testing the alternator?

[blazinjames]

how do i properly test an alternator on a 95 4L jeep cherokee?

does my battery have to be fully charged to test properly?

 

[xjbubba]

One easy way, if you have a digital volt meter (DVM), is to test the battery voltage with the engine off; s/b ~12.6v (battery charged, and in good condition). Start the engine and check the battery again; s/b ~13.2 to 13.8, depending on the state of the battery.
If you see the numbers, above, it indicates your alternator, regulator, and wiring are probably OK.
The battery does not have to be fully charged to test the alternator. The initial state of the battery will effect the alternator's charging rate. The fact that it does, indicates a properly operating alternator/regulator.
As an example, if the battery was low, voltage, let's say ~12.4, when you start the engine, the alternator will regulate the voltage to the high end of its range, or ~13.8-14v max; this would be normal. As the battery recharges, you would see the alternator output slowly drop to ~ 13.2 volts--indicating the maintenance output level of the alternator, again, normal.
This is not an exhaustive test of the alternator's ultimate output capability, but tells you that it is "probably" OK.

 

[blazinjames]

I just test the battery when the jeep was off it read 11.64, then i tested it running it was 11.20

then i turned on the lights, heater and radio and it was readin 9.22 so i figure the alternator is shot?

 

[FloydXJ]

Make sure you are getting good contact, but if those are the actual numbers, it is definitely shot, and the battery could use a change as well. If your battery light in the dash is working, it should be on when the engine is running if it is working properly.

 

[birchlakeXJ]

Here is a similar thread to read.

http://www.naxja.org/forum/showthread.php?t=1025013

 

[Johnnie Walker]

Is your belt tighten properly? I would say have a parts store test your alt. And have them trickle charge your battery overnight.
But it sounds like both of them are f'd up.

 

[Shorty]

Is your belt tighten properly? I would say have a parts store test your alt. And have them trickle charge your battery overnight.
But it sounds like both of them are f'd up.

I'd hold off on calling the battery F'ed up until you charge it and test it. If it had 11.4V and still cranked the engine, it's probably just discharged.

 

[xjbubba]

I just test the battery when the jeep was off it read 11.64, then i tested it running it was 11.20

then i turned on the lights, heater and radio and it was reading 9.22 so i figure the alternator is shot?

Those readings are totally wacky!! It's hard to believe a battery that read 11.64 volts could start the engine?? Then, when running, the alternator output is less than the battery voltage? Doesn't make sense.

 

[birchlakeXJ]

Don't use battery voltage readings to determine condition of the battery or the alternator. Inconclusive and you don't want to chase your tail.

*Test output of alternator with a meter or have it tested at a shop
*Load test the battery.

 

[winterbeater]

I just test the battery when the jeep was off it read 11.64, then i tested it running it was 11.20

then i turned on the lights, heater and radio and it was readin 9.22 so i figure the alternator is shot?

Clearly the alternator is not charging the battery. Best bet is to take it off and have it tested. It is probably bad.

The other less likely possibilities are the voltage regulator (in '95 built into the PCM IIRC), or the wires/connections between the alt., batt., or regulator.

 

[winterbeater]

I just test the battery when the jeep was off it read 11.64, then i tested it running it was 11.20

then i turned on the lights, heater and radio and it was readin 9.22 so i figure the alternator is shot?

More than likely, it IS a shot alternator. I'd pull it off and take in for a bench test.

The other less likely possibilites are a bad voltage regulator (IIRC built in to the PCM by '95) or bad wires/connections between the three major components.

 

[Matthew Currie]

Those readings are totally wacky!! It's hard to believe a battery that read 11.64 volts could start the engine?? Then, when running, the alternator output is less than the battery voltage? Doesn't make sense.

A lower reading when running makes sense if the alternator isn't working at all.

 

[marcoleo]

I'd the rectifier shot and when the engine was running the voltage drop down (renix with internal regulator)

 

[joe_peters]

Those readings are totally wacky!! It's hard to believe a battery that read 11.64 volts could start the engine?? Then, when running, the alternator output is less than the battery voltage? Doesn't make sense.

Starter's are dumb as a box of rocks, they want amps more than volts, so the voltage can be very low and they will still turn the engine over enough to start it.

The electronics will start screwing up when the voltage drops below 9 volts, but the engine may still start but run like crap with voltage below that threshold.

So, no, a battery with 11.64 volts static has more than enough voltage to start the engine if there is enough amps to operate the starter.

The higher static voltage, 11.64, than running voltage, 11.20 volts, makes sense as the engine is pulling down the battery more than the alternator is charging it--with the eventual effect that the voltage shortage will reach the critical point and it won't run anymore.