Question
QUESTION: My system: 5-zone system, 2 valves original to the house (Nelson I believe), 3 I added (RainBird) controlled by RainBird ISM-6 controller.  

Problem: Fault errors ("ERR" on LC) on random zone(s) during run.  Sometimes zone 2, zone 4, sometimes zone 2 and 3.  Problem has appeared on all zones.

Troubleshooting so far:

1) Since problem is on all channels, and since it appeared about a week after a storm, I thought maybe the controller got hit by lightning (wild guess).  Replaced controller with a new ISM-6.  Same problem, no change.

2) Since problem is on all channels, and the control cable has buried splices along the run, I thought the cable was to blame.  Replaced with new cable, no splices in the field, only connnection is at valve box with WELL insulated wire nuts to keep out moisture and avoid shorts.  Same problem.

3) All valves tie to common ground wire.  If the ground wire on one solenoid valve had shorted, any valve could fault along common ground, even if the valve itself wasn't shorted.  I disconnected all valves, and re-connected them one at a time, independently, and run the system multiple times with just one valve... hoping problem valve would reveal itself.  No problems.

It appears the problem only happens when all the valves are connected normally and when it runs through a program.

Grasping at straws: The older Nelson valves, when checked a multimeter at the ISM box, show lower impedance than the newer RainBird valves.  Is it possible that the lower impedance valves pull more current (inrush and steady state) and could overload the controller?

LOST!

ANSWER: Sorry for my slow response.
Thanks for the great info. It helps immensely. Have you tried running the system with only the Rainbird valves (all 3)and not the Nelson valves connected? Then try the system with only the Nelson and not thee RB's.
It is possible the Nelsons are drawing more amperage and causing the controller to sense a short. Rainbird's controllers have a very sensitive "fault" circuit and are designed to work with modern solenoids. You may have mistakenly stumbled on a combination Rainbird's engineers didn't anticipate. Or the lightning affected the to Nelson valves and not the RB valves.
For sanity sake, I would replace the Nelson's and be done with it.

Matt

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply Matt!  See #3 above-- I connected each valve independently to the controller to check them.  I tied each valve, one at a time, to zone 1, then jumpered all zone terminals together and ran the sprinkler as normal.  Each time a zone switched, it turned off the zone, then turned it right back on (since the valve was controlled by all zones).  This allowed me to check normal run time as well as 5 cycles of the valve to look for transient issues.  In each case, for each valve independently, the system ran through its program without a single problem.

I hooked it back up like normal last night and ran it.  I got a zone fault the first try.  Yes, the first try.

It's worth noting that I've had as many faults on zones with RB valves as with the Nelsons (the zone that faulted last night was an RB).

Next, I hooked a multimeter inline with the common ground and ran the system to check on current draw.  The Nelsons pulled about 3.4mA, RBs pulled about 2.6mA.  This sounds like your suggestion... but my last fault was on a low-current RB valve.  

One more thing... during one zone change last night (when it was setup "normally") my meter caught a very brief hop of 13mA inrush when one of the RB valves fired.  It did not fault, but it does make me wonder if inrush is a problem.  I'm not sure what the fault circuit current limit is for the ISM.  Could it be this RB controller has problems with RB's own valves?  Argh!

If my RB controller is overly sensitive, I'd rather regain sanity by replacing it with a more robust controller.  Much easier than retrofitting two Nelson valves glued onto a PVC line in a small hole in the ground.  But until I KNOW this is the problem, I hate to throw down ~$100 for a new controller just to find it has the same problem.

Any other thoughts?

THANKS for the prompt reply, and for your patience with my long messages.  You're a great help!

Answer
I'm still thinking of resolutions. But, how's your electricity supply? Have you tested the 120 volt recept?
My thought is this: a brown-out situation would lower the controllers voltage to less than 24 volts. At lower voltage the amperage would go up causing a fault. You could also test the 24 volt supply to the controller. A Rainbird transformer usually put out 25.5-27.0 volts. Try test the 24v inputs on the controller while running through the zones.
Let me know how it turns out.

Matt