Engine/Transmission (1994 - 1998) Is there a way to test alternators

[NCousins]

On my '97 12v about 160k on it, I have occasional charging, most of the time it isn't, or it was not charging above 12. 1v. I lost the alternator after I changed the fuel plate, at the same time I cleaned all the battery terminals. I do not think the two are connected, but??.



I went in and added a external voltage regulator, to bypass the PCM, now I have no charge, the engine will not turn off with the key. Have to pull the power the to regulator (the power is switched) to kill the engine. Cut the dark green wire (PCM), as shown on the wire chart from hayes manual.



Have pulled the brushes from the alternator, they are worn, but have good contact. The outside brush has a deeper grove in the shaft then the inside brush.

 

[kenny61]

Any alternator shop can bench test em I tink some auto parts stores such as pep boys or autozone have the bench testers as well

 

[JDKammin]

Is your tach working correctly? If not, your tach and alternator have a common interest, the sensor on the front of the engine at the dampner.

 

[LRutigliano]

You can test your alternator on the vehicle with a digital VOM. Battery voltage with the engine off needs to be 12. 5 to 12. 6 volts (fully charged) when starting the test.



Start the truck and let it idle and check the voltage at the batteries. It should be 13. 5 to 14. 5 volts. Start turning on loads such as the lights, blower motor, wipers, etc and it should stay at 13. 5 to 14. 5 volts. If it drops to less than 13. 5 volts, you most likely have a bad alternator, but do the next step...



You can increase the engine RPM and see if it comes back, some alterators will not provide enough amperage at idle speed for the number of electrical loads you have turned on.



If the battery voltage reads less than 13. 5 volts, the alternator is bad.



Hope this helps,

Louis

 

[kenny61]

Not to start an arguement ... when testing it on the truck you are testing the the whole charging system altenator, pcm,wiring and whatever else regulates the charging. Bench testing it checks only the altenator removing the other variables

 

[rattle trap]

Not to start an arguement ... when testing it on the truck you are testing the the whole charging system altenator, pcm,wiring and whatever else regulates the charging. Bench testing it checks only the altenator removing the other variables

This is partially true. You can test one major function. An internal failure is nearly always brush or diode based. Once brushes are done... no charge what so ever, regardless of field voltage (which you can test on vehicle)



The other failure, of course diode based, usually low voltage.



How do you know if your diodes are bad w/o bench testing? Get out your lovely DMM, and select a/c voltage. Place leads on battery as normal DC test. Diode failure will display a presence of A/C voltage. Got more than about . 2 vac? Go ahead and rebuild or replace at your discretion, the bridge is on the way out.

 

[kenny61]

Did not know that. On the older cars I remember you could ground or jump the field to by pass the voltage regulator but I got no idea what to do on these new fangled cars they got so many things that effect something else its hard to pinpoint what the heck is causing the problem.

I'm gonna have to remember the ac voltage thing

 

[Breed]

I was having all type of trouble with my alternator until I replaced the brushes from Larry B's also replaced the starter from there and put in a Add-on starter relay kit from them and every thing works just fine now. I would call Larry B's, LarryB's Dodge starting problems solved here.
253-272-1444 or e mail him at -- email address removed -- and he will answer all your electrical question for the Dodge truck and he's pretty inexpensive and stands behind his products. He is a expert on Dodge Starters and alternators. I had a mess until I spoke to him and he's just good. I put in a new DENSO starter with the big contacts and he overnighted them to me so I got them the next day. Easy fixes and simple directions.

 

[rattle trap]

Did not know that. On the older cars I remember you could ground or jump the field to by pass the voltage regulator but I got no idea what to do on these new fangled cars they got so many things that effect something else its hard to pinpoint what the heck is causing the problem.



I'm gonna have to remember the ac voltage thing

The a/c voltage presence or diode failure, if you will, is usually caused by incorrect jumper cable hook up, or heavy load at idle.



Interestingly enough, you can often hear this a a high frequency buzz. The buzz is often mistaken for bad case needle bearings. The larger roller/ball bearings are more likely to growl.



I found this out only after accidentally doing a 100amp load test at idle. Bad stuff there. Never want full load below 1500-2000 rpms. Good reason to raise idle when jump starting as well.



The diodes are quite sensitive to heat generated by the windings and the diodes themselves. They do make external heat sinked diodes capable of handling 250-400amps, but the extra $200 is hardly worth the cost.

 

[NCousins]

thanks guys for the info. pulled the alternator, took it in to be tested, they could only get 9. 2v off of it, so it seened like a good time to replace. did not know that larry b sold replacement diodes.



I had all ready replaced the alternator, before I read about check for AC voltage, so I can not check that now, that woulod of been interesting.



I could not tell in there was a high frequency buzz, can'nt hear anything over the engine, probibly because of exhuast leak at the manafold.



thanks again for all the interesting ideas, that I have to file away. .