Use a jumper cable put one end on the positive terminal on the battery and touch the other end to the terminal on the motor. If the motor doesn't run, it is the problem, if it runs something else is wrong. The pump motors are automotive starter motors and any shop that repairs starter motors can repair them.

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Rich
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(215TL) - 1995 21' TowLite
(2209T) - 2009 22' TowLite
99 Ford Explorer 4DR 4X4

Make sure you have cleaned the ground wire connections at the battery and on the trailer frame. As Poprichie said jump from the battery positive lead to the pump motor stud and the pump should run. If it doesn't then the motor is bad. Bad ground connections have been the cause of many problems on RV's. Next, if you don't have a manual pump to lift the top off of the safety lock so you can release it, depending on the size of the Hi Lo, If small enough 4 strong guys might lift the top the fraction of a inch needed to pull the release cable and you then activate the electric lowering valve or open the manual lowering valve on the pump slightly to then lower the top. Do not open the lowering valve until after you have pulled the release cable. You do not need any pump, manual or electric to lower the top. The weight of the top is what causes it to lower when the electric or manual lowering valve is opened. As stated by another member, the motor is a starter motor and any good auto electric shop should be able to rebuild or replace it.

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Jim L & Faye
2805C
2003 Silvarado diesel crew cab
central Virginia

Doesn't seem like a bad motor would make your interior lights go out too?

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2001 Towlite 24TD
2003 Chevy Avalanche Z71
Past campers: 1989 Funlite 21', 1997 TrailManor, 1979 Holiday Rambler MH., 1982 Palomino PopUP

If you have no interior lights and no power to the pump motor seems you have lost the plus side of your power or have lost the ground. Jump from the positive battery post to the pump motor stud and also connect a jumper cable from the negative battery post to the pump motor frame. Make sure you have good connections with the jumper cables. See if the motor runs now. How are you testing your battery to know that it is good? Since I own a 2005 Hi Lo I do not know the power system on a 79. I do know the system from the battery to the pump and the above check will tell you if your pump motor is good or bad assuming you have a good battery. This check bypasses all switches, relays, circuit breakers, fuses and wiring. You cannot check the condition of a battery just by checking the voltage. A bad or almost dead battery will show 12 volts.

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Jim L & Faye
2805C
2003 Silvarado diesel crew cab
central Virginia