Essential Guide to the 1938 GMC T155 Ignition Coil
Keep your classic GMC running strong with the right ignition coil information.
- The 1938 GMC T155 requires a specific 6-volt ignition coil unless the electrical system has been converted to 12-volt.
- Symptoms of failure often appear when the engine is hot, such as stalling and refusing to restart until cooled.
- New aftermarket coils from brands like Standard Ignition are the most reliable and practical replacement option.
- Replacement is a simple, DIY-friendly task that typically takes less than 30 minutes.
Understanding the Ignition System in Your 1938 T155
The ignition system on your classic GMC T155 is straightforward compared to modern vehicles. It uses a single ignition coil that supplies spark for all cylinders through a distributor. Because this truck was originally built with a 6-volt positive ground electrical system, it requires a specific 6-volt ignition coil. Using a 12-volt coil on a 6-volt system will result in a very weak spark, if any. Conversely, using a 6-volt coil on a truck converted to 12-volts will cause the coil to overheat and fail quickly without a special part called a ballast resistor.
Symptoms of a Failing Ignition Coil
An ignition coil can fail in a couple of ways. It can fail completely, leaving you stranded, or it can fail intermittently, especially as it gets hot. Here are the common signs:
- No-Start Condition: The engine cranks but never fires up. This can happen if the coil has failed completely and isn't producing any spark.
- Engine Stalls When Hot: The truck starts and runs fine, but after 20-30 minutes, the engine sputters and dies. It won't restart until it has cooled down significantly. This is a classic sign of a coil failing from 'heat soak'.
- Backfiring: Unburnt fuel enters the exhaust and can ignite, causing a loud pop or bang.
- Rough Running and Misfiring: A weak spark from a failing coil can cause the engine to run poorly, shake, or lose power.
- Visible Damage: Look for cracks in the coil's housing or signs of oil leaking from the body of the coil.
Choosing the Right Aftermarket Ignition Coil
For a vehicle like the 1938 GMC T155, finding a New Old Stock (NOS) or original equipment (OEM) ignition coil is nearly impossible. Even if you found one, the nearly 90-year-old materials inside would be unreliable. A new aftermarket coil is the most practical and reliable choice.
Standard Ignition is a well-established brand that has been making quality ignition components for decades and is a common choice for vintage vehicle restorations. Their products are designed to meet or exceed original specifications, ensuring reliable spark and performance for your classic truck's simple ignition system.
Part Cost Comparison
| Part Type | Price Range | Notes |
|---|---|---|
| New Aftermarket (Recommended) | $32.57 - $38.58 | Provides modern reliability for a vintage system. Standard Ignition is a trusted brand. |
| New OEM | Not Available | Original parts for this vehicle are no longer manufactured. |
| Used OEM | $10 - $30 | Very risky due to age. Internal insulation breaks down over time, making failure likely. Not recommended. |
Frequently Asked Questions (FAQ)
How do I know if I need a 6-volt or 12-volt coil?
Your 1938 GMC T155 was originally built with a 6-volt system. If your truck still has its original generator and a large 6-volt battery (with 3 caps), you need a 6-volt coil. If a previous owner has upgraded the truck to a modern 12-volt system with an alternator and a 12-volt battery (with 6 caps), you will need a 12-volt coil, likely with an external ballast resistor.
Can I test my old ignition coil?
Yes, you can perform a basic test with a multimeter. A 6-volt coil should have a low primary resistance, typically between 1.4 and 1.6 ohms across the two small terminals. You can also check for a spark by holding the main coil wire (with insulated pliers) about 1/4 inch from the engine block while a helper cranks the engine. A healthy coil should produce a bright blue spark. A weak, yellow or orange spark indicates a bad coil.
Is replacing the ignition coil a difficult job?
No, this is a very easy job for a DIY mechanic. It involves disconnecting the battery, removing two small wires and the main high-tension wire, and unbolting the coil from its mounting bracket. The entire process usually takes less than 30 minutes.
Are there any recalls or technical bulletins for this part?
No. Modern safety recalls and Technical Service Bulletins (TSBs) from organizations like the NHTSA did not exist for vehicles made in the 1930s. Any known issues are based on common knowledge from mechanics and restorers working on vehicles of this era.
Technical Specifications
System Voltage: 6-Volt Primary Resistance (approximate): 1.4-1.6 Ohms Style: Canister, Oil-Filled
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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.