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How to tell if the alternator is charging properly?

dbreid

dbreid

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All,
I had a no charge issue a while back in my off road truck, and while driving to the trail, I wound up being forced to drain both batteries (I run duals) to death. No problem...

I bought a new 78AMP alternator, and bought two Yellow Top Combo Deep cycle batteries. The dual batteries are switched by a 12vguy controller in cab, and everything seems to be wired properly.

Now, I am somewhat paranoid that the charging circuit isn't working, so I watch my VOLT in dash gauge like a hawk. As I drive it around, it seems to hover just below the 13V mark. How can I tell if the alt is charging properly? I am about to take a trip, and want to make sure it is running well before I go, and I dunno if I am being "fooled" by the new optimas into thinking everything is OK.

Thoughts?

-Dan
 
Well, 78A isn't a lot, so depending on loading, that may well be why your last one died. Figure your HEI or whatever ECU/ignition, lights, and whatever other assorted accessories are running all the time are gonna take away from that 78A pretty quick, which doesn't leave much to charge. The output max is also at higher RPM's (i.e. highway speed) so if you're puttering around town at night (i.e. lights on) you may not be charging much at all.

Anyway, it's easy to tell if you're charging. Look at the gauge with the motor off (you may need the key on to get it to register.) That's the battery voltage when not being charged. Should be 12, maybe 12 and a half volts.

Now start the motor. The voltage should jump up to 13, 13 and a half. This means the alternator is doing its thing. If the voltage *doesn't* jump, you have a problem (though it may not be the alternator; could be gauge, wiring, etc.)

You should check voltage on both batteries, the details of which depend on your switching/isolation apparatus and that can get complicated. Worst case get yourself a $5 Harbor Freight voltmeter and just check the batteries at the terminals now and again. Sure, the cheapos aren't real accurate, but even those can tell the difference between ~12V (battery sitting) and ~13.8V (charge) :D

Now, for the purists -- 4x4High among them -- I *know* I am oversimplifying things. However, the charge current *is* a function of the voltage difference from charge to steady state, so it's a quick and easy test.

-- A
 
Put a meter on you battery and check voltage while it is not running, 12.5 volts or so, Then start it up and check your voltage again, it should be in the 14v range.
Tarey
 
Better to test off the back of the alternator. Voltage loss between the battery can (and will be sometimes) up to 1/2 volt, and at the fuse panel, can be as large as 1 volt less than what the alternator is putting out.

Single terminal on the back of the alternator. Test it with no load (just engine running) and with everything you can think of that draws a large amount, turned on, high. Heater and high beams are usually it, unless you have electric fans.

It's somewhat arbitrary, but really voltage shouldn't change much as load increases. If it does, your alternator is too small, or not working correctly. Just did this with mine...mine seemed to work fine until the engine warmed up, then the output of the alternator started declining until it wasn't charging anymore.

Volt gauge, if consistent, can be used as an indicator of failure, as dremu stated. Just like to know where "normal" needle position is, and what that relates to at the alternator voltage. (IE gauge reads 13V when the alternator is really putting out 14V)
 
dyeager535 said:
Volt gauge, if consistent, can be used as an indicator of failure, as dremu stated. Just like to know where "normal" needle position is, and what that relates to at the alternator voltage. (IE gauge reads 13V when the alternator is really putting out 14V)
Click to expand...

I meant to emphasize this; the dash gauges are notoriously inaccurate, so 12V on the gauge could be plus or minus, say, 20% of the actual reading. However, you should still see a reliable difference between engine on and off, i.e. charging and not, so as Dorian says, you just learn the two positions on your particular gauge and you're good.

-- A
 
This is all very helpful. THe list of things I have that draw power are:

1.) Twin Flex-A-Lite fans (they are on a manual override switch, as well as dual stage thermo switch... they come on for brief periods like a hurricane, then cut off... truck stays very cool)
2.) Headlights
3.) Driving lights (KC), which I ararely use
4.) CB
5.) Turn Signals
6.) LED rock lights (4 of the roundeyes ones.. low draw).

Warn Winch when I use it.

That's about it.

I'll check the draw with the power being sucked. Thanks!

-Dan
 
An old fashion (and cheap) way to check if your alternator is working is to shut off engine, place flat screwdriver blade on center rear of alternator ( by the rear bearing ). The screwdriver should not stick to the bearing case. Now start the enging and repeat test, the screwdriver should stick ( like a magnet ) to the rear bearing. That means your alternator is working.
 
Alright, I just went out and checked the setup, and it looks like the alternator is NOT charging. On my main battery I got 11.8V and it didn't jump when I started the truck. On the aux, I got 12.5V, and it didn't jump when I ran the truck either...

Aside from grounds, of course, what is the best thing to check now? It is a brand new alternator, and pretty much the only things I "hooked up" when I installed it were:

Screwed the nut on the back for the red wire.
Attached the two slot connector plug
Bolted it in
Tightened the (new) belt.

What is the best way to determine what is wrong? I am fine with buying another alternator, but I am not even sure the one I have is bad, and am wondering if some other aspect of the charging system is messed up.

It is possible to plug the connectors in wrong?

-Dan
 
dbreid said:
Alright, I just went out and checked the setup, and it looks like the alternator is NOT charging. On my main battery I got 11.8V and it didn't jump when I started the truck. On the aux, I got 12.5V, and it didn't jump when I ran the truck either...

Aside from grounds, of course, what is the best thing to check now? It is a brand new alternator, and pretty much the only things I "hooked up" when I installed it were:

Screwed the nut on the back for the red wire.
Attached the two slot connector plug
Bolted it in
Tightened the (new) belt.

What is the best way to determine what is wrong? I am fine with buying another alternator, but I am not even sure the one I have is bad, and am wondering if some other aspect of the charging system is messed up.

It is possible to plug the connectors in wrong?

-Dan
Click to expand...

Hard to get the plugs wrong, as they're keyed.

Personally, I'm suspicious of the secondary connector; one pin supplies juice through the alternator light on your dashboard (or if not so equipped ... dangit, comes from somewhere else, I forget.) It provides an exciting voltage to turn the thing on, so to speak. (I love being turned on and excited ... just not by electricity :haha: )

Also make sure that the two pin connections are clean, as they get scuzzy pretty easily and we know what that does to electricity.

With the engine going, check the voltage at both terminals of the two-pin blade, and post up whatcha get ... by then I'll have remembered which wire does what (red and brown?) or someone else will have posted up.

Could be your battery isolator-switch thing, too; did you install it, did someone else, did it used to work, etc?

Other observations: I assume the alternator is rebuilt, and not actually NEW new. Anyway, most parts places have a thing to spin test it, so you can take it it to a convenient one and they'll test it for you. Most likely it's a connection issue, but no sense in chasing gremlins that aren't there.

And put your main battery on a trickle charger or something; leaving it for any length of time at less than 12V is bad for it. I gather those funky spiral-cell ones don't take kindly to being over-charged, so I'd try and avoid jumping it or using a stupid high setting on a charger, especially as it gets low like that.

-- A
 
I will go right now and test those pins....

I bought the switch from 12vguy. It is an in cab switch, and a solenoid that switches them together, separate (isolated) or the backup jumpstarting the first one.

It used to work beautifully for a year, and I haven't changed any of that.

And I bought the alternator from one of those online parts places like 1Aauto, or whatever. Not sure if it is new, but it is new to me, and came in a box with a warranty (insert Tommy Boy jokes here). :)

Be right back!
 
I suspect a bad alternator. Check voltage right at the large terminal on the back with the engine running, you should get anywhere between 13.5-14.5 volts. If you don't get that then the ALT is bad.
 
Here's a picture of what I am working with:

alt_01.jpg


Battery Voltage with the truck off = 11.67
Battery Voltage with the truck ON = 11.67 (Aux battery is isolated)
Voltage from ground to either pin on the alternator (the things I am pointing to in the pic) = NOTHING
Voltage from either of the female plugs (in my hand in the pic) = 11.67
Voltage from the red terminal on the back of the alt = 11.67

Thoughts?
 
That ALT is dead. Return it for another one.

Also, move that ground cable to the engine block. Whats with the tape on the end of that ground cable?
 
Also, here's another... I noticed that the pins seemed to be a little scorched when I pulled the plug (which doesn't seem to be keyed, btw):

alt_02.jpg


Note also that I took the reading Scott asked for with the plug OFF. I will go try again with it plugged in.

-Dan
 
For reference, I took the measurement to the Red terminal on the back, and it was still the same with the plug in.
 
OK, so now I have other questions:

1.) Does it matter which way the plastic plug is plugged in? They don't seem to key keyed?
2.) As long as I am buying a new alternator, is there one that will fit this truck and be bigger and put out more at low RPM? Obviously loss of power isn't an issue on the 454, and I suppose given enough time, I'd do lots of research, etc, or buy a re-wrapped Alt. But I am leaving for camping/wheeling Thursday night, so I'll need to basically pick one up at NAPA/Kragen/wherever tomorrow. What should I be asking the parts monkeys for?

3.) Where on the block is the best place to ground? I will move that cable (maybe just buy a new one altogether as long as I am at the store.... Thoughts?
 
As for 3), there should be plenty of places to ground it. Check for an existing ground on the block itself and the cylinder head(s). I have both grounded, but with ground straps. On the block, I'm grounded on the fuel pump cover.
 
dbreid said:
1.) Does it matter which way the plastic plug is plugged in? They don't seem to key keyed?
Click to expand...

This one I am still not sure of?

dbreid said:
2.) As long as I am buying a new alternator, is there one that will fit this truck and be bigger and put out more at low RPM? Obviously loss of power isn't an issue on the 454, and I suppose given enough time, I'd do lots of research, etc, or buy a re-wrapped Alt. But I am leaving for camping/wheeling Thursday night, so I'll need to basically pick one up at NAPA/Kragen/wherever tomorrow. What should I be asking the parts monkeys for?
Click to expand...

The answer to this seems to be in this thread!

http://coloradok5.com/forums/showthread.php?t=230956

dbreid said:
3.) Where on the block is the best place to ground? I will move that cable (maybe just buy a new one altogether as long as I am at the store.... Thoughts?
Click to expand...

Gotcha. It is dark now. I'll look for a good spot tomorrow.
 
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