Troubleshooting system where one bank never shows more than a few watts. Have two sets of four 100 amp panels going to two MPPT I 30 in an RV. On the bank with the issue app only shows around 15V even though measures around 40V at junction on roof and also at other end of wires where it connects to the controller, but as soon as wires are connected to the controller it drops to approx 15V even when measured directly at pins where wires connect. Voltage on the other bank matches what is measured at roof on the app and also when checked at connection to controller. I assumed it must be a controller issue so I swapped the inputs to the other controller just to verify and again had the same issue.
Doesn’t really make any logical sense to me, but I’m a solar novice and could use some help.
Can you add the exact solar-panel models you are using? There’s no panel to my knownledge beeing a 100A Panel 
It is totally normal, that the Open-Circuit-Voltage (no loads) is way higher than with something connected to the panels. 40V going down to 15 however is kinda high difference.
Do you have the panels connected in series or parallel? Do they have the same orientation?
I am guessing you mean 4 x 100W panels as the match the 30A MPPT.
The usual reason for a big drop in voltage from open circuit voltage to just above battery voltage is a poor connection somewhere in the PV wiring, MC4 connectors or in one of the panels internal wiring between solar cells or you have connected one of your panels backwards. The poor connection or reverse panel has high resistance, when open circuit there is no current flowing so the voltage is passed through the poor connection. Once you connect to the MPPT current starts to flow, even if it is very low and the voltage drops over the poor connection.
Check your wiring, your panel polarity, your connectors. You can also try your panels one at a time to see if you have a problem with one of them.
Yes,100W panels that was a typo. I pulled all the connections apart and checked each panel was producing between 19-20V. System is 3 years old and has been working fine up until recently. Hard to examine the wiring without pulling the rv apart, guess I could try running temporary wires from the roof to the controller to see if the result is the same to rule out wiring. Yeah, seeing a consistent 35-40 volts at the connector and then it dropping to around 15 the second it is connected seemed drastic to me.
The trouble is checking the open circuit voltage of each panel is of no use in this type of situation.
Thinking about this, a better test is to measure the short circuit current of each panel. You need a multimeter that will measure DC amps up to about 10A. Connect the ammeter to the panel.wires and you should get the specified current. If one does not produce current you have found the problem. Many resources on line showing how to measure this. As system has worked reverse polarity panel is unlikely.
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Thanks I’ll give that a try and see what I come up with.