Type “A” vs. Type “B” automotive relays
If you’ve ever plugged in a “correct” relay and the car immediately acts haunted. Latched circuits, mystery battery drain, or a load that will not turn on, there’s a good chance you ran into the Type A vs. Type B trap. They look the same, they fit the same socket, but they swap the positions of terminals 30 and 86. That’s all it takes to make a sane wiring harness do weird things.
Quick refresher: what the numbers mean
DIN terminal numbers are consistent across relay brands:
- 30 = common feed (usually battery +)
- 87 = normally open (NO) contact
- 87a = normally closed (NC) contact (only on 5‑pin changeover relays)
- 85 & 86 = the coil terminals (polarity matters only if there’s an internal diode)
Orientation note: Diagrams and pin callouts below are from the bottom (terminal side) view, which is how relay base layouts are specified.
The curveball: Type A vs. Type B layouts
Electrically, Type A and Type B ISO “mini” relays do the same job.
- Type B (the “common” aftermarket layout): Your mental model likely matches this one.
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Type A (the “gotcha” layout): Same case, same 5 blades, but 30 and 86 are swapped.
- Because the plastic keying is identical, a Type B relay will plug into a Type A socket—and vice‑versa—and quietly re‑wire your circuit.
What goes wrong when they’re mixed
If the socket was wired for Type A and you plug in Type B (or the other way around):
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Coil gets battery+ all the time: The harness’s 30 (battery feed) lands on the relay’s 86 (coil) instead of the contact common.
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Back-feeding / latching: The control lead that should drive the coil ends up on the contact common, so the relay may stay latched or behave inconsistently.
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Battery drain: A typical 12 V ISO mini coil is ~90 Ω.
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Current: A
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Power: W
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Over 24 h that’s ~3.2 Ah enough to flatten a battery in days, especially if the car already has a small standby draw.
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If the relay has a suppression diode: Reversed coil polarity or an unintended path through the diode can pop a fuse or stress an ECU driver.
Real‑world example: BMW E36
Plenty of E36s (and other ’80s–’90s BMWs) use sockets wired for what many catalogs call Type A. Drop in a generic Type B (often a black case replacing the BMW green one) and you can see exactly the symptoms above: relay stuck “on,” load won’t work, battery dies after sitting. The fix isn’t color—it’s matching the base layout to the socket.
Don’t mix up “Type” with “Contact style”
These are different decisions:
- Base layout (Type A vs. Type B): where 30 and 86 physically sit.
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Contact style:
- 4‑pin NO (30–87)
- 5‑pin changeover (30–87a–87)
- 5‑pin dual‑NO (30–87–87b, no 87a)
You must match both the base layout and the contact style.
How to ID the right relay in 30 seconds
- Flip it over and read the little schematic. Find where 30 and 86 are physically located.
- Check your socket: Which cavity is fed by battery+? That must land on 30—not on 86.
- Look for suppression: If you see a diode symbol on the case, remember polarity matters (commonly 86 is +, 85 is –).
- Double‑check contact style: Do you need 87a (changeover) or dual‑87?
Two‑minute bench test (no wiring diagram needed)
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Find the coil: Measure resistance pairs; the ~60–120 Ω pair is 85/86.
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Identify 30: With the coil unpowered, continuity between 30 and 87a (if present) tells you which blade is 30.
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Confirm action: Power the coil (observe polarity if a diode is present). You should hear a click; continuity moves from 30–87a to 30–87.
Bottom line: Type A and Type B ISO mini relays look alike but swap 30 and 86. Mix them and you’ll get latching, weird behavior, and battery drain. Before plugging in: flip the relay, confirm the socket’s battery feed lands on terminal 30, match contact style (87/87a or dual‑87), and note suppression (diode = polarity matters). Don’t trust case color or “5‑pin” alone. A 30‑second check beats hours of chasing ghosts.