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How do I know if the alternator is dying?

1K views 12 replies 7 participants last post by  TFLundyB275 
#1 ·
Last 7 or 8 starts were laggy. This am it just burped a couple times. After checking connections and cleaning them...put a charge on the batteries and got a fast start.

The alternator is 10 years old (batteries 2 years old)and that is my first guess but is there a way to check without removing it and taking to the parts store?
 
#2 ·
If you have a voltage gauge the running volts will be close to 14, on a stock gauge that will roughly halfway between the 13 and the bottom of the high voltage which has red under it.

If the alternator is weak it will charge properly when there is no load on it, no fan, no headlights but once you turn those on it will drop to 13 or lower.

If you have a chance, swing by a parts place, or phone them, and ask if they will do a battery test for you. Most places will do free test's to sell you batteries. You'll just have to unhook 1 battery so they can test 1 at a time.
 
#11 ·
Thanks for the advice. I stopped by AA and they did a systems check. He insisted it was not necessary to unhook a battery.:confuzeld

He said results were..batteries are good and alternator without a load was 13 and with a load was 12. Am I on the cusp of failure? Did he even do the test correctly?

It started fine x2 today. Will see what the morning will bring

About my dash gauges. Pretty much none of them work anymore, including the voltage one.
 
#4 ·
Ok, so you don't have a factory voltage gauge then.

This is the factory one.

Gauge Auto part Speedometer Tachometer Measuring instrument



You can use a multi meter at any point in the truck you can find power, eg the cigarette lighter, fuse box, loose assorted wires under the dash, across either battery. Pretty much any part of the dash will be ground if you do it in cab.

Set the meter to 20 and under DC, it may just be labeled with a 2 lines, 1 solid 1 dashed. Te black lead will go to the common and the red to V Ohm Ma and as usual red is positive, black is ground.

The wavy 1 is AC and don't use the amperage setting if you never done it before.

Measuring instrument Product Electronics Tool Technology
 
#5 ·
You may have a bad battery. If the gauge is showing proper voltage around 13 then youll probably have to pull the batteries to have them tested. Not running iirc you should be at around 12.8 volts but this needs to be checked with the Batts disconnected and like said 13 to 14 while running. Check that your Alternator belt is still there:eek:
 
#12 ·
Yes, belt is there and seems pretty snug.About a half thumbnail slack when pressed.

Advanced Auto system check said 13 without a load and 12 with a load and batteries are good. He didn't disconnect anything, so not sure how accurate testing was.
 
#7 ·
Never heard that tip, I like it! Get a cheap multimeter, even the free one from Harbor Freight will work. Like the others said, you should be charging somewhere around 14v. But you may have one or more bad batteries too.
 
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#9 ·
My alt died last thursday. Everything died inside no lights in the cab, only one semi hard start (slow crank) but the truck kept running. When I got to the shop I put my meter on the bat and it read 7vdc. 14. something with the new one installed and after charging the batteries.
 
#10 ·
On factory wiring, the cab power is picked up off the alternator. Whats interesting about this is the fact that it should of pulled power from the battery. Interesting....
 
#13 ·
Without a load, and no other issues present (recently drained battery, etc.) the voltage should be 14.3-14.5. Anything under that proves there is a charging issue. You said the belt is tensioned, having good connections, a good set of batteries- after a few minutes (worst case) run the charge should be those figures. Even with a slightly slacked belt you will get there with no load.

For comparison, if you left your headlights on for a good while and managed to start the engine, your voltage will be low for a good while, in some cases perhaps even with 10 minutes of run time even with the charging system in perfect condition.

A test with "load" is difficult to figure as depends on the load, condition of the belt/pulley/tension, condition of alternator, alternator amperage, how depleted batteries are, RPM of alternator, etc.

Simply, I would highly suspect the alternator is going bad. Its known to happen.

If you decide to swap out, I recommend you go a new or high quality remake. Make sure the amperage is 78, I believe that will be your best bet. The other options are 66Amps, but you may find that it doesnt put out enough in some cases, although you can get by with it. A 94Amp is available, but you will increase the chances of belt slippage and squeal.

Make sure you are running a 5725 belt. Don't use anything less.
 
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