Relay numbers, and how to jump a relay socket
Every relay has numbers moulded into its base. Once you know what they mean you can test any relay in the car, jump a relay socket to prove a pump works, and tell a bad relay from a bad circuit in a couple of minutes.
What the numbers mean
The numbers come from the DIN 72552 standard that BMW and other German makers use. The same numbers appear on the factory wiring diagrams, so a relay's terminals can be matched straight to the drawing.
| Number | What it is | In plain words |
|---|---|---|
| 30 | Power in | Battery positive, live all the time. It is the common contact: the power that the relay switches to the load. |
| 87 | Output, normally open | Not connected to 30 until the relay is switched on. This is the wire that goes to the pump, fan or lamp. |
| 87a | Output, normally closed | Connected to 30 while the relay is off, and disconnected when it switches on. Only on 5-terminal relays. |
| 86 | Coil, one end | Usually the positive side of the coil. |
| 85 | Coil, other end | Usually the ground side of the coil. On a car with an engine computer this is often the wire the computer grounds to switch the relay on. |
Other terminal numbers you will meet on the E30 diagrams:
- 15 is switched ignition power, live with the key in RUN or START.
- 31 is ground.
- 50 is the starter control wire, live only while the key is held at START.
The coil of some relays has a diode or resistor built in. Those relays only work one way round, so respect the 85 and 86 polarity when you test one.
How a relay works
A relay is a switch operated by a small electromagnet. A small current through the coil (85 and 86) pulls a metal arm across, which lets a large current flow from 30 to 87. That is why a fuel pump can be switched by the engine computer, which could never carry the pump's current itself.
The E30's key relays
These come from the factory wiring pages for the 2.5i. The 325e's circuits differ in places, so use its own pages for that car.
| Relay | 30 | 86 / 85 (coil) | 87 | Wiring |
|---|---|---|---|---|
| Main relay (DME relay) | Battery junction block, 4 mm² wire. Also feeds the computer's pin 18. | 86 is fed from the same supply. 85 goes to computer pin 35, which grounds it. | Computer pin 37, the injectors and idle actuator through splice S112, and the fuel pump relay's coil. | 1360-5 2.5i Injection Electronics (Main Relay / Fuel Pumps) |
| Fuel pump relay | Battery junction block, 1.5 mm² wire. | 86 is fed from the main relay's 87. 85 goes to computer pin 3, which grounds it when it sees the engine turning. | The fuel pump feed: C101 pin 13, C114 pin 2, then the pumps. | 1360-5 2.5i Injection Electronics (Main Relay / Fuel Pumps) |
| Start relay (automatic cars) | From ignition switch terminal 50. | 86 is fed through the range switch pin 9. 85 is grounded at G200. | The start wire to the starter terminal 50. | 1240-0 Start (2.7e/2.5i, automatic, except convertible) |
| K5 and K7 unloader relays | Power distribution box feed. | Switched from the start line, so they act while cranking. | Loads that are shed during starting. | 1240-1 Start (2.7e/2.5i, manual, except convertible) |
The two engine relays work as a pair. The main relay has to switch on first, because its 87 feeds the fuel pump relay's coil. So if the main relay is dead, the fuel pump will not run either, and it can look like a pump fault.
The relays sit in the power distribution box on the passenger side of the engine bay, near the air flow meter. Their exact positions vary with year and market, so identify them by their wires and the markings on the relay and cover rather than by position alone. The Power Distribution Box Layout page shows the box.
Finding the right socket holes
Do not trust a drawing of the socket if you are not sure what you are looking at. Confirm the holes yourself.
- Pull the relay and read the numbers moulded on its base. They show where each terminal sits on the relay.
- Match the relay's terminals to the socket: the hole that its terminal 30 plugs into is socket 30, and so on.
- Confirm 30 with a meter or test light. On the fuel pump relay socket, the 30 hole should have battery voltage all the time, with the key off.
- If the relay is a 4-terminal type, it has no 87a. If it has 5, 87a is the one next to 30.
Jumping a relay socket
Jumping means linking 30 to 87 in the empty socket with a wire. That does what the relay would do when it switches on: it connects battery power to the load. If the load now works, the load and its wiring are fine, and the fault is the relay or whatever switches it. It takes about two minutes.
- Ignition off. For extra safety you can disconnect the battery while you make the connection, then reconnect it.
- Pull the fuel pump relay and identify socket holes 30 and 87 as described above. Confirm that 30 is live.
- Make a jumper. Use 14 to 16 gauge wire with an in-line fuse of 10 to 15 A, and fit proper male spade terminals. Do not push bare wire into the holes, because it spreads the terminal and makes the socket unreliable.
- Fit it into 30 and 87 only. The fuel pump should start to hum straight away, and you can hear it from the rear of the car.
- Remove it as soon as you have your answer.
| Result | What it means | Next |
|---|---|---|
| Pump runs | The pump, wiring, fuses and supply are good. The relay or its control is the fault. | Swap the relay. If that does not fix it, test the coil side: 86 and 85. |
| Pump stays silent | The fault is between the socket and the pump, or the pump itself. | Check voltage at 30, the feed connectors, the pump ground (G300) and the pump. |
The same trick works on other relays, with one big warning for each:
- Main (DME) relay: bridging 30 to 87 powers the engine computer and the fuel pump relay's coil with the ignition off. It is a good test, but remove it straight away.
- Start relay (automatic): bridging 30 to 87 makes the starter crank the engine. Check the car is in P and the handbrake is on first.
- Fan, horn and other load relays: 30 to 87 makes the load run, just as with the fuel pump.
Start with the guide if you are not sure where to begin: the no-crank / no-start guide.
Swap and bench tests
Swap test. Several relays in the box are the same type. Swap a suspect relay with a known-good one from a circuit that is not involved (the horn or fan relay, say). If the fault moves with the relay, the relay is bad. It is the quickest test and often the only one you need. Relays that work cold and fail hot are common, so swap one that has been warm.
Bench test. With the relay out of the car, use fused jumper leads from a 12 V supply.
- With no power on the coil, a meter between 30 and 87 should read open, and between 30 and 87a (if present) should read closed.
- Connect the coil, 12 V on 86 and ground on 85, to hear a click. Now 30 to 87 should read closed, with almost no resistance, and 30 to 87a should open.
- Measure the coil's resistance between 85 and 86. A figure of roughly 50 to 120 ohms is typical. An open circuit or near zero is a dead coil. Relays with a built-in diode or resistor read differently depending on probe polarity.
- A relay that clicks but does not pass current has worn or burnt contacts and should be replaced.
What not to do
- Do not use an unfused jumper. A short in the circuit with an unfused wire can overheat the wiring and start a fire.
- Do not jump random holes. Linking the wrong two terminals can feed battery power into the computer or ground a signal wire, and has caused damage on E30s. Confirm each hole first.
- Do not leave a jumper in place. A pump running with the engine off can flood the engine or leak fuel.
- Do not push thick wire into the socket. It stretches the terminals. Use a spade terminal.
- Do not fit a larger fuse because one keeps blowing. Find the cause.
Written from the factory wiring diagrams on this site and general diagnostic practice. Voltages, resistances and pressures are typical figures, so confirm them against your car's manual. Take care with fuel, high voltage and the battery.