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Alternator questions- short circuit protection?

hubs97xj

NAXJA Forum User
Location
Fort Wayne, IN
Just ran an errand in the Jeep, and saw my check gauge light on, and saw that my alternator was sitting on 9. Drove 5 minutes, came home, turned it off, then restarted. Needle jumps back up to ~14, no warning light.

Now, it's been pouring rain here most of the night, and I've had this happen before, after crossing a flooded intersection- water up to the floorboard, plowed into it doing 30. Needle dropped, light came on. Same deal then (2 or 3 years ago)- drive it home, wait a bit, restart, no problems.

I'm curious- do the alternators have some sort of integrated short protection, or is mine probably dying? Is there something related to the voltmeter/ckt that might short and show no charge, without the alternator failing? What are some likely causes here?

Newer belt and battery, spec'ed tension, no other issues that I'm aware of. Got to make a 3 hour trip this afternoon, and getting stranded on the road is obviously something I want to avoid. I'll probably hit the local auto electric shop and have them look it over to be safe, but I thought I'd ask here as well.
 
Slippery when wet may be appropriate.

If the alternator consistently has those symptoms I would have it tested. If it only happens in wet conditions, I would check the age/condition and tension of the serpentine fan belt.
 
I have had that happen on several ocasions. No Biggie, turn the key off then re-start and away I go until I drive through high water again. It usually happens when the belt slips. The power steering also go out at the same time except the steering is restored within 60 seconds after coming out of the water.
 
I'm curious about the gauge- with the key in "run", motor off, it shows battery voltage. I figured something must be shorting for the gauge to drop off with the engine running- unless it's showing only the alt output when it's running... in which case, the water and the freaking 100# of tension the Krikit's showing me...

Belt's apparently shot, but that's easy enough. Have to do some poking around and figure out where it's reading the voltage. It's not acting the way my other cars did with dead alternators (either fried, or when belts let go, etc), which leaves me scratching my head.
 
Open frame auto alternators will get wet and stop working in the pouring rain, or fording creeks. Once the water spins out of the brushes and off the commutator, the shorting stops.

It's normal.
 
First things first - the IP gage is a notorious liar, and suspect readings should always be verified using a "known good" multimeter.

Moving on...

When your alternator gets wet (as yours is, and usually with "road water" with lots of crud and grit mixed in it,) you're dealing with a few problems:
  • The internal connections get wet
  • The brushes stick for a little bit
  • The belt gets wet and can slip
  • The bearings get wet (they're not sealed)

And probably others I can't recall as well.

The internal connections get wet, which knocks down output (if it should surge, there is a fuse in the output circuit that is meant to protect the rest of your electrical system. However, it usually "goes dead" rather than surging.)

Mud, grit, oil, and other things in the water can get into the brush holder, which causes the brushes to stick. They're solid bits of carbon that are held against the slip rings on the rotor with very light springs (so they're free to bounce,) and they can bounce off and not bounce back until they're dry. Or you rinse them out (using electrical contact cleaner is preferred, but clean clear water can be used in a pinch. Just make sure you run it hot afterwards to dry it out, as the springs can rust and break - then you're boned.) The alternator drying out usually takes five to ten minutes.

The belt getting wet causes the other problems as well - power steering loss, the water pump doesn't get driven as efficiently, the belt slips on the alternator (how much depends on electrical loading,) and the aircon can cut out for a minute or so. This usually goes away inside of ninety seconds.

The bearings getting wet doesn't cause a direct problem with output, but can lead to premature unit failure. Just bear that in mind.
 
When, for whatever reason, alternator output drops on a running motor, the motor and electrical devices draw increased current from the battery, and a meter connected across the battery will reflect a drop in voltage below the normal at-rest voltage of the battery. This is a function of "ohm's law" and is perfectly normal.

It is difficult to compare with the situation when the alt failed on your other vehicles. For example, when a belt broke with all its attendant clattering , you probably didn't spend a lot of time staring at the voltage gauge while the motor continued to run. Regardless though, the same principle applies to any battery and load situation.
 
When, for whatever reason, alternator output drops on a running motor, the motor and electrical devices draw increased current from the battery, and a meter connected across the battery will reflect a drop in voltage below the normal at-rest voltage of the battery. This is a function of "ohm's law" and is perfectly normal.

Was thinking about what I was seeing, and had another look. Went out and checked the gauge again- it wasn't on the bottom of the gauge this morning (it falls below 9 when off), but it was sitting at 9- unless I've missed something else, that's exactly what I was seeing- the increased load on the battery.

Thanks for the info on the alternator- good to know, and I'll feel a lot better not wondering if my alt is going to die when I'm going to be three hours from home. I figured there would be something to protect the charging system, but I didn't know what, how, or why.
 
Truly poor wiring, marginal cooling system, low mounted alternator--why do we love this thing called XJ?

Because they are fun, and we are a bunch of sickos that like aggravation, pain, and humiliation--just like my first marriage.
 
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