How to diagnosis a problem

Welcome to

  • Register now and join the discussion
  • Friendliest RV Community on the web
  • Modern site for PC's, Phones, Tablets - no 3rd party apps required
  • Ask questions, help others, review campgrounds
  • Get the most out of the RV Lifestyle
  • Invite everyone to and let's have fun
  • Commercial/Vendors welcome


RVF Regular
Dec 11, 2020
RV Year
RV Make
RV Model
Dutch Star 3858
RV Length
38 ft
Thought I would share this bit of knowledge I learned while I was a Service Manager for Chevy Dealerships.
Try to find a "theory of operation" for what ever it is your working on. I you have a understanding of how something operates,
you are ahead of the game in find a problem.
GM has gone as far a stating a a "theory of operation" and showing a wiring diagram of the component and that is all they give
a tech to repair a vehicle now.
Even if you can not find information specific to what you are working on, a general "theory of operation" ( say how a vehicle AC system cools)
goes a long way to diagnosing a problem and maybe where to check.

Here is how to diagnosis a "battery" light that came on, on the dash or a battery that will not turn over the engine.
First that "battery" light does not necessarily mean that the battery is bad.

2 tools everyone should have is a test light, and a multimeter.

Ok, first thing to do, make sure the fan belt still there. If that's good, take your multimeter, or volt meter and check the battery voltage.
Make a note of what it is (Say 12.5 volts with engine not running).
Start the engine, recheck the battery voltage. Should be more than is was when the engine was not running (12.8 to 14.5)
If there is no change or it is lower, your alternator is not charging. Say with engine not running, the battery shows 10.5 volts, the engine turns over slow, buts starts.
With engine running, the battery voltage is still 10.5 volts. Your alternator is not charging, you need a new alternator and the battery charged.
IF your battery shows 10.5 volts with the engine not running, but 13.5 volts with the engine running, then most likely, your battery is bad. Have the battery load tested.
NOTE: the battery needs to have a good charge to properly load test it. So it may have to try and be charged before a load test, to test it properly.
MOST of the time, a "battery" light that comes on, is the alternator not charging.

Here is another tip on how to diagnosis a battery that keeps going down, but checks good. (This goes for chassis battery or house batteries)
Let's check the house batteries.
The battery need to have a decent charge in it.
1st you will need that test light you bought.
Now touch one end of the test light to the negative side of the battery post. Touch the other end to the positive post
See how bright the light is, That is a full 12 volts making the light bright.
Now, Disconnect the negative (-) battery cable or cables from the battery ( on house batteries, not the cable that connects the batteries together, rather the cable that grounds the battery to the to the frame).
Take 1 end of the test light and connect it to the battery cable you removed. Connect the other end of the test light to the negative post on the battery. ( some one may have to hold it to the battery or use a pair of vise grips or clamp to attach it. Make sure the key is not on and all power and switches are off.
IF the light is bright like it was when you touched the test light between the negative and positive battery terminals, you have a "draw" on the battery.
Leave the light attached to the cable and negative battery post. Have some one go to the fuse panel for that battery (say the house fuse panel for the "house" batteries), and start pulling 1 fuse at a time.
Pull ONE FUSE AT A TIME and note the test light. I like to leave the fuse out until the light goes out. Some times the may be 2 or more fuse circuits that are the problem. Not often though. Did it go out or get dimmer? If so THAT circuit has a "draw" on it. IF it got dimmer, mark that fuse location and keep pulling fuse until the light goes out. Let's say one fuse made the light go out when the fuse was pulled. Put the fuse back in and see if the light come back on. Yes? Leave the fuse plugged in and Find what all that fuse powers. Go to each item powered by that fuse, make sure the power is turned off on each item (like a closet light). If all the switches are "off", then you are going to have to disconnect each item until the test light goes out. IF say a closet light makes it go out when power is disconnected from it, the that light assembly is probably bad and need replaced. IF none of this works, you have something else connected to that fuse circuit or you have a shorted wire. THOSE are hard to find.
Get a multimeter that has a symbol on it like this that checks continuity