​How Wiring Harness Connectors Can Damage A Good DDEC ECM

Fernando DeMoura
October 2018

Years ago I remember these toy animals that would grow three or four times in size when dropped into water. They were simply animal shaped sponges that would absorb water and expand. When these toys would expand they would soften and tear easily. No amount of drying in the sun could bring them back to their original size. The same thing happens to the inner and outer silicone seals protecting your ECM in your wiring harness connectors when the seals are exposed to fuel for long enough. These seals protect the harness and ECM pins from contact with water. If these pins stay wet and are powered up for long enough they will turn into green dust.

I did an article about a year or so ago talking about why ECMs are not completely waterproof. Most ECMs have a hole in them, a breather, to equalize air pressure inside the case when the ECM heats up, cools, and changes elevation. With DDEC III and DDEC IV boxes the breather is the same hole the engine side harness bolt threads into. Fuel, oil, and water intrusion coming through the breather on a DDEC isn’t as common as other seal failures in DDECs but earlier this month I saw two no start/no communication DDEC failures that ended up failing twice within a few months because of water intrusion through the connector breather. I’ve tried plugging this breather before but just like a crankcase building excessive crankcase pressure the seals and orings blow out as soon as things heat up. You wouldn’t want your technician blocking your crankcase breather and you don’t want me to block the breather in your DDEC so as of today there is no way to make a DDEC III or DDEC IV completely waterproof. Also, don’t pressure wash the case or the connectors. These seals will not hold back that much pressure even if they are new.

I repaired these two boxes a second time at my cost but explained to these guys that they needed to change their wiring harnesses because I wasn’t going to cover repairing these boxes a third time.

Three things you can do to avoid this kind of ECM damage.

First, inspect your seals and replace your outer seals if they have absorbed fuel or if your inner seals are bad replace the wiring harness. Second, check the connector housings on the wiring harness. If the nylon is cracked or warped because someone over torqued the bolt, replace the harness. Third, make sure the bolt isn’t cross threaded and make sure the bolt seats in the connector housing. If it looks like it didn’t seat the connector housing will act like a straw and suck water right into the ECM when the air inside the ECM cools off.

Written by Fernando DeMoura, Diesel Control Service
Phone 412-327-9400.