​No Start Problems After Installing A DDEC ECM Part 2

Fernando DeMoura
December 2017

Last month I wrote a little about a very common no start problem after a DDEC ECM has been installed. To read the article, look up the November 2017 edition of Movin’Out.

Before we can talk about the other common reason DDEC ecms don’t start after being installed we need to talk about the harness side pins.

It’s no secret that DDEC IIIs and DDEC IVs are my favorite engine control system but I’ll admit this control system has some weaknesses. The harness side pins used in older DDEC II, III, and IV harnesses are not as good of a design as you’d find on Cummins or Caterpillar control systems. These terminals rely on a small amount of spring pressure applied by the harness side pin tongue to keep good contact with the flat pins on a DDEC. Particles of sand or grit on the tongue can prevent the DDEC pin and the harness pin from making contact. This is why I’m not a big fan of dielectric grease on DDECs. The grease makes these particles stick to the contact points on the pins. Another problem with these pins is the tongue can get crushed down on installation or by a technician testing the pin. If this happens to the ignition pin or any of the timing input pins, then you get a no start problem. Often the ECM takes the blame and it’s understandable as to why.

When a technician tests critical circuits like the ignition or the timing sensor pins he will likely use two test probes from a multimeter. Test probes are essentially round pins that have no trouble hitting the tongue of the harness side pin. After he verifies they are ok he’ll then reconnect the harness to the ECM, then check to see if the circuit in question is powered or carrying a signal. If it’s dead it’s natural to assume the problem is in the box. In reality the flat pins of the DDEC never came in contact with the harness pins even though the harness is fully seated. That’s why it’s important to check that the tongue on the harness side pins are up and the flat pins on the DDEC side are aligned. I like to use a cheap dental pick to lift these tongues. I have to do this at least once a week to my test bench harnesses because I plug into many different DDECs and my bench harnesses has seen a lot of action.

Getting back to timing…if your DDEC doesn’t have a signal from its crankshaft sensor (TRS) and bull gear reference sensor (SRS) it doesn’t know your trying to start it. If it thinks crankshaft rpm is 0 it will not fire an injector. These pins are located in slots S1 and S2 for the SRS (bullgear) and T1 and T2 for the TRS (crankshaft). Before sending me a DDEC for repair because you think yours is bad, do us both a favor and take a look at these pins.

Written by Fernando DeMoura,

Diesel Control Service LLC.


Phone 412-327-9400