400 Cummins Alternater Problem

Without using a digital voltmeter, we will all be guessing!

With engine off, check voltage at batteries-- both banks. 12.7= fully charged battery.

Start engine. Voltage should rise to 13.5- 14.5 VDC at both batteries.

If not, check at your battery isolator. Both at the lug IN from the alternator and at the lugs OUT to the two battery banks.

Let us know what you find-- if those check out, likely a wiring issue at/to the dash.

Diagnose, don't just replace parts.

And make sure you are NOT on shore power or have the generator on, as that can cause the alternator light to come on.

why would he be replacing a alternater if lite was on only when on shore power?
People run gen sets while driving down the road all the time.I have never seen my alt lite come on?? Iam 100% sure he certainly means alt lite on when engine running + or driving down road

My alternator light will come on when hooked to shore power... does not do it when going down the road with generator running.
If your starter solenoid was stuck open you could not start the engine and it's a normally open switch anyway. Go buy a $5 digital meter and do the tests that Brett outlined.

prism wrote:
why would he be replacing a alternater if lite was on only when on shore power?
People run gen sets while driving down the road all the time.I have never seen my alt lite come on?? Iam 100% sure he certainly means alt lite on when engine running + or driving down road

It really isn't that uncommon to "confuse" idiot charge lights when multiple charge sources are present. If the charge voltage of, say a smart inverter/charger is above that of the alternator, it can certainly trigger the light.