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The backup alarm on our Nissan Platinum II forklift does not work. When we put it in reverse, there is no beeping like there should be. This is a safety problem we need to fix. How do we test for power and ground at the siren? If we have power and ground, what else can cause the backup alarm to not work.

Hello and welcome to fixaforklift.com!!

On any back up alarm there is always one power wire and one ground wire coming the battery. The best way to test it is to use a volt meter, you can probe the black wire coming from the volt meter to the negative battery cable and use the red probe cable to check power going to the power side while truck has the key on but engine off, It HAS to be in the reverse direction.

Put the red cable on the power side of the back up alarm and you should see battery power.

If you see battery power

then you should have to check for battery ground. probe the ground side of the alarm and check continuity on battery ground,
if there is no contunuity find the break in the ground wire and repair or you can find any known good ground and attach a wire going there.

If you find power and ground then the back up alarm is not working and should be replaced!!