So the starter does not turn the engine when the key is turned to start?
Although your coach may be different than mine, the following is common to most Class A coaches so I apologize in advance if this information doesn't exactly match your 2009 Phaeton. This information applies to my Freightliner XCM Chassis.
If the starter motor fails to engage there are four areas that need to be checked to isolate the cause. These areas are Starter Power, Start Relay/Solenoid/Starter, Ignition ON Circuits, and Ignition START Control.
Starter Power is provided by the Chassis Batteries. Areas of concern are:
Low Battery Voltage caused by discharged battery or a defective battery. Chassis battery voltage should be over 12.7 volts when fully charged.
Loose connections or corrosion, particularly battery grounds. Trace the heavy RED (+12 volt) and BLACK (Ground) battery cables from the batteries to the starter. The RED cable will pass through one or more Fuses.
Bad starter power fuses. Your coach could be different but on my coach the chassis battery power lines pass through two fuses. The first fuse is a 300 Amp battery terminal fuse mounted to the +12 side of the chassis battery.
The next fuse in the starter line is located in the a large power distribution panel with heavy cables and a large 150 Amp starter power fuse that goes directly to the starter.
On engine, locate the starter motor and check a solid ground cable. Some starters use the starter frame ground but if there is a starter ground cable check both ends for a solid connection.
The starter motor should have a good ground and a solid 12.7 or more Volts on the heavy starter power connection at all times. If good, the next thing to check is the Ignition ON circuits.
If you have the knowledge and skill, it is possible to bypass all of the start circuitry and directly energize the starter with a jumper to determine if the issue is in the starter or in the starter control circuits.
If the starter is failing, sometimes a sharp rap or two on the starter case can get a failing starter to engage once or twice but if so, the starter needs to be replaced as soon as possible.
Ignition ON - When you turn the Ignition Switch to the ON position, several events should take place. The instrument panel should light up. The Engine Control Module should power up, for Diesel engines, engage the Intake Grid Heater, and extinguish the start heater indicator on the instrument panel. The Transmission Control Module should power up and enable the Transmission Panel at the driver’s controls.
In order for these events to take place, a sequences of fuses and relays provide power to the Instrument Panel, Engine and Transmission Control Modules (computers).
The fuses and relays involved will have names similar to the following:
At the dashboard, an Ignition Switch Power Fuse provides +12 volts to the Ignition Switch contacts.
An Ignition Relay Power Fuse +12 volts to the Ignition Relay contacts.
The Ignition switch can then provide power to Accessories, such as the radio and there may be an Ignition Relay Coil Power Fuse to energize the Ignition Relay.
The Ignition Relay can then take +12 volts from the Ignition Power Fuse to Instrument Panel Fuse(s). In addition it provides power to several Ignition Fuses, usually in the engine compartment, for the Engine computer, Transmission Computer, and additional Ignition relays. We are now ready to engage the starter switch.
There may be several fuses in the engine compartment with names like XMSN BAT or TCM BAT that provides a constant +12 volts battery power to the Transmission Control Module to preserve computer settings. Likewise, there may be fuses called ECM BAT (or something similar) to provide a constant +12 volts battery power to the Engine Control Module to preserve computer settings.
Ignition START Control – To engage the starter we need a start signal from ignition key which may locked out if the engine is already running, and/or if the transmission is not in neutral.
If the engine is already running there may be a Starter Disable Circuit that energizes a Starter Lockout Relay and prevents the start circuit from energizing.
In or near the engine compartment, if the Transmission Control Module is powered up AND the transmission is in Neutral, the Transmission Control Module will use the Neutral Start Relay to enable the start control to be passed to the Start Relay near or on the Starter Motor and if the Starter Motor is good, the starter engages the engine and attempts to start it.
Your coach should have relay and fuse similarly named (and possible abbreviated names) to provide these functions.
Even if you don't have spare fuses or relays, in many cases you can find more than one identical fuse or more than one identical relay that can be used (with all power shut down) to swap with the suspect fuse or relay.
Only swap one suspected item at a time and test the results. If the symptoms change then swap those two components back and verify that the issue has returned. Sometimes, simply removing and reinserting a fuse or relay will clean the contacts enough to make the system work. Take notes of all your troubleshooting steps in case the symptoms return.
Hope this helps!
Frank & Corrine Miller - 2011 Discovery 40X
380HP Cummins ISC 8.3L Engine, Freightliner XCM Power Bridge Chassis