Richard, I had problems with BOTH my wipers sticking and moving VERY slowly when I first bought my coach. The previous owner had the drivers motor rebuilt with little change. After a year or two I just couldn't take it so I ordered a new pair of units from Newell. They are sold as an assembly with the motor, gear, arms and blades. They are made by Bosch and I paid over $1,500 counting shipping for the pair.
In retrospect, I suspect I wasted much of that money as the new ones worked better but were still not great. Here is what I think is happening based on my analysis (your results may vary):
1) One of the arms on each blade attaches to the motor, the other attaches to a rotating shaft. That shaft is somewhat exposed to the elements and gets corrosion on it making it difficult to turn. That puts an undesirable load on the motor. Adding penetrating oil to that shaft on a routine basis seems to help.
2) The wires going to the motors may not be large enough and voltage loss could be an issue even though there is a relay in front of the passenger's seat for the wipers. I find that since my headlights are typically on when the wipers are running, my voltage on the chassis batteries isn't as good as I would like (12.8-13.0) with the engine running. I may have a voltage regulator problem (that you likely don't have) but I find that running the generator when I need the wipers brings the chassis voltage up to 13.0 - 13.4 and the wipers work much better. When I have the time, I will likely see about resizing the wires from the relay to the motors.
One thing the new wiper units did solve was the wipers going off the side due to slippage on the shafts. If the attachment nuts are left somewhat loose, they can strip down the ribbing on the shafts and they will slip. If they are over-tightened, they tend to bind.
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