Bosnian Heavy Trucks Review Transmission Torque Requests and CAN Data When Engine Fueling Is Limited During Shifts Despite Normal Injector Tests

Modern heavy trucks do not control the engine and transmission independently. During gear changes, the transmission controller may request a temporary reduction in engine torque so the shift can occur smoothly and protect driveline components.

In Bosnia and Herzegovina’s heavy-truck repair market, this communication can create an injector-like complaint when fuel quantity drops during shifting even though the injectors themselves test normally.

Torque Reduction Can Be Intentional

During an automated or electronically controlled shift, the transmission may send a request through the vehicle communication network.

The sequence can be:

Shift required → Transmission requests torque reduction → Engine ECU reduces fuel → Shift completes → Torque is restored

A short reduction can be normal.

The diagnostic problem begins when torque remains limited or the request becomes inconsistent.

Look at the Timing of the Symptom

Technicians should determine whether power loss occurs:

• only during shifts;
• immediately after shifts;
• in specific gears;
• under heavy load;
• or continuously.

A symptom tightly linked to transmission activity points toward cross-controller communication rather than the injector alone.

Compare Torque Requests

Where diagnostic tools provide the data, useful parameters may include:

• driver-requested torque;
• transmission-requested torque reduction;
• engine permitted torque;
• actual engine torque;
• fuel command;
• and gear state.

The goal is to understand who is asking the engine to reduce power.

CAN Communication Is Part of the Control Chain

Engine and transmission controllers exchange information through the vehicle network.

Problems involving:

• network communication;
• invalid messages;
• controller configuration;
• wiring;
• or transmission faults

may influence engine torque strategy.

A fuel-delivery complaint can therefore originate outside the engine ECU.

Why Injector Testing Can Be Normal

If the ECU intentionally reduces injection quantity, the injectors may simply execute that command correctly.

This is a crucial distinction:

Reduced injection command is not the same as insufficient injector capacity.

Separate Normal Shift Strategy From Abnormal Limitation

Technicians should not treat every torque reduction during shifting as a fault.

The important questions are:

• Does torque return after the shift?
• Is the reduction appropriate to the transmission state?
• Are transmission faults active?
• Does the engine remain limited after the request should have ended?

Heavy-Truck Diagnostic Value

In vehicles with automated transmissions, diesel troubleshooting increasingly requires cooperation between engine and driveline diagnostics.

A workshop focusing only on the fuel system may miss the controller that initiated the torque reduction.

Industry Interpretation

The Bosnian heavy-truck example illustrates a cross-controller diagnostic model:

Transmission state → CAN torque request → Engine torque permission → Fuel command → Injector actuation

Before replacing injectors, technicians should confirm whether the ECU was actually allowed to request more fuel.

FAQ

Can a transmission controller reduce engine fueling?

Yes. Electronic transmission systems can request engine torque reduction during certain operating conditions.

Does normal injector testing rule out a low-power complaint?

No. Other controllers may still be limiting the fuel command.