I have a ML121RT Low Voltage 121-Watt Power Pack with Timer hooked up with three 20 watt lights attached - going north. I want to add one more 20w llight going south. Can I attach an additional wire to the A/B terminals to add this dedicated lone light? If not, how do I add the other light?

Yes possibly:
Did you unplug transformer before adding light.
Does wire size match distance and gauge requirements?

Copy following link:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Gene
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http://waterheatertimer.org/Malibu-power-pack-stopped-working.html#troubleshoot

Gene
Water Heater Timers Save Money

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Copy following link for power pack troubleshoot resources:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

-set power pack to ON and plug power pack into cheap wall timer
-take apart motor and clean gears, and add white grease
-use 5-year warranty coverage

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If clock has slowed down or is running reversed or gets stuck on the tripper, then clock motor has gone bad.
There are no replacement clock modules for this product, but product comes with warranty.
Work-around: Buy cheap plug-in timer, set power pack to override, and plug power pack into cheap timer:

Resources: including warranty links, tips, trippers, etc:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.

http://www.intermatic.com/products/professional%20lighting/light%20expressions/power%20packs/ml121rt.aspx
Intermatic site with product

http://www.intermatic.com/~/media/files/intermatic/products/instructions/professional%20lighting/ml121rt%20trilingual.ashx
Manual for your product

When power pack malfunctions, you have to buy another one

I have looked for solutions at Intermatic, at malibulights.com and the internet

Your next power pack:
Match up wattage of bulbs with wattage of power pack
15 - 20 watt bulbs = 300 watts
-Replace bulbs as soon as they burn out or other bulbs will burn out and power pack will burn out
-When working on bulbs, unplug power pack or it may burn out

There is a break in the main wire between the 2nd and 3rd lights. Has anyone been digging or working in the yard lately?
While the transformer is fully loaded, six 20 watt bulbs won't overload it.

If other lights are working the power pack is OK. Usually, as in your case, if changing the bulb doesn't work, check the connection where the light ties into the main run.

These should work:
http://www.nextag.com/timer-trippers/shop-html