How does the DEF Derate slow the Cummins Engine

[jerryd13]

So, the DEF system correctly or incorrectly has sent a DERATE COMMAND and you now are operating a vehicle down a busy highway at 5 MPH, just a bit faster than walking.


So, what happens to make your Cummins engine be throttled back?


? Does the ECM Slow the fuel delivery, if so by what means.
? Does the ECM mechanically restrict the throttle lever.
? Does the ECM reduce the rate of electrical fire pulses to the injectors.
? What doe the ECM actually change to slow or restrict the Cummins engine.

[Archer2]

That’s a good question. I am not 100% sure of how Cummins accomplishes the deration. But I have a strong suspicion that at least part of the answer is by manipulating the Variable Geometry turbocharger control. I say this because we found out that the emissions control system operates on a separate J1939 network than most of the rest of the vehicle control systems. The VGT is also on the same, separate network as the DEF sensor along with the DPF and SCR systems.

[jharrell]

Originally Posted by jerryd13

? Does the ECM Slow the fuel delivery, if so by what means.

Fuel delivery is controlled by injector timing. The common rail is pressurized and the amount of fuel delivered to cylinder is based on how long the injector is open. The ECU controls the injectors timing.

Originally Posted by jerryd13

? Does the ECM mechanically restrict the throttle lever.

There is no throttle lever unless you go back to mechanical engines like the 12v, otherwise engines have a throttle position sensor on the engine actuated by cable from pedal that gave the ECU a value, newer engines use a pedal position sensor on the pedal itself sending demand to ECU. Both basically tell the pedal position to the ECU and it decides what to do with it.

Originally Posted by jerryd13

? Does the ECM reduce the rate of electrical fire pulses to the injectors.

Yes this is how the ECU controls fuel delivery, modern diesel have multiple injection events, the computer opens the injector multiple times per stroke for varying lengths to precisely control fuel burn. By reducing the total time the injector is open less fuel is delivered to the cylinder lowering power output.

Originally Posted by jerryd13

? What doe the ECM actually change to slow or restrict the Cummins engine.

Reduce fuel delivery using injector timing and lower boost pressure using the VGT and waste gate.

[jerryd13]

So, my quest is to provoke thinking and questions that will assist all Diesel RV owners to fully understand the unsafe DERATE of the DEF system when the Computer, ECM gets which ever specific condition it has been programmed for constituting a DERATE.


Some likely events, these are my guess only, I am looking for accurate
for sure DEF ENGINE DERATE CONDITIONS, on various year 2011- 2018
Cummins 8.3 ISC engines, and if the chassis matters, please note the chassis that the DEF equipment is related to. I know we have different DEF component manufacturers between Freightliner and Spartan, and perhaps others, this mumbo jumbo allows for more confusion. Plus, as far as I know, there is no model or type number on most DEF systems, so if you reply, please be specific, brand, model and year of RV and Chassis and which engine.


If we the owners communicate we can better understand the causes of DEF DERATE, and find useful solutions to both prevent and or get a safe exit path in times of a cricis.

• LOW DEF FLUID LEVEL
• INCORRECT DIFFERENTIAL AIR FLOW THRU PARTICULATE FILTER
• INCORRECT NOX SENSOR SIGNAL
• DEF DOSING PUMP FAILURE
• DEF DOSING VALVE FAILURE
• DEF INJECTOR FAILURE
• INCORRECT TEMPERATURE AT VARIOUS SENSORS.