RV Chassis Battery and Charging: No Crank, Slow Crank, No Charge

Which of the three you have

Before anything is measured, the sound of the engine narrows it to one of three faults.

• Nothing happens when the key turns. No crank, no click, or a single heavy click. That is the starting circuit: the battery, the cables, the relay or the starter.
• It cranks, but slowly. The engine turns over more slowly than it used to, or it turns and then stops. That is almost always the battery or the connections to it, and it is the case a jump start fixes.
• It cranks normally and does not fire. The starting and charging circuit is working. That is a fuel, ignition or engine-management fault, and it is not this page.

If the dash shows a charging message, that is a fourth thing. Ford's message is "Check Charging System", and the manual's own gloss is that "The charging system needs servicing." The engine may still start and run on the battery alone, which is why the message is easy to ignore and should not be.

The chassis battery, and why it is not the house bank

A motorhome carries two batteries with two jobs, and they are not interchangeable. Winnebago, which wires both sides, describes the difference: "a chassis battery (commonly referred to as the cranking battery) is designed to provide high amperage / current to start an engine." The house bank is a deep-cycle set, built to be drawn down slowly and recharged, and the two want different treatment.

That is also why they are kept apart. The same document explains what happens when they are not: "The constant 14+ volts from the alternator tends to overdrive a deep-cycle battery and results in a creation of undesirable heat." A device sits between them, and its whole job is to decide when the alternator is allowed to charge the house bank and when the two are separated.

The owner-level check on the chassis battery is the same as on any battery, and Spartan's manual states it in one line: "Inspect batteries for any sign of loose connections or corrosion at the terminals. Assure the battery is securely mounted." A terminal that is loose or furred with corrosion behaves exactly like a battery that is going flat.

The isolator: what closes it, and when

The device between the banks goes by several names, which is why searching for it is hard. Winnebago lists them: "the Dual / Boost solenoid, the Battery Boost solenoid, the Dual / MOM solenoid, the Aux (Auxiliary Start) solenoid, and the Emergency Start solenoid." They are the same part doing the same job.

Intellitec publishes the exact voltages its controller works to. The isolator closes on this condition: "When this voltage goes above 13.3 volts for approximately 12 seconds, as happens when the engine is running normally (normal alternator output voltage is approximately 14.4 volts), it will close the isolator relay providing charging current to the coach battery. When the ignition switch is turned off, the relay will open immediately."

Two things follow from that, and both are diagnostic. The first is that the house bank only charges when the chassis system is above 13.3 volts, so a weak alternator shows up as a house battery that will not charge rather than as a chassis fault. The second is the overload behavior: "If the voltage should fall below 12 volts for more than two seconds while the ignition is on, the relay will drop out to feed all the alternators available output to the chassis battery to keep the engine running." Intellitec names the symptom: "The resultant flickering of lights would alert the driver of the system overload." Lights that pulse while driving are not a lighting fault.

When the coach battery will not charge, Intellitec's own troubleshooting order starts at the chassis side: "With engine running, chassis voltage must be above 13.5 volts", and "If less than 13.3 volts, check vehicle's charging system." If the voltage is there, the next check is the relay itself, and the maker gives the tolerance: "If the drop is greater than 0.3 volts, replace relay."

The boost switch, and when it will not work

The boost switch is the owner's own tool for a flat chassis battery: it joins the house bank to the chassis battery so the coach can start itself. Intellitec publishes the condition it needs: "If when starting, the chassis battery voltage falls below 9 volts for about 1/2 second and the coach battery is above 9 volts, the isolator relay will be closed and held until the starter switch is released."

That sentence explains the case where the switch does nothing. It needs the house bank to be above 9 volts. A coach that has been sitting with the house batteries flat has nothing to give, and the boost switch will feel dead because it is. Charging the house bank first is the fix, not replacing the switch.

The alternator, and the dash message

The alternator's output is a range rather than a number. Winnebago puts present-day alternators in the low to mid 14-volt band, and calls that "a voltage threshold that present day chassis / starting batteries find friendly." Intellitec gives the working figure from the other end of the same circuit: "normal alternator output voltage is approximately 14.4 volts" A charging system that reads in the twelves while the engine runs is not charging, whatever the dash says.

Ford's fuse chart names the alternator's own circuit. Fuse 29 is "10 A" and protects "Alternator 1 A-line." A charging fault that is really a blown fuse looks exactly like a failed alternator, and the fuse is the cheaper of the two to check.

The loads the alternator carries are the same ones the coach runs on, and the watts to amps calculator turns an appliance label into the draw each one becomes.

The dash message lives in a different table. Ford files "Check Charging System" under a heading called "Battery and Charging System", which is the group the message belongs to rather than a fuse position.

What drains a chassis battery while it sits

A chassis battery that goes flat over a few weeks of storage is usually doing what it was left to do rather than failing. Two things are worth checking.

The first is the isolator itself. Intellitec's test for a drain is specific: "Check voltage on module with ignition off", where it should read zero, and "the relay should be open with no voltage applied to coil." A relay that is stuck closed joins the two banks permanently, so every house load draws from the chassis battery as well.

The second is storage itself. Spartan's manual is direct that a battery left alone will not stay charged: "If your chassis is not equipped with such a battery charger, you must provide a means of keeping the batteries properly charged during periods of inactivity." A maintainer is the answer, and it is cheaper than a battery.

How long a bank carries a given load is the battery runtime calculator's question, and the answer decides whether a maintainer is enough or the bank is finished.

What is not owner work

Three jobs sit past the line this page draws.

• The isolator module itself. Intellitec's own instruction is that "All servicing of this module should be done only by a qualified Service Technician." The module is connected directly to both batteries, and the maker warns that its full power is available at the terminals.
• The alternator. The belts and the connections are owner checks; the alternator is a service part, and on a motorhome it is reached from underneath or through the floor.
• The starter. A no-crank fault that survives a known-good battery and clean terminals is a starting-circuit fault, and the starter on a motorhome is a heavy part in an awkward place.

What is owner work is everything before those: the terminals, the battery's state of charge, the boost switch and its condition, the charging voltage at the battery, and the fuses.

What it costs

The cost of a starting fault is decided by which of the three it is, and the cheap ones are the common ones.

At the bottom is the diagnosis and what it usually finds: a corroded terminal, a battery that needed charging, a maintainer that was never fitted, a fuse. None of that is a major part.

In the middle is the battery itself, which is a normal wear item and a normal price, and the isolator relay, which is a modest part on most coaches.

At the top is the alternator or the starter, and both are the outcome to arrive at last rather than first. The checks above decide whether either is actually needed, and the charging message on the dash is the one that points at the alternator rather than at the battery.

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