Ignition Contact Set Guide for the 1939-1940 GMC AC150
Keep your classic GMC running strong by understanding this critical ignition part.
- For a 1939-1940 GMC AC150, new ignition contact sets are only available from aftermarket brands like Standard Ignition.
- Symptoms of failure include hard starting, rough running, stalling, and loss of power.
- Properly setting the point gap during installation is essential for correct engine operation.
- Replacing the condenser along with the points is a standard and recommended practice.
Symptoms of a Failing Ignition Contact Set
Ignition points are a wear item and require regular replacement. When they start to fail, you'll notice clear signs in how your truck runs. 🎬 Watch this guide on classic car ignition point symptoms. The contact surfaces can burn or pit over time, and the rubbing block that opens the points can wear down, causing poor performance.
- Hard Starting or No Start: This is one of the most common signs. Worn or misaligned points can't create the proper electrical signal to generate a strong spark, making it difficult or impossible to start the engine.
- Engine Misfires and Rough Running: If the points deliver a weak or inconsistent signal, the spark plugs won't fire correctly. This leads to a rough, shaky idle, hesitation during acceleration, and an overall lack of smooth engine operation. 🎬 Learn how to fix a mystery misfire in classic cars.
- Stalling: Worn points can struggle to maintain a consistent spark at low RPMs, causing the engine to stall at idle or when you come to a stop.
- Reduced Power: A weak or intermittent spark from failing points means inefficient combustion. Your truck will feel sluggish and lack acceleration power because the engine isn't running on all cylinders effectively.
Choosing a New Aftermarket Ignition Contact Set
For a vintage truck like the 1939-1940 GMC AC150, new original equipment (OEM) parts are no longer manufactured. Your only option for a new part is the aftermarket, which serves this need well.
Brand Focus: Standard Ignition (Standard Motor Products)
Standard Motor Products (SMP) is a long-standing and well-regarded manufacturer of ignition components. For decades, their "Blue Streak" line was considered a premium, high-quality option, often exceeding original part quality. While SMP now offers different product tiers, they remain a reliable choice for classic vehicle parts like ignition points. For your GMC AC150, a Standard Ignition contact set is a widely available and trusted replacement part.
Estimated Costs
| Part Type | Price Range |
|---|---|
| New Aftermarket (Standard Ignition) | $21.44 |
| Shop Labor | $100 - $200 |
Installation Pro-Tip
Properly setting the "gap" on new points is the most critical step of the installation. This small air gap, measured with a feeler gauge when the points are fully open, determines how long the coil has to charge. For a 6-cylinder GM engine of this era, a common starting point for the gap is around 0.016 to 0.019 inches. 🎬 See how to set your ignition points gap quickly. An incorrect gap will lead to poor running, misfires, and hard starting. It's also a good practice to replace the condenser at the same time as the points.
Frequently Asked Questions (FAQ)
What are ignition points?
Ignition points are a set of mechanical electrical contacts inside the distributor that act as a switch. They control the timing of the spark that ignites the fuel in your engine.
How often should I replace the ignition contact set?
Ignition points are a maintenance item, not a lifetime part. They typically last around 20,000 miles but should be inspected regularly for pitting, burning, or wear on the rubbing block. Given the age of the vehicle, an annual check-up is a good idea.
What is the point "gap" and why is it important?
The point gap is the maximum distance between the two contact points when they are opened by the distributor cam. This setting is critical because it controls the "dwell," or the amount of time the coil has to build a charge. An incorrect gap leads to a weak spark and poor engine performance.
Should I also replace the condenser?
Yes, it is highly recommended. The condenser absorbs excess voltage to prevent the points from arcing and burning out prematurely. Since you are already working inside the distributor, replacing both the points and condenser at the same time is standard practice.
Helpful Videos
Technical Specifications
Point Gap: Approximately 0.016" - 0.019" (Consult a service manual for exact specification)
We Have This Part in Stock
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.