​DDEC III and DDEC IV fan problems.

Fernando DeMoura
July 2016

It’s right in the middle of June as I write this article and temperatures in many parts of the US are over 100 degrees. I can hear an engine-cooling fan roaring in the distance from my building here at the ECM lab. The sounds I hear are coming from the I-76 PA Turnpike and lots of trucks are working hard and lots of fan clutches are cycling today. Anytime your engine fan clutch is engaged you’re losing fuel mileage. This is one reason why most trucks made since 1998 have an ECM controlled fan. As engine RPM increases so do the losses in performance due to the increased load of turning that fan. Chassis dynamometer tests have shown that this loss can be as high as 40 horsepower at the wheels! This isn’t always a bad thing if you can control it. Most ECMs can be programmed to engage the fan when the Jake is on high to increase the hold back power of the Jake. A properly functioning ECM controlled fan is smarter than old switch based systems and only engages when the fan is needed. An ECM controlled fan can also engage if the charge air temperature gets too high helping the engine make more power so there is a fuel mileage and performance advantage to having an ECM controlled fan.

Here’s the downside…they’re more complex and can be harder to diagnose and repair without the proper tools. A common problem I routinely fix are fan control problems with DDEC IIIs and early DDEC IVs. DDEC IIIs and early DDEC IVs have two circuit boards. One circuit board lies on the bottom of the ECM case and is responsible for fan and Jake control. If oil, fuel, or water gets into the ECM this board is the first to go. Newer DDEC IVs don’t need this lower circuit board and are less likely to have this problem but if your fan won’t shut off there is a chance it could be the ECM. An easy way to check is to unplug the fan solenoid and supply power to the fan solenoid and see if it releases the fan.

Most applications are set up so that if there is an electrical or ECM failure the fan clutch engages. This is something every driver should know. If you ever have an overheating situation and your engine fan isn’t engaging all you have to do is unplug the two pin electrical connector to the fan solenoid. As long as your fan clutch isn’t bad your engine fan will engage and stay engaged overriding any electrical or ECM controls. If you have an older truck with a switch based fan system and you aren’t afraid to run some wires you may be able to convert to an ECM controlled fan and gain mileage and performance depending on what ECM you’re running and how it’s programmed.

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