BMW E30 318i Fuel Injection Basics

These notes helped me cure a no-start condition on my 1984 BMW 318i.

The E30 318i Fuel Injection is Bosch L-Jetronic and is fundamentally the same as the fuel injection aspects of the Motronic used on the 325, 325e, 325i as well as the larger-engined 528e, 533i, 535i, 633CSi, 635CSi, 733i, 735i and so on.

However, there are some important subtleties, including the role of the fuel pump relay. On the 318i it plays an unusually central role. When the relay is suspect, I make sure it has positive feed. Then, I hot-wire the positive to one of the terminals to provide positive voltage to the fuel pump, and to another terminal to provide positive voltage to the fuel injectors. The relay has two other terminals too, not relevant for when I hot-wire it. In normal life, these provide the essential input that make the relay work.


The L-Jetronic computer a.k.a. electronic control unit (ECU) is above the glove box.  Its cable looks like the following picture, with pin “1” near the incoming cable:

E30318IBWhen I troubleshoot a no-start situation, I ignore subtleties like the ECU input signals as to barometric pressure, the coolant temperature, and whether the air conditioning is on.

I focus on:

Pin 1: does it provide positive voltage when ignition is on, and does it flash when the engine turns over, in synch with engine speed?

Pin 2: Throttle position switch positive for wide open throttle

Pin 3: Throttle position switch positive for closed throttle

Pin 4: Starter positive

Pin 5: Ground a.k.a. negative

Pin 9: Positive from fuel pump relay

Pin 12: Ground provided for fuel injectors … the main output function of the ECU.

Pin 13: Ground a.k.a. negative

Each of these can be easily tested at the cable, to see if they behave as they should: step on the accelerator pedal all the way to see if you have positive voltage at pin 3, let off and see if you have positive voltage at pin 2, and so on.

I don’t take ground connections for granted. My car had a bad ground connection. The ground connection by the fuel rail plays a key role in this.

The nice thing is that you don’t have to worry about subtleties like how many Volts are involved. The system is really simple as such. When something is supposed to be positive, you either have 12V or you don’t.

Using these basic principles, I was able to get my 1984 BMW 318i to basically fire on start-up. Many other hurdles remain, but this is still a huge milestone.

Meanwhile, I learned that the fuel pump relay for the BMW 318i (with the M10 engine) is a 7-prong relay, and that the 5-prong relays for the M20-engined E30 car won’t work.  So, I ordered a new part from Pelican Parts for $31  and we’ll see what happens.

As to the wiring for the fuel pump relay, it accepts input from the starter when energized (black and yellow wire, code 50) and the ignition coil ground (black wire, code 1).  When the starter is energized or the engine is running, the fuel relay considers that its “green light” to energize. This means that it connects the “always positive” feed (red wire, code 30) to the positive of the fuel injectors (red-and-white, code 87) and the positive of the fuel pump (red-and-white, code 87b). There is also a ground connection (brown, code 31) and a positive feed when the ignition is on (green, code 15).

The basic idea is to prevent the situation where the car has been in an accident, with the key on and the driver unconscious while there’s an engine fire, and the fuel system is literally adding fuel to the fire.  So, unless the engine is turning or the starter is engaged, the fuel pump is shut down.  That’s the logic behind all this, as I understand it.  The internal circuitry of the fuel pump relay is electronic, so we’re way beyond the simplicity of a typical relay.


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