Bosch invented the automotive relay in 1956 and remains the gold standard for OEM and aftermarket applications worldwide. Whether you're replacing a failed relay or building a custom electrical system, here's everything you need to know.
What Is an Automotive Relay and Why Does It Matter?
A relay is an electrically-operated switch. It allows a small current (from a switch, fuse box, or control module) to control a much larger current (to a motor, compressor, fan, or lighting circuit). Without relays, every high-current circuit would need heavy-gauge wiring all the way to the dashboard switch - impractical and dangerous.
Relays also protect switches and control modules from high-current damage. Your AC compressor, cooling fan, fuel pump, starter, and horn all use relays.
Bosch Relay Types
Mini Relay (Most Common - 12V, 30A/40A)
The standard ISO mini relay used in virtually every modern vehicle fuse box. Fits the universal 5-pin relay socket. Available in normally open (NO) and change-over (CO) configurations.
- Bosch 0332019150 - 12V, 30A, normally open (4-pin)
- Bosch 0332019151 - 12V, 40A, normally open (4-pin)
- Bosch 0332014456 - 12V, 30A/40A, change-over (5-pin)
Best for: direct replacement in any modern vehicle fuse box, lighting circuits, accessory wiring
Normal Open Power Relay (Heavy Duty)
For high-current applications requiring more than 40A. Used in cooling fans, starter circuits, and high-draw accessories.
- Bosch 0332002156 - 12V, 75A, normally open. One of the most popular heavy-duty relays for aftermarket applications.
Best for: electric cooling fans, winches, high-power lighting (LED light bars), compressors
Time Delay Relay
Adds a programmable delay before opening or closing. Used in glow plug circuits (diesel engines), turbo timer applications, and HVAC systems.
Flasher Relay
Controls turn signal and hazard light timing. Electronic flashers (replacing old thermal flashers) are required when upgrading to LED turn signals to maintain correct flash rate.
How to Read a Bosch Relay Part Number
Bosch relay part numbers follow a consistent format:
- 0 332 - Product family (relays)
- 002/019/014 - Sub-series (power/mini/change-over)
- Last 3 digits - Specific variant (voltage, amperage, contact configuration)
5-Pin vs 4-Pin Relays: What's the Difference?
4-pin relays are normally open (NO) - the circuit is open at rest and closes when the relay energizes. Used for most on/off applications.
5-pin relays are change-over (CO) - they have both a normally open AND a normally closed contact. When de-energized, the NC contact is connected. When energized, the NO contact connects. Used when you need to switch between two circuits.
Pin numbering is standardized:
- Pin 85 & 86: Coil terminals (12V control signal)
- Pin 30: Power input (battery/fused power)
- Pin 87: Normally open output (connects when energized)
- Pin 87a: Normally closed output (5-pin relays only)
How to Test a Bosch Relay
- Remove the relay from its socket.
- Use a multimeter set to continuity/resistance. Test between pins 85 and 86 - you should read 70-90 ohms (coil resistance). Open circuit = failed coil.
- Apply 12V to pins 85 and 86. Test continuity between pins 30 and 87 - you should now read continuity. No click or continuity = failed contacts.
- Remove power - continuity between 30 and 87 should disappear.
Common Relay Failures and Symptoms
- No click when energized: Failed coil - replace the relay
- Click but no output: Burned contacts - replace the relay
- Relay stuck on (won't de-energize): Welded contacts from current overload - replace relay and check for short circuit
- Intermittent operation: Corroded contacts or loose socket - clean socket terminals or replace relay
Shop Bosch Relays
We carry a full selection of Bosch mini relays, power relays, and change-over relays. OEM quality at aftermarket prices with fast shipping.
