350i 4CU injection wiring queries

[mk1fan]

Evening All.

Reading through the clear guides on vintagemodelairplane.com I am a bit confused.

As I have understood, the resistor pack is supposed to drop the voltage feed from 12v to 3v to feed the injectors. This feed is constant with the ignition on.

The injectors are then earthed at the ECU by the ECU to fire the injectors as required.

Reading through the injector testing guide on the above website, however, the first test is checking the voltage at the ECU plug. It states that the voltage should be within 0.5v of the [12v] battery. As this is downstream of the resistor pack, should it not be 3v?

I've checked the voltages on a couple of 'working' 350i and the pins of the resistor packs are 13v in and 12.5v out - which is what Wendy is showing too (when running).

Is the guide correct that the ECU plug readings should be within 0.5v of the battery?

[adam quantrill]

From what I can see of the wiring diagram the resistor pack goes to battery + (via a couple of relays).

When the 4CU fires an injector (bank) it "grounds" the 4CU side of the injector.

The voltage across the injector at that point is determined by the resistor pack resistance and the injector resistance (the ratio of these two to be more precise) and battery voltage.

As the resistor pack are 6 ohms and the injectors around 1.2 ohms you'll get around a 5:1 ratio so for a nominal 12V batteryaround 2V across the injector, but for 13.5V a little more.

[adam quantrill]

Wedge epistles says the injectors are 2.4 ohms in which case the volts across the injector (when it's being fired) will be a bit higher... 3.5 to 4V.

[mk1fan]

So is the guide incorrect? Should the voltage at the ecu plug be 3v or 11.5v??

[Penelope Stoppedit]

If a resistor is in an open circuit ***Example ECU disconnected*** there will be no current flow and the voltage measured at each end of the resistor will be the same. When a resistor is in a closed circuit there will be current flow through it and the voltage measured at each end of the resistor will differ and be dependant on the value of the resistor being measured
It's all about open and closed circuits

The test you are carrying out with the ECU plug disconnected is an open circuit test and if there is 12 volt supplied to the cable going to the resistor there should be 12 volt passing through the resistor to the ECU plug

[mk1fan]

So using a multimeter run directly to an earth not the circuit earth.

ECU plug disconnected. Ignition on.

The ECU plug should show 11.5v.

As a follow up separate question. How come the multimeter does not form a closed circuit?

[adam quantrill]

Ahh - the multimeter DOES form a closed circuit. However, one the volts setting any multimeter worth its salt will be hundreds of kilo ohms to mega ohms, depending on sensitivity. Compared with the low resistance of the other circuit elements the voltmeter will drop the most volts across it, and hardly anything will be across the 6 ohm resistor, because very little current is flowing.

[Penelope Stoppedit]

This is a very well explained write-up about the affects of connecting a voltmeter across a resistor
https://www.allaboutcircuits.com/textbook/direct-c...

[mk1fan]

Thanks, I'll have a read this evening. However, I'm not connecting a voltmeter across a resistor, I'm connecting it after.

[blitzracing]

You will only see the voltage drop at the point the injector is firing- ie the ECU has grounded one side of the injector so the injector is now open. You wont see this voltage drop unless you have the engine running and using an oscilloscope to see the injector open period. All other readings are effectively open circuit, so no current flow, no significant voltage drop across the resistor.

[mk1fan]

I have been lent and ostreachometer so I'll move on to that after doing the continuity, voltage and resistors tests (as per the guide).

Thanks.