Solar Water Pump Repair: What Can Be Fixed and What Should Be Replaced

Solar pump repair is mostly field work: cable joints, PV connectors, fuses, the check valve, riser pipe joints and a worn pump end can all be put right without buying a new system. Two parts usually cannot: the sealed submersible motor and the electronics inside the controller. Those two are normally replaced, and the question that matters is whether the warranty still runs.

This guide starts after the fault has been found. If you are still working out which part has failed, use our symptom guide to solar pump faults first.

What Can Be Repaired and What Gets Replaced

Lorentz states that there are no user-serviceable parts inside the motor or the controller of its PS2 pumping systems. Franklin Electric tells installers not to repair a unit unless its manual describes the step.

Repair in the field. Cable splices, PV connectors, fuses and surge protectors, check valves, riser pipe joints, float switches and level probes, loose terminals, and a single cracked module swapped for a matching one.

Repair by module. The pump end is the stack of impellers and bowls above the motor. It unbolts from the motor and is replaced whole, or rebuilt with new stages, bushes and shaft.

Replace as a unit. The submersible motor in most solar sizes, and the controller. Repair of either belongs to the manufacturer or an authorised service centre, and an attempt by anyone else can end the warranty.

Where the repair line sits on a solar pumping system. Manufacturers state there are no user-serviceable parts inside the motor or the controller. Source: LORENTZ PS2 manual; Franklin Electric AIM manual 2026; MIMAH engineering practice.

Test Before You Pull Anything

A pump pulled for no reason costs a day and risks dropping the string, so test from the surface first.

A PV array is live whenever the sun is on it. Open the isolator, wait at least two minutes for the controller to discharge, which is what Lorentz requires, and then disconnect the motor cable at the controller. Measure insulation resistance from each motor lead to earth with a 500 volt tester. If the motor has electronics built into it, read the manual before you put 500 volts on it.

Franklin Electric's manual gives readings that sort the result. With the drop cable attached and the motor in the well, 2 megohms or more is normal for a new motor, 0.5 to 2 megohms is a motor in good condition, and under 0.5 megohms means insulation damage to find and repair. Those are Franklin's figures, so use your own maker's table if it publishes one.

Then measure winding resistance line to line. Franklin says a higher than normal reading on any pair points to an open winding, a bad cable or a poor joint, and a lower one means a shorted motor.

The manual also lists readings for a motor alone, out of the well: 200 megohms or more for a new motor, and 10 megohms or more for a used one fit to reinstall. Once the string is up, split it at the first joint and test each side to tell a bad motor from a bad cable.

Insulation resistance readings with a 500 volt tester, for the motor alone out of the well and for motor plus drop cable in the well. Source: Franklin Electric AIM manual, 2026 edition.

Pulling a Borehole Pump Without Losing It

Pull on the riser pipe, never on the cable. Lorentz specifies a cable fixing every 3 metres and a loop about every 40 metres to keep tension away from the motor cable.

A safety line should already be on the string. Water Mission's solar guide recommends polypropylene rope or braided stainless steel cable, and Lorentz warns against nylon because it takes up water and weakens. The line must be able to hold the whole string with its water. If there is none, fit one on the way back in. It is there to catch a falling string, not to lift one.

Know the weight before you choose the kit, because the riser comes up full of water. Lorentz tabulates that water at 2.0 kg per metre for 2 inch pipe, 4.6 kg for 3 inch and 8.2 kg for 4 inch. Suppose 60 m of 3 inch riser: the water alone is 60 x 4.6 = 276 kg, before pipe, cable and pump. With 2 inch pipe it is 60 x 2.0 = 120 kg. Neither is a job for a rope and a few helpers. Lorentz recommends a crane or hoist. On a light string that can be a chain hoist on a tripod, both rated above the full load.

Clamp every length. Lift one length, set a pipe clamp across the top of the casing so the string cannot slide, then unscrew.

Keep people clear. Lorentz is blunt: never stand under a suspended load.

Stop if it will not move. A jammed pump is a job for a rig.

Pump End: Worn Stages and Sand Damage

Sand wears the running clearances of a pump end until it can no longer build head. Lorentz limits sand content to 50 ppm for its pumps, which is 50 grams of sand in each tonne of water. It says more than that shortens pump life considerably, and a pump blocked by sand is not covered by the warranty.

Replacing worn stages with parts from the same maker is a reasonable repair when the motor, shaft and housings are sound.

If the borehole still produces sand, a new pump end only buys time. Redevelop the well first, as we explain in our guide to solar borehole pumping, and then repair.

Lorentz says helical rotor pumps are damaged by running in reverse or running dry once wetted.

Submersible Motor: Rewind, Replace or Send Back

Submersible motors come sealed and filled. Franklin's are water lubricated with a non-toxic water-based solution, and Lorentz delivers its submersible pumps prefilled with water. An oil-filled motor has a sealed oil cavity instead. In either type, a motor opened in the field has lost its fill and its seals, so do not open one.

Whether a motor can be rewound depends on how it was built. Franklin lists rewindable motors only from 75 hp (55 kW). Grundfos calls its 4 and 6 inch MS motors hermetically sealed, and its rewindable MMS range starts at 5.5 kW in a 6 inch frame (brochure in Swedish). Those are two makers, so read your own data sheet. For most small solar pumps the choice is a new motor or a return to the maker. If yours is marked rewindable and a new one is weeks away, a workshop that knows wet windings is a fair option.

One motor repair is worth doing on site: a damaged motor lead. Franklin says always to use a new lead, because old rubber sets and may not reseal, and to tighten the jam nut to the stated torque, 20 to 27 newton metres for a 4 inch motor with a jam nut. Some larger leads are not field replaceable.

Check the cable before you condemn any motor. Lorentz calls splice failure a common source of error, and a leaking splice looks exactly like a motor fault on the tester. Warranty on pumps and motors commonly runs 2 to 5 years, according to Water Mission's guide, so read the clause before anyone touches the motor.

Cable Joints and Splices

A cable joint is the weakest point in a submersible string, and also the one part you can always repair properly.

Lorentz warns that bad splicing can damage the controller and the pump, and excludes that damage from warranty. Use a splice kit rated for submersible duty and sized for the cable. Cut back to clean copper, follow the kit's instructions, and test the joint before it goes into the water.

Remake a doubtful joint instead of patching it, because each joint is a place for water to enter. Replace the cable when the sheath is cracked, when it has been pinched under the well seal, or when readings stay low after the joint is remade.

Controller and Inverter: Repair or Swap

A failed controller is rarely a field repair. Lorentz sets two rules that are easy to break: no switch between motor and controller, and no connecting the motor wires to a controller that is switched on. Either can destroy the unit, and the warranty excludes it.

On site you can replace DC fuses and surge protectors, clear dust, insects and water from the enclosure, tighten terminals and restore settings. Board-level repair is for the manufacturer or an authorised centre, and for a generic imported inverter a swap is usually the practical answer.

Swap like for like, because controllers are matched to motor type and voltage. Our article on solar pump VFDs and controllers covers why. Write the settings down before a failure: frequency limits, dry-run trip, restart delay.

Water Mission's guide lists typical warranties at 5 years for inverters and power conditioning. Our guide to solar warranties explains what usually voids them.

Rising Main and Check Valve

Whenever the string is out, replace the check valve. It is cheap and it fails quietly.

Franklin asks for at least one positive-sealing check valve, favours a spring-loaded or built-in type because they close quickly, and says never to use a swing type with a submersible motor. It also asks that every threaded joint be tightened to no less than 10 foot-pounds per horsepower of the motor, which is about 13.6 newton metres per horsepower. For a 3 hp (2.2 kW) pump that is at least 3 x 13.6 = 41 N m.

Replace corroded couplings and any length with damaged threads while the rig is there.

PV Strings and Connectors

Connectors are the repair most solar pump owners will meet on the array. Sandia National Laboratories' field work on PV connectors sampled fielded connectors and found operating temperatures from a normal 25 to 27 degrees Celsius up to 95 degrees Celsius on very hot ones. It lists poor installation practice and a lack of compatibility standards between makers before 2020 among the causes of failure.

So replace a suspect connector as a matched pair from one maker, crimp it with the proper tool, and never force two brands together. An hour after the repair, in full sun, compare it with its neighbours using an infrared thermometer.

Replace a cracked module with one of the same electrical class, because in a series string the weakest module sets the current. Then compare open-circuit voltage across the strings, as set out in our guide to solar PV commissioning tests.

Repair or Replace: A Rule You Can Apply

The best-known percentage rule comes from electric motors. A 2008 Plant Services article describes an old rule of repairing only when the cost stayed within 57 percent of a new motor, and argues the line has moved to the mid-60s because a good rewind now holds its efficiency. That article concerns large industrial motors on long running hours, with a 100 hp motor as its opening example. It does not cover submersible motors or pump ends, and its case rests on an energy bill a solar pump does not pay, so we do not carry its number across.

Our working rule compares cost with life: repair when the repair cost as a share of new cost is smaller than the repaired part's expected life as a share of a new part's life.

Take a hypothetical pump end. A new one costs 100 units and is expected to last 8 years in this borehole, which is 100 / 8 = 12.5 units a year. A refurbishment with new stages and bushes costs 55 units and is expected to give 5 years, which is 55 / 5 = 11 units a year. Repair wins, narrowly. If the same refurbishment gives only 4 years, it costs 55 / 4 = 13.75 units a year and replacement wins.

A hypothetical pump end showing why repair cost per year of expected life decides the question. The units are illustrative, not prices. Source: MIMAH engineering practice.

The rig day can change the answer. If either option needs the same pull, add it to both. With a hypothetical 30 units for crew and rig, the 5-year refurbishment costs 85 / 5 = 17 units a year and the new pump end 130 / 8 = 16.25, so replacement wins after all. A shared cost always favours the option that lasts longer.

Age enters through that life figure. In our judgement, a part in the first third of its design life can justify up to about 60 percent of new cost, because most of its life is still ahead. A part in its last third rarely gives more than a third of a new part's life, so the ceiling drops to about a third. In the middle third, work it out from the rule. These are our own working numbers, not a standard.

A part that has failed twice from the same cause goes out, because the cause is still there. And where a spare is weeks away and the village has no other water, speed beats the percentage.

Spares a Remote Site Should Hold

Water Mission's guide recommends considering spares at commissioning where supply chains are weak. Hold what is cheap and fails often.

Electrical. DC fuses, a surge protector of the right rating, two cable splice kits, connector pairs of the installed make with the crimp tool, and a spare controller where the nearest supplier is more than a day away.

Hydraulic. A spring-loaded check valve, a few lengths of riser with couplings, a wellhead seal and a spare level probe.

Tools. A 500 volt insulation tester, a multimeter with a DC clamp, pipe clamps, a polypropylene safety line, and the logbook of commissioning readings.

Test the Repair Before You Leave

Prove the repair with readings, and write every one in the logbook with the date.

Test insulation as the string goes down and again at the wellhead. Franklin's table puts a used motor and cable in good condition at 0.5 megohms or more.

Check rotation next. For a centrifugal pump, compare flow in both phase orders and keep the one that delivers more. Lorentz allows a new helical rotor pump only 5 seconds of dry testing in total, and none once it has been wetted. On a three-phase motor, Franklin's checklist asks for current unbalance of 5 percent or less and current under nameplate on every lead.

Then lift the dry-run probe out of the water and confirm that the pump stops and restarts after its delay. Finish by reading the flow meter at midday and comparing with commissioning figures, or with the pump curve at the real head.

Decide Before the Rig Arrives

Before anyone pulls the string, confirm the fault at the surface and list what is cheap and old and can come out in the same pull. Price repair, part replacement and a new set, including the crew and the rig day. Then fix the cause, whether sand, cooling or a loose connector, or the new part will fail the same way.

If you want a second opinion on a fault, or a repair done to a written procedure with the readings handed over, our renewable energy team can help. Get in touch with the pump make, the symptoms and your last readings.