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Dave,
It seems worse when the weather is hot and when I have driven the car a good distance. When I shut it off and try to start it after it sits for a time, the starter spins very slowly. I had the charging system checked over the years - no problems detected etc. Ideas??
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As I said, "the switch" is frequently the cause of strange electrical problems.The simplest check would be to put the cable lug which is presently on the switch B (battery) terminal on the same terminal as the E (earth) cable lug. This will bypass the switch. Or if the battery earth cable is long enough, remove it from the switch & attach it directly to the frame.
You didn't ask, but the battery cables should be no less than 2 gage copper cable.
If this doesn't cure the problem there is a chance that the starter or solenoid has problems.
If you are profficient with voltmeters, you can measure the voltage from battery earth terminal to frame while cranking the engine. There should be no more than 2/10 th (0.2) volt reading. If it is higher, there is too much resistance in the switch or cable connections.
Similarly, attach a volt meter between the battery hot terminal & the starter main terminal. While cranking, there should be no more than .8 volts drop. If there is more, there is too much resistance in the battery main cable, it's connections, or the soleniod contacts.
Obviously you need a low range volt meter for this.
D