# Twelve Volts Of Sag Explains The Dead Slide A slide that hums, moves an inch and stops is usually a power problem rather than a mechanical one. Rested at the battery the bank reads healthy. Loaded by two slide motors it sags to twelve volts at the controller, and the controller stops rather than damage itself. ## A resting reading proves almost nothing about a slide Owners arrive having already tested the batteries, and the test was a meter across the posts with everything switched off. That reading tells you the state of charge and nothing about the bank's ability to deliver current. A tired flooded battery, a sulphated AGM and a healthy bank all read within a tenth of a volt of each other at rest. The difference between them only appears when something asks for real current, and a slide room is one of the largest single loads on the house side of a coach. So the useful reading is taken while the slide is trying to move, with the meter leads at the motor or the controller rather than at the battery. On a healthy system that number stays high and steady through the whole travel. On a marginal one it collapses the instant the motors load, recovers the moment the switch is released, and then reads perfectly fine again by the time anybody thinks to measure. That recovery is precisely why the fault gets misdiagnosed: every measurement taken after the fact looks normal. ## Cable length, crimps and a return path nobody designed for age Voltage lost between the battery and the motor is lost as heat in the run, and there are three places it goes. Length and gauge come first, because a slide at the rear of a forty foot coach may sit thirty feet of cable away from a bank in a front bay, and the factory sized that cable for a new battery and a new motor with no margin for either aging. Connections come second: every crimp, every blade fuse holder, every ring terminal on a bus bar adds a small resistance, and a coastal coach has a green tint inside crimps that were never sealed. The return path is third and it is the one owners never look at. Current has to get back to the battery negative, and on many coaches that happens partly through the frame, through a bolted ground stud, through a bracket painted at the factory. A slide motor pulling hard through a corroded return sees the same voltage loss it would see from an undersized feed, and no amount of attention to the positive side finds it. Measuring both sides separately, with the load applied, is what separates the two. - Battery post to controller, measured under load, not at rest - Controller to motor, measured under load on the same travel attempt - Motor negative back to battery negative, measured as a drop rather than as continuity - Each blade fuse holder checked for heat after a travel attempt - Every bolted ground stud on the run inspected rather than assumed ## Undervoltage lockout looks exactly like a failed motor Modern slide controllers protect themselves. A Schwintek or Lippert In-Wall controller running two motors in sync watches current and voltage on both channels, and when the supply falls below its threshold it stops, because attempting to drive a motor on low voltage draws more current, not less, and cooks windings. What the owner sees is a room that starts, stops, and then refuses to try again for a while. What the owner concludes is that the motor or the controller has failed, since both are the parts that visibly did nothing. Some controllers report the reason and most do not report it in any form an owner can read. On a system with a diagnostic display or a fault code sequence, the code often separates an undervoltage stop from a stall or an obstruction, and that single piece of information is worth an hour of labour. On a system with no display at all, the substitute is a meter on the supply during an attempt, which is the same measurement done manually. Either way the finding has to come before parts, because a controller replaced on a sagging supply behaves identically to the one it replaced. ## Plugged in and still short of voltage The most confusing version of this arrives from an owner sitting on shore power. The coach is plugged into a pedestal, so the assumption is that power is not the issue, and the slide still refuses. On most coaches the slide is fed from the house battery regardless of what the shore cord is doing, and the converter sits alongside the battery rather than replacing it. If the bank is dead or disconnected, the converter alone often cannot supply the inrush current two slide motors want, so the room behaves exactly as it would with no shore power at all. A converter with reduced output makes it worse in a way that is genuinely hard to see. It keeps the coach lit, the pump running and the fridge board happy, so nothing looks wrong until the largest load in the vehicle asks for current. That is the overlap between this fault and a charging problem, and it is why a slide complaint on a coach that lives plugged in at a storage lot ends up as a converter finding more often than owners expect. The measurement is the same: read the supply at the controller while the room is trying to move. ## Two readings that separate power from mechanism Anyone with a basic meter can narrow this down before booking. First reading: leads on the battery posts, someone holds the slide switch, and watch what the number does in the first two seconds. A drop of a few tenths is normal. A collapse toward twelve volts or below is the finding, and it points at the bank or at everything upstream of the motors. Second reading: leads at the controller or motor supply, same test, same moment. A number materially lower than what the battery showed under the same load means the loss is in the wiring between them. The pattern of results is the answer. Battery holds and controller holds means the supply is fine and the problem is mechanical, which moves the job toward the drive assembly, the rails or an obstruction. Battery collapses means the bank. Battery holds while the controller sags means the run: cable, fuse holder, connection or ground. Photograph the meter display each time with the vehicle in frame, note the ambient temperature, and bring the photographs. That set of readings routinely turns a two hour diagnosis into something much shorter, and diagnostic time is billed by the hour either way. - Reading one: battery posts, under load, watched for the first two seconds - Reading two: controller or motor supply, same attempt, same conditions - Both readings photographed with the display legible - The state of charge before the test, and whether the coach was on shore power - Whether the room had already been attempted several times that day ## The parts owners buy before they measure A predictable set of parts arrives with coaches whose owners went shopping first. A motor, because the motor made a noise and then stopped. A controller, because a forum thread said controllers fail. Sometimes both, plus a set of batteries bought at a warehouse store on the theory that new batteries cannot hurt. That last purchase is the one that stings, because a new bank installed onto an undersized or corroded run sags in exactly the same way the old bank did, and the owner has now paid for batteries and still has a dead slide. Parts are also frequently correct. Schwintek motors do fail, and a room that has been forced against an obstruction long enough will strip drive teeth. The point is only that the measurement is cheaper than the part in every case, and it is non-destructive, and it is refundable in the sense that diagnostic time is credited against an authorised repair while a fitted electronic component is not returnable. Owners who bring a bag of new parts and a set of readings get a much shorter conversation than owners who bring only the bag. ## Signs the low reading is the result and not the cause A sagging supply is sometimes downstream of a mechanical problem rather than upstream of it. A room binding in its opening, a rail out of adjustment, a seal rolled under, or a coach parked out of level all raise the force the motors have to produce, and higher force means higher current, and higher current pulls the voltage down through a run that was adequate for a room moving freely. The reading looks identical to a weak bank. What separates them is behaviour: a power fault gets worse as the bank discharges through the day, and a mechanical fault appears at the same point in the travel every time. That last detail is the most useful thing an owner can observe. A room that always stops nine inches out, on a full bank, on shore power, in the morning, is telling you about geometry rather than electricity. A room that went out fine yesterday and refuses at the end of a long weekend without a pedestal is telling you about charge. Write down where in the travel it stops and how many inches, because that number decides whether the coach is booked as an electrical ticket or as slide mechanism work, and those are different bays and different rates. ## Related - [/services/roof-slide-awning/slide-out-repair/](https://ocrv.guru/services/roof-slide-awning/slide-out-repair/) - [/services/mechanical-systems/battery-charging-system/](https://ocrv.guru/services/mechanical-systems/battery-charging-system/) - [/services/mechanical-systems/electrical-repair/](https://ocrv.guru/services/mechanical-systems/electrical-repair/) - [/prices/](https://ocrv.guru/prices/) --- Source: https://ocrv.guru/blog/systems-and-power/twelve-volts-of-sag-explains-the-dead-slide/ OCRV Center, 23281 La Palma Ave, Yorba Linda, CA 92887. (949) 799-3387. Serving Laguna Niguel, California. All work performed in shop. No mobile or roadside service. Licences: California Bureau of Automotive Repair ARD00288521; EPA CAL000367879. Published ranges are ranges, not quotes. Every figure traces to https://ocrv.guru/prices/.