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​DDEC IIs Part 2: A Design From The Past

By Fernando DeMoura

March, 2017

There is a myth about DDEC IIIs and IVs that if one of the two 12 volt supply connections is lost half the injectors will shut down. This is not true. The two 12 volt supply connections that feed DDEC III and IV ECMs are connected together internally. Still I had always wondered where this myth came from.

It wasn’t until I started bench testing DDEC IIs that I found out why some technicians believe this.

On a DDEC II the high current injector driver circuits are somewhat electrically isolated so when one connection is lost one of two injector “banks” shuts down. This will instantly shut off injector number 1,2, and 3 Or injector number 4,5, and 6. At first I thought this injector driver design was illogical. It wasn’t until further study that I realized this was actually a form of redundancy. The DDEC II has two of everything on the injector driver circuit board. A failure on one side would cause you to lose half your injectors but would not leave you stranded. I’ve grown to admire the DDEC II’s design. It has symmetry and all components are given plenty of space to dissipate heat. It’s clear it was designed by a human on a drafting table and not a computer running CAD.

The DDEC II has half the amount of digital processing components as the DDEC III but the parts are large, hard to find, and expensive. The DDEC II has heavy aluminum heat sinks that make solid contact with the case. Parts are installed with heavy solder joints that pass through the circuit board like a 40 pin Cat ECM. Compared to the surface mount components used on DDEC III and IV circuit boards the connections in a DDEC II are much more durable. DDEC IIs have no batteries to go bad and their rubber seals and heavy cases do a good job at keeping oil and water out. The manufacturing of a DDEC II was no doubt slow and expensive compared to later designs but this ECM might just be the most reliable of all the DDEC designs. This ECM was designed to be serviceable and repairable but before I can offer repairs as a service I need to find suppliers for these old parts.

When considering the DDEC IIs operating system, the timing control and fuel settings are basic. Timing advance options are limited but as far as I can tell it will do what it’s told if asked nicely. The first performance programs I plan on doing will simulate the 550 hp GU programs I do for DDEC II to DDEC IV conversions. If my oscilloscope doesn’t know the difference neither will your injectors. If a DDEC II will turn an injector on and off at the exact same time as a DDEC IV running a DDEC II conversion program, then engine performance will be the exactly the same.

Written by Fernando DeMoura, Diesel Control Service LLC., Web: www.dieselcontrolservice.com Phone: 412-327-9400