Taking a long time to load

Replace trippers.
If timer dial is rotating and keeps good time, and manual override turns load on-and-off, then replace the trippers.
http://www.grainger.com/Grainger/INTERMATIC-Timer-Tripper-2E054?Pid=search

Stationary pointer points at 6:00 and moving it does not affect timer function.
The pointer is reference point for setting current time.
http://waterheatertimer.org/pdf/How-to-operate-and-set-T104-timer.pdf

Other resources:
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timer.html
http://waterheatertimer.org/Intermatic-trippers-and-parts.html#T100

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Problem is not clear.
What is turning on-off every 15 minutes? The clock motor? Or the Load controlled by the timer?

If the dial is not rotating and keeping good time, then replace Intermatic T104 240Volt WG1573 clock motor
If the Load is turning on-off every 15 minutes, then that sounds like possible electrical short and not the timer, but more information is needed. Add a comment.

Open following link for troubleshoot checklist:
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timer.html

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Here are two links that will be handy to keep on file:
http://waterheatertimer.org/How-to-troubleshoot-Intermatic-timer.html
http://waterheatertimer.org/How-to-wire-T104-Intermatic-timer.html

T104 is 240Volt timer.
When you open the door, there should be piece of translucent plastic covering the wiring.
As long as plastic cover is in place, it is safe to touch visible timer parts.
If you have a child who might stick something into the timer, then lock timer door.
As a general rule with electrical devices, do not stand on bare ground or wet surface or lean into or touch anything metal while working on timer. Put down dry boards. Same with your circuit breaker panel.

The T104 yellow dial rotates just like a clock.
The dial has ON and OFF trippers attached to outer edge of dial.
As these trippers rotate around they move on-off lever that turns the pool pump or Load on and off.
ON tripper moves lever one direction, and OFF tripper moves same lever other direction.
The manual override lever clicks back and forth with audible sound each time tripper moves on-off lever.

Move manual override back and forth and it will turn Load on and off.
Override should snap crisp and clean.

To set timer, lift and rotate dial until current time lines up with stationary silver pointer.
Stationary silver pointer is pointed at 6:00.

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HPM 24 hour timer has segments located around dial.
http://waterheatertimer.org/Woods-timers-and-manuals.html#HPM-dial

http://waterheatertimer.org/images/hpm-24hr-timer-500.jpg

Segments appear to move up or down.
Other similar timers: Segments down = Load ON; Segments up = Load OFF
Press several segments down.
Plug timer into wall.
Plug 240Volt load into timer.
Manually rotate timer dial clockwise to see if segments down turn load on or off.

There is also a 2-position slide-switch located on top of timer.
Move slide switch to each position to see which is need for timer to turn load on-off.

After positions are defined, push segments down or leave segments up for times you want load to turn on.
Rotate dial clockwise until current time lines up with arrowhead located on inner dial.
Timer dial rotates as time passes, and segments move a plastic lever that turns load on and off

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You posted under Intermatic T104 timer.
If you have different timer, information below may not fit your timer.
Add a comment with correct timer brand and model number.

1) Click manual override on-off to see if Load turns ON, and that override snaps crisp and clean and is not sluggish or hard to move.
If override is not functioning well, then timer is bad. Buy new timer.
2) If dial is rotating, note current time, and watch if dial is keeping correct time over full day.
If dial is running slow, then buy T100 series 240Volt clock motor for T104 timer.
3) If dial is keeping good time, then tripper might be worn and need replacement

Link below shows Intermatic T100 series parts:
http://www.grainger.com/Grainger/timer-accessories/timers/electrical/ecatalog/N-8hi

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Does the manual override turn Load ON-OFF? If manual override is broken or doesn't operate, it may be time to buy new timer mechanism and snap into existing box.
http://www.amazon.com/dp/240Volt-20
http://www.grainger.com/Grainger/INTERMATIC-24-125V-Hour-Dial-Timer
Do timer dials rotate as time passes? Do dials keep proper time? If not, you need new clock motor. Grainger sells clock motors and trippers:
http://www.grainger.com/Grainger/timer-accessories/timers/electrical/ecatalog/N-8hi
Are the tripper pins still attached to the dial? If tripper pins are worn down, they need replacement.

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You posted under Paragon 2606 Panel-mount countdown timer.
If this is not your timer, add a comment.
http://waterheatertimer.org/Paragon-timers-and-manuals.html

2600 series is mechanical interval timer.
26006-00 can handle 10 amp loads.

Interval timer is a countdown timer that keeps Load turned on for set amount of time.
You set the time by rotating dial

To start timer, Rotate dial from 1 minute to 30 minute.
Rotating dial turns on Load and begins countdown.
Timer turns OFF Load after set amount of time expires.

Manual for P1353:
Programming instructions for P1353 start page 30
http://waterheatertimer.org/pdf/Intermatic-P1353-manual.pdf

The on-off button is merely an override of existing program, and then programming resumes at next event setting.
I have read this manual looking for a simple way to turn off timer functions so no pool equipment operates, and it appears that you have to turn off circuit breaker.

This is the same as any mechanical timer such as T104 or T101, and is not brand-specific as all mechanical box timers follow same pattern. Mechanical timers have override button, but the timer reverts to regular schedule at next tripper, just as the P1353 does.

Keep in mind that each load on timer can be wired separate from the timer clock. So you can turn off the load wire going to each load, and still have power to clock.
http://waterheatertimer.org/images/P1353-terminals-566.jpg
If you cut power to each load, then it is not necessary to reset current time. Programs remain inside timer even if power is completely cut to timer.

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Internal components of manual electrical timers do wear out (in my case, within 2 years), which will trip an AFCI/GFCI breaker at the same time every day, even before the circuit switches on. This regularity of the problem (at 5:50 PM every day) was the clue that the timer was at fault.