📋 Quick Facts
Time
10 minutes
Difficulty
Easy
Tools
2 tools
Cost to DIY
$10-25
Most automotive relays follow the same SPDT 4-pin or 5-pin pinout: terminals 85 + 86 are the coil (control side), 30 is the battery feed, 87 is the output to the load, and 87a (5-pin only) is the normally closed output. Once you know the pinout, testing takes 5 minutes.
🛠 What You'll Need
- Digital multimeter (shop a digital multimeter on Amazon)
- Jumper wires (12V) with alligator clips (shop jumper wires on Amazon)
- Fuse/relay puller (shop relay/fuse pullers on Amazon)
- Owner's manual for relay-box diagram
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⚠ When NOT to DIY thisWhen bench-jumping a relay, connect the LOW side first (terminal 30 from a fused supply) and only THEN energize the coil. Reverse order can spark and damage the relay. Always use a 10-30A inline fuse when bench testing.
🎯 Expected Readings (Pass/Fail Reference)
| Coil resistance (between 85 and 86) | 60 - 120 ohms typical |
| Open coil | OL / infinity = bad relay |
| Continuity 30 - 87 with coil OFF | OL (open). Normally open contact |
| Continuity 30 - 87 with coil ENERGIZED (12V on 85-86) | 0 ohms (closed). Audible click on energize |
| Continuity 30 - 87a with coil OFF (5-pin only) | 0 ohms (closed). Normally closed contact |
Numbers are typical. Always cross-check against your factory service manual for the exact spec.
📝 Step-by-Step Test Procedure
- Identify the relay and read the pinoutPull the relay. Look at the bottom or side - most have a diagram showing pin numbers (30, 85, 86, 87, sometimes 87a) and a coil/contact schematic.
- Swap test (fastest)If the car has an identical relay nearby (e.g., horn relay = fuel pump relay = headlight relay), swap the suspect with a known-good one. If the failure follows the relay, the relay is bad.
- Coil resistance testSet multimeter to 200 ohms. Probe terminals 85 and 86. Should read 60-120 ohms. Infinity = open coil (bad). Near 0 ohms = shorted coil (bad).
- Contact continuity, de-energizedSet to continuity. Probe terminals 30 and 87. Should be OL (open circuit) with coil off. Probe 30 and 87a (5-pin): should be closed (0 ohms).
- Energize and listen for the clickConnect 12V from a battery (with a fused jumper) to terminals 85 (+) and 86 (-). You should hear and feel a sharp click. Continuity 30-87 should now be 0 ohms (closed). No click = bad coil or stuck contacts.
- Voltage drop check (in the car, fast troubleshooting)With relay in place and circuit demand on (e.g., key on for fuel pump), back-probe terminal 87. Should read battery voltage (within 0.5 V). Low voltage = corroded relay socket or burned contacts.
- Inspect the relay socketPull the relay; look inside the socket for green corrosion or bent pins. A bad socket mimics a bad relay - clean with electrical contact cleaner.
- Replace the relay if any test failsMost auto relays are universal 4 or 5-pin Bosch-style. Always match the amp rating (usually 30A or 40A). shop universal automotive relays on Amazon.
✅ Pass / Fail Criteria
✅ PASS
60-120 ohm coil, audible click on 12V, 30-87 closes when energized, no corrosion, no burned smell
❌ FAIL
Open or shorted coil, no click on 12V, contacts will not close (or are stuck closed with no signal), burned smell or melted housing
🔧 If It Fails - What To Do Next
Replace the relay. Universal automotive relays are $5-15. If the same relay socket corrodes repeatedly, clean it with contact cleaner or replace the connector. See How to test a fuse if you suspect a related fuse problem.
🔗 Related Guides
❓ Frequently Asked Questions
What are the four pins on a relay?
85 and 86 are the coil; 30 is battery + input; 87 is the switched output. 5-pin relays add 87a as a normally closed output.
How can I tell if a relay is bad?
Coil resistance outside 60-120 ohms, no click when 12V is applied to 85-86, or contacts (30-87) that will not close when energized.
Can I bypass a bad relay temporarily?
Yes - jumper terminal 30 to terminal 87 in the relay socket to force the circuit on. This is a diagnostic-only fix; do not leave it that way (no overload protection).
Are all 4-pin relays interchangeable?
Mostly yes, as long as the amp rating and coil voltage match. Always check the rating stamped on the relay.
Why does my fuel pump relay keep failing?
Usually a tired fuel pump pulls high current and burns the contacts. Replace the pump if relays repeatedly fail.
Will AutoZone test a relay?
Most stores have a free under-hood relay tester or will swap an identical relay to confirm. Just ask.