So, I have to ask... I have done ZERO PD on the problem - but today I went to crank my M936 and all I got was solenoid chatter. Turned off both switches, muttered a few choice words. Then I turned on main power, turned on the ignition switch, then attempted to engaged the starter a second time. Same only less aggressive solenoid chatter. Shut down both switches again the same way I always do.
The batteries were dead when you delivered my truck. No big deal. I pulled them out and brought them in the building. Checked water levels and slow charged both of them overnight. Next day, carried them out and installed them. Truck has cranked reliably since then (roughly 10 weeks) . At least once or twice a week it gets cranked. It always fast spins over and cranks right up. Last time I cranked it was Tuesday (4 days ago). I have gone as much as a week without any problems...
Hate to admit that I didn't actually pay much attention to the gauge while the truck was running beyond the very first day after installing the batteries. It was in the green, so I smiled. Reading here seems to indicate that they aren't very reliable - so I figured "good enough". Where it is positioned I almost have to lay down to see it... My plan had been to deal with that situation later. The pictures below seem to indicate that there REALLY IS a problem and the gauge REALLY IS GOOD.
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It seems worthy of mentioning that every time I crank it, it runs for about 20 or 30 minutes. About half of that time is at about 1300 rpm. I have been performing shutdown in reverse of start. Ignition off, pause to let the engine stop, then Main Power off.
I also snapped a quick picture of what appears to be the stock generator or whatever it is that is supposed to cause the batteries to charge. (below)
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I have plenty of tools to test 12v systems but nothing other than a circuit tester for 24v. That was easy. New bulb and presto, 24v tester. I figure tomorrow I can disconnect the batteries and charge them individually again and reinstall. That should fix the problem based on last time... Any particular suggestions on a good tool to verify charging output?