 Ignition Coil: A Guide to Replacement and Diagnosis
Keep your classic GMC's 228 inline-six firing reliably with the right ignition coil information.
- The ignition coil is essential for your 1946 GMC's 6-volt system, but symptoms of failure can also point to a bad condenser or poor ground connections.
- Always check the simple, inexpensive parts first, like the condenser and wire connections, before replacing the coil.
- Replacement is very easy for a DIYer with basic tools and can be done in under 30 minutes.
- A modern aftermarket coil from a reputable brand like Standard Ignition is a reliable and recommended choice for this vintage vehicle.
Is Your Ignition Coil Failing? Common Symptoms
On a classic truck like the 1946 GMC EF350, the ignition system is straightforward. A failing coil will usually give you clear signs. Look for these common symptoms:
- No Start or Hard Starting: The engine cranks but won't fire up, or takes a very long time to catch. This can happen if the coil is completely dead and producing no spark.
- Engine Misfires: The truck may run rough, shake, or sound like it's sputtering or coughing. This happens when the coil provides a weak or inconsistent spark, so not all cylinders are firing properly.
- Backfiring: A loud bang from the exhaust can occur when unburned fuel from a misfiring cylinder ignites in the hot exhaust pipe.
- Sudden Stalling: The engine may die unexpectedly while driving or idling if the coil overheats and fails, then may start again after it cools down.
- Weak Spark: If you test the spark and it looks thin and yellow or orange instead of bright blue, the coil is likely weak.
Before You Replace the Coil
On these 6-volt systems, other cheaper parts can mimic a bad coil. Before spending money, check these first:
- The Condenser: Located inside the distributor, a bad condenser is a very common failure and causes the exact same symptoms as a bad coil. It's a good idea to replace it first.
- Grounds and Connections: A rusty coil bracket or loose, corroded wires can prevent the coil from working. Ensure all electrical connections are clean and tight, and that the distributor and coil have a good ground to the engine block.
Choosing a New Aftermarket Coil
For a vehicle from 1946, finding a “New OEM” part isn't realistic. Your choice is between a modern aftermarket part, a rare and expensive New Old Stock (NOS) part, or a used original. For a critical component like an ignition coil, a modern aftermarket part is often the best choice, providing reliability with new materials.
Brand Spotlight: Standard Ignition
Standard Ignition is a well-regarded brand in the aftermarket that often engineers its parts to be more durable than the originals. Their coils are manufactured to high standards, often featuring better internal components and seals to prevent issues like moisture intrusion and overheating that caused old coils to fail. For a workhorse truck, even a classic one, this modern reliability is a significant advantage.
Installation Overview
Replacing the ignition coil on your EF350 is a simple job that most owners can do themselves in under 30 minutes with basic hand tools.
- Disconnect the Battery: Always disconnect the negative battery terminal first.
- Locate the Coil: It's a small canister, usually mounted to the engine block or firewall, with a large wire going to the center of the distributor cap.
- Disconnect Wires: Label the two small wires going to the (+) and (-) terminals before removing them so you can reconnect them correctly. Pull the large high-tension wire from the top of the coil.
- Remove the Old Coil: Loosen the bolt on the mounting bracket and slide the old coil out.
- Install the New Coil: Slide the new coil into the bracket and tighten it. Reconnect the small primary wires and the main high-tension wire.
- Reconnect Battery: Reconnect the negative battery terminal and start the engine to test.
| Item | Estimated Cost |
|---|---|
| New Aftermarket Ignition Coil | $32 - $39 |
| Shop Labor | $80 - $150 |
| DIY Total | $32 - $39 |
| Shop Total | $112 - $189 |
Frequently Asked Questions
Do I need a special coil for a 6-volt system?
Yes, absolutely. You must use an ignition coil specifically designed for a 6-volt system. A 12-volt coil will not produce a hot enough spark and will cause poor performance. Conversely, using a 6-volt coil on a 12-volt system will cause the coil to overheat and fail quickly.
Does my truck have a ballast resistor?
Many 6-volt systems of this era did not use a ballast resistor, but some did. A ballast resistor is a ceramic block that reduces voltage to the coil after the engine has started. If your truck has one, it should be replaced if it's cracked or damaged. If it doesn't, you must use a coil that does not require an external resistor.
Can I test my old ignition coil?
Yes. You can perform a spark test by holding the main coil wire (with insulated pliers) about 1/4 inch from a metal part of the engine while a helper cranks it. You should see a bright blue spark. You can also test the primary resistance between the small (+) and (-) terminals with a multimeter; for a 6-volt coil, it should typically read between 1.0 and 2.0 ohms.
Why is my new coil getting very hot?
Coils get warm during normal operation. However, if it's too hot to touch, it could be due to a few issues. The most common is leaving the ignition key in the 'on' position without the engine running. It could also be caused by incorrect wiring or a faulty component elsewhere in the ignition system. Ensure you are using the correct coil for a 6-volt system without an internal resistor if your truck doesn't have a ballast resistor.
Technical Specifications
Cylindrical Canister Style, Oil Filled, 6-Volt System, For use with external resistor or no resistor depending on specific coil design, Compatible with points-style ignition distributors.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.