Ad-blue line

AdBlue lines connect the urea tank, supply module and dosing injector in the SCR (Selective Catalytic Reduction) system. A leak, restriction or faulty line heating disrupts the supply of urea solution, causing dosing errors and emission faults. On MAN and Mercedes trucks, buses and vans, these faults can eventually lead to torque limitation or starting restrictions. Targeted checks prevent a correctly functioning supply pump or dosing injector from being replaced unnecessarily.

There are suction lines, pressure lines and, depending on the system layout, return lines. Versions may be unheated or fitted with integrated electrical resistance heating elements. AdBlue contains 32.5% urea and freezes at around -11 °C. The line heating and any automatic purge function must therefore match the original system configuration. Replacing a heated line with a standard hose causes supply problems in winter conditions.

Always select parts using the vehicle’s chassis number. Visual comparison or selection by model name alone is never sufficient. On MAN TGS and TGX models with D26 engines or Mercedes Actros models with OM 471 engines, for example, line routing varies according to emissions specification, wheelbase and tank position. The exact aftertreatment configuration must also be checked for Lion’s City, Citaro, TGE and Sprinter.

Important selection parameters include internal diameter, length, minimum bend radius, connection fit, seal geometry, and pressure and temperature resistance. Heated versions also require the correct supply voltage, connector pinout and heating characteristics. Operating pressure varies between dosing systems; use only the vehicle-specific specification. Line plastics, such as polyamides approved for this application, and sealing materials must be resistant to AdBlue. Do not use universal fuel hoses or fittings made from copper, brass or uncoated steel.

White urea deposits around quick-release couplings indicate dried fluid, but may also be left over from previous spills. Other signs of faults include chafing at chassis brackets, heat-induced hardening near exhaust components, kinked lines and corroded heater connectors. A suction-side leak can draw in air without any obvious external fluid loss. This prevents the supply module from building up sufficient pressure.

1. Read the fault codes and recorded operating conditions. Compare the requested and measured supply pressure, fluid temperature and heater control. Check whether the problem only occurs in freezing conditions.
2. Allow hot exhaust components to cool down. Carefully clean off external crystal deposits and inspect connections, protective sleeves and mountings again during the specified actuator test.
3. Check pressure build-up and retention according to the manufacturer’s procedure. Where specified, use an AdBlue-resistant pressure gauge with a suitable adapter. A pressure drop does not automatically prove that a line is leaking: also rule out internal leakage in the supply module or dosing injector.
4. With the heating circuits disconnected and de-energised, measure their resistance using a multimeter. Compare it with the manufacturer’s temperature-dependent specification. Then check the supply voltage under load and measure current draw with a clamp meter; a correct no-load voltage does not rule out excessive contact resistance.

Allow the specified run-on or purge cycle to finish completely and depressurise the circuit according to the workshop procedure. Wear gloves and eye protection. Cap open connections immediately to prevent dirt ingress. Replace damaged line assemblies and the specified seals; do not repair integrated heating sections with universal hose couplings.

Install the lines without strain in the original holders, observe the minimum bend radius and replace any missing heat shields and chafe protection. Check that quick-release couplings are fully locked. Then carry out the specified filling, bleeding and dosing tests, and verify leak-tightness, heater operation and any purge function. This ensures that the line is not simply replaced, but that the cause of repeated failure is also addressed.