​Is Your DDEC III Series 60 Throwing A Fit At 1800 Rpm? The Cause Might Surprise You.

Fernando DeMoura
December 2018

About 8 years ago I was part of a team whose focus it was to power a 389 glider with a 12.7 Series 60. Back before harnesses with converter boxes came to market this was something that as far as we knew hadn’t been done before. Our local Pete dealer said it couldn’t be done. “There hasn’t been a Detroit in a Peterbilt in over a decade.” The dash of the 2009 389 is computer controlled and the only way to get it to talk to the 1998 DDEC was through multiplexing. It was my responsibility to span that 11-year technology gap and get the ECM and the computer in the 389 dash to talk to one another. At that time multiplexing was new to me and I wasn’t sure what kinds of problems I’d see. The Series 60 was getting a special camshaft and injector combination. The injector height settings were also being tinkered with.

Our team was a group of professionals that were passionate about their work and one day the shop foreman and I got into a heated discussion. “Something electrical in that 389 is screwing with the injection timing!” I told him the bump idle, the park brake, and throttle were the only input controls that could change the engine’s injection timing south of the firewall. It wasn’t till years later I realized I was wrong.

Three weeks ago, a local owner operator told me he needed to get his hands on a DDEC III as soon as possible. He came by the lab with his DDEC III ECM in hand. He said his box was bad and wanted to use it for a core credit. “If I run it up to 1800 it starts to smoke and backfire. When I tried another ECM, it ran all the way up to 2100.” If possible, I like to see the core BEFORE I sell a replacement ECM to anyone. I like to verify that the core is the cause of the problem. That way I know by selling you a replacement the problem you have will be fixed.

Instead of just selling him an ECM I decided to run his core ECM through a simulation at full load turning 1850 rpm. It ran fine. After looking through his core’s diagnostic logs I saw an inactive vehicle speed sensor failure fault that had recently been active and stayed active over the last 35 engine hours.

If you don’t have a speed limiter enabled in your DDEC programming a vehicle speed sensor failure will just set a fault, but if you do have one enabled this fault will cut back the fuel until rpm is down to 1800 rpm or even as low as 1600 rpm. When a vehicle speed sensor fault goes active a DDEC IV will cut all electrical power to the injectors to get the RPM down but a DDEC III doesn’t do that so gracefully. The DDEC III reduces injection duration but the injectors don’t actually shut off. Instead they fire off randomly and generally have a fit. This is normal for a DDEC III even though it sounds like the ECM is in its death throws on both my test bench and on a live engine. I told my customer his ECM was fine and cut him some 18-gauge stranded, 2 conductor cable and sent him on his way. If a new sensor doesn’t fix his problem the harness that runs over the transmission and picks up the tailshaft signal will do it.

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


A fresh run of 18-gauge stranded wire ran between the transmission tailshaft sensor and pins E2 and E3 on a DDEC ECM can often cure vehicle speed sensor signal issues.

Flash code 54 is a vehicle speed sensor fault that indicates a condition that can cause a DDEC III to fire its injectors erratically at 1800 RPM.

The loss of the vehicle speed signal above 1800 RPM is causing this DDEC III to fire injectors #1, #2 and #4 at the same time.