Alternator chassis battery question

Yachts · 06-03-2013, 03:46 AM

Seemed like lately if I drive with just chassis batteries ,no merge ,no genset sometimes my voltage would drop to 10v.. Yesterday it ran fine.. Tonight driving home with lights on ,voltage dropped again.. I started genset and hit battery merge and back up to 12-12.5 v.. I had alternator tested couple of weeks ago they said it was fine.. Could my chassis batteries be bad?

folivier · 06-03-2013, 12:16 PM

Check the voltage at the batteries with your voltmeter. The dash gauge could be bad.
Check the battery cables, they could be loose or corrosion build up. I've seen the cables corrode inside the insulation, when bending them you could feel they were different. Cut the insulation and there was corrosion inside.

Wheeler · 06-03-2013, 03:04 PM

Most likely it is something simple a loose wire or corrosion like Forest said.

77newell · 06-03-2013, 04:22 PM

Yachts: I'm going to assume the voltage that is dropping as you drive is the chassis voltage and not the house battery voltage. When you turn on the generator and merge the voltage rises to the normal level. This says to me that the gauge and wiring to the gauge is probably working fine. The battery terminals are probably OK since both the gen/merge returns things to normal and you did not report any hard/slow starter which I would have expected would be more noticeable at high starter amp draw than with just the alternator pushing current.

Assuming (yea, maybe not the greatest idea but we have to start somewhere) the alternator is fully functional, then the current is getting cut off somewhere between the alternator and the batteries. You may be able to further confirm this by observing the battery charge rate which I would expect is negative - a discharge (which is a dissed charge). If all this is as I have suggested, then I would start measuring voltage at the alternator with the engine running - beware the moving parts - and follow on down the wire to the battery until I found the lower voltage at which point I know what is broken. You want to measure at the bolts sticking through the wire terminals rather than the terminal itself to make sure the electricity is getting out of the wire and terminal to the intended destination because as others said above it is not uncommon for the terminal to be the problem.

Please let us know what you find so the rest of us can more easily solve our problems should a similar situation occur.

Yachts · 06-04-2013, 02:56 AM

i will .. im going to have a friend of mine who is a mechanic do some checking and i will report back

Yachts · 06-06-2013, 01:52 AM

Well I have the answer.. Loose belt.. I replaced the belt right after I bought the coach and had not checked it..It was a little loose tightened it up by wrenching out the alternator.. Waalaa. 12.5 to 13 volts..now just one issue left on it to fix height ride valve adjustment.. Please help in that section..

folivier · 06-06-2013, 02:04 AM

Yeah always need to re-check the tension after it runs a bit. Glad it was an easy and cheap fix.

77newell · 06-06-2013, 02:17 AM

Well geez, all us "experts" was wrong. It never occurred to me to check the belt, good catch. This turned out to be a classic example of of a since-you problem - since you fixed it it doesn't work.

folivier · 06-06-2013, 02:25 AM

Hey Jon, you got new belts on your alternator. And I did re-tighten them. But you better check them yourself

Yachts · 06-06-2013, 02:33 AM

Hey are you on lake st Clair?

77newell · 06-06-2013, 02:36 PM

Forest: hey thanks for the headsup, I'll check it this weekend.

Yachts: we live about 15 miles west of Lake St. Clair. I haven't been on the lake yet this year but the last couple years I've crewed on a sailboat for a Wednesday evening racing series and hope to get back out there next week.