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> Wipers on low speed keep slowing down ... on full speed works okay
Probably not a bad wire, otherwise the problem would be more intermittent and/or would permanently degrade. The fact that full speed is working normally says a lot.
The 240 wiper motor uses a three brush system and I presume the 740's of that vintage are similar. One brush is offset from 180 degrees and is used for low speed. I'm betting you'll find the spring behind that brush has broken (they're kinda frail) which allows the brush to spark which creates heat which creates resistance which slows down the motor.
Although it is a bit tedious, you can remove the wiper motor assembly, open up the motor and carefully pry the end cap off the brush holder. I've successfully used a section of the flimsiest ball point pen spring I could find as a replacement (cut off about 1/3 then stretch out to suitable length). Check the other brush springs too -they're probably brittle and will break as soon as you touch them. Total cost: zero. Frustration potential: high
While you're in there, check to see that the motor magnets glued inside the case aren't rusty and loose. This is a common 240 problem that also leads to motor slowdown and overheating. Use a good quality epoxy if needed.
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