Re: Electrial issues with 1992 U225 Cummins 5.9
Reply #3 –
I concur with @wolfe10 on this one. I had a similar issue with one of my trucks a few years back; whenever I turned on a load my temperature gauge went up, oil pressure gauge dropped off, and voltmeter dropped off. I was cash-poor and time-rich at the time and I didn't want to start off replacing the alternator and battery right away so I tried pulling every ground strap I could find at frame, engine block, body, and dash, cleaned their lugs/screws and their mounting locations, applied a good conductive grease (NO-OX-ID "A Special") and reinstalled, plus ran a couple extra ground straps for good measure. All my gauges returned to correct operation after that and never so much as looked at me funny ever again.
Of course, as soon as possible thereafter I went out and bought a set of mechanical gauges because oil pressure in particular is one thing that I don't ever want to be relying on an electronic sensor to know... but that's a bit beside the point.
I'd suggest getting a multimeter and putting it on your battery, then start your coach, make a note of the v your gauge is reading, compare it to the v your multimeter at the battery is reading, then turning on one of those loads - say, the heater- and again make a note of the v your gauge is reading, go back out and see what your multimeter reads. I'd lay money that while your gauge is reading ~11.2v, your multimeter will still be sitting happy at ~13.5v... in which case you know it's your gauges reading incorrectly, not your alternator or battery. And if the gauges are reading wrong, I virtually guarantee it's a bad ground somewhere.
A good place to start is to first make sure that your connections at the batteries are good... can't count the times I've seen a loose clamp on a battery NEG post. Then, a good long visual inspection of all wiring and ground points that you can find... it's not at all unlikely that there'll be an obviously failed/cut/worn through/fallen off ground strap. If nothing comes up in an initial inspection, then... well, if you have the time, start pulling them one-by-one, cleaning the mounting locations and the lugs, and reinstalling.
My money is that it's either a loose NEG right at the battery or it's a loose/missing/corroded engine block ground strap, but man... considering how many ground straps my little truck had, I don't want to even hazard a guess about how many must be on a coach 
Topic: Electrial issues with 1992 U225 Cummins 5.9