In order to finalize the ECU tester board design I needed to know how much current the ECU requires. I couldn’t find any documentation on this so I decided to try simulating the circuit reverse engineered by @CaptainSLC and @cushjbc (see D-Jetronic 1973 450SLC Schematic).

The injector drivers have a separate voltage supply on pin 24 and this powers the heavy-duty components so all I needed to do was simulate the injector drivers part of the circuit. The rest of the ECU should have minimal power requirements in comparison.

In order to do that I needed an electrical model of a Djet injector. This page: The ECU and this page: 914 D-Jetronic Parts and Troubleshooting indicated they can be modelled by a 3.77 to 4.16mH inductor and a 2.5 to 2.8 Ohm resistor in series.

Here is the model I used:

Here is an injector waveform captured by @cushjbc from his car:

Here is the output from my simulation (note – there is no conditioning or modification of the waveform from the sensors as I did not simulate those):

It’s not identical but close enough for my purposes.

Here is the current consumption of the circuit:

Each time an injector group fires the ECU draws 3.14A of current on pin 24.

Here is the current consumption of a single injector:

Each injector draws 1.29A.

Injectors are grouped together into pairs, with one pair on a single ECU pin (3, 4, 5 or 6). The current consumption for a group is 2.58A:

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