Richard
the Zodiac tri uses a peristaltic pump to pump acid to the pool and measures pool acidity with PH probe. Problem is dispensing acid and measuring it. The pump flex tube gets squashed (it parks the tube flex rollers on the tube) and it dispenses less and less till it gives "PH" errors. Then you really need to calibrate the probe often.
Then you need to have the acid container close to pump and acid fumes corrode everything...and I mean everything. I barley managed to resurrect my controller PCB from said corrosion. I had transistors corroded legs
...some sort of electrolysis gone mad (the PSU stby pwr causing fume attraction).
Oh also the PH probe coax and BNC...often get very loose I've seen the centre pin of the BNC corroded and disconnected or poor connection to the controller. Inside the controller the BNC connects by pigtail to a sub PCB (part of the Tri add on) and this connects via main PCB with IDC type cables. These IDC get corroded out from fumes as well...this I know
Anyway to cut to the chase I and many others have given up on acid dispensing and do it manually now. Its was to complicated for the controller to do this with cheapest parts available the manufacturer used. I had PH errors pretty much from the start...as others did. Zodiac tri is notorious for this issue.
Usually your acid pump fail is not the pump just the squeeze tube or pump head. The pump head plastic degrades & it breaks. By now you would also have seen backup battery fails and replaced this?
I've been discussing the Zodiac system with a few others for some time on the whirlpool forum, thread with quite a few repairs:
https://forums.whirlpool.net.au/forum-replies.cfm?t=1389867&p=-1&#bottomWorth a laff of a read. If you need more info happy to assist, I'll not be giving up on mine because I'm way too stubborn!
Brendan