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SAE J112a โ Electric Windshield Wiper Switch Testing and Compliance Guide
SAE J112a (Revised Oct 1977) specifies the temperature test, endurance test setup, and performance requirements for electric windshield wiper switches and combination wiper/washer switches. This recommended practice ensures that switches withstand extreme conditions and repeated use while maintaining electrical integrity. Below we break down the key requirements, test setup, and common pitfalls.
🔍 Key Test Parameters and Conditions
| Parameter | Requirement | Details |
|---|---|---|
| Temperature Test | 10 cycles at design load after 1h soak at 75°F (24°C), 165°F (74°C), -25°F (-32°C) | Same switch for subsequent endurance test (Section 4.2). |
| Voltage Levels | 6.4V (6V sys), 12.8V (12V sys), 25.6V (24V sys) | Measured at input terminal. Power supply must meet dynamic regulation and ripple ≤300 mV p-p. |
| Endurance Cycles | 10,000 complete cycles at 75°F ±10°F, then 1h ON in low position. | Cycle includes position sequencing with specified travel/dwell times. |
| Travel Time | 0.1–0.5s between positions (1.0–3.0s if rheostat) | Must include dwell time in each position and suitable OFF dwell for motor parking. |
| Dwell Time | 0.5–1.0s per position | Longer in OFF allowed for motor parking. |
| Voltage Drop | ≤0.30V (average of 3 readings, excluding rheostat) | Measured before/after endurance and after soak tests. If wiring integral, include 3 in. wire. |
These parameters create the baseline for switch validation and must be carefully implemented in any test setup.
🛠️ Voltage Drop and Durability Criteria
The voltage drop measurement is a critical indicator of contact health. After 10,000 cycles and temperature soaks, the switch must still exhibit low resistance to ensure efficient operation and prevent excessive heating.
Design Insight: To ensure longevity, select contact materials that remain stable over 10,000 cycles and at temperature extremes. Use bifurcated contacts or redundant contact points to maintain low resistance even after wear. Copper alloy contacts with proper plating (e.g., silver or gold flash) are typical. Also, ensure the switch mechanism does not degrade the contact force over time.
⚠️ Common Mistake: Failing to use the same switch for the temperature test and the endurance test. Section 4.2 explicitly requires using the same switch for both to ensure realistic degradation. Additionally, power supply regulation and ripple must be controlled to avoid false failures.
Frequently Asked Questions
- Why must the same switch be used for temperature and endurance tests?
This ensures that the cumulative effect of thermal extremes and mechanical cycling is assessed on a single device, reflecting real-world stresses. The standard mandates this in Section 4.2. - What is the required voltage drop limit after testing?
The average voltage drop across terminals must not exceed 0.30 V (excluding any rheostat), measured at design load. This limit maintains efficient power transfer to the motor. Measurement must include 3 inches of wire if wiring is integral. - How are combination wiper/washer switches tested?
The wiper portion must meet SAE J112a, the washer portion must meet SAE J234, and if mechanically coordinated, they must be tested simultaneously using the same switch. The combined switch must satisfy both standards. - What are the power supply requirements for endurance testing?
The supply must deliver the specified test voltage with dynamic regulation within 1.0 V excursion, static regulation within 2%, and ripple below 300 mV p-p. These prevent supply-induced test errors and ensure consistent load conditions.
