​Is Your DDEC III Or DDEC IV ECM Holding Out On You? Here’s How To Tell Without Going To The Shop

Fernando DeMoura
August 2018

I get a lot of calls from Series 60 owners who aren’t satisfied with their engines power. Most of the time the DDEC takes the blame and it’s understandable as to why. Most Cummins and Cat ECMs set a check engine light if the ECM is holding back fuel because of a derate or as it’s more commonly known in the automotive industry as going into limp mode. DDEC ecms don’t always set faults when they hold back power. For years I’ve seen this baffle technicians and Series 60 operators alike

Here are just a few common derates that won’t set a check engine light in most DDEC software versions:

•Vehicle speed sensor errors - especially if progressive shift or vehicle speed sensor tampering protection is enabled. RPM and maximum torque can be limited if the signal from the sensor on the tail shaft of the transmission starts to get screwy.

•ABS errors - derate requests coming from the ABS because the ABS thinks the drives are slipping. There is a glitch with Diagnostic link software that sometimes identifies ABS faults as DDEC faults even though the DDEC check engine light is turned off.

•Automatic transmission throttle control/derate requests – If you have an automatic and depending on your configuration you’re often not in direct control of your throttle positioning sensor, and if you are your transmission can still override your input through the datalink. So transmission errors can cause your DDEC to hold back power.

•Low Boost sensor reading – This one is my favorite and is by far the most common reason your DDEC will hold out on you. It could just be the sensor, it could be the wires going between the sensor and the DDEC, it could even be the DDEC’s hardware or software but usually it’s just a boost leak.

You might think mailing me your ECM or bringing your truck into a shop would be the best way to find problems like this but that’s not the case here. Most of these problems aren’t obvious without a road test or a dyno run. So what’s the easiest way to know if your DDEC is cutting power even though you don’t see a check engine light? A basic bare bones datalink reader. If you don’t already have one a basic J1708 Scangauge will display percent throttle and percent load. Last I checked it was about $150 on amazon. There is also an improved version co developed by Keven Rutherford and Scangauge known as the Scangauge KR. The best price I’ve found is actually on my former employer’s website Pittsburgh Power. Last I checked they had it for $197. These datalink readers are a no brainer. They will save you time and money by narrowing down the search for your problem.

The key is in these two parameters.

Percent Throttle (TPS) – How much power you want (It’s normal to see this number max out at 97 percent.)

Percent load (LOD) – Percentage of available power in that DDEC program that the DDEC is giving you.

Keep an eye on them as you drive and you’ll see if your DDEC is holding out on you.

Written by Fernando DeMoura
Diesel Control Service LLC.