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Alternator not charging

cabledawg

cabledawg

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Ok, so I thought I had gotten another bad alt from AZ, so I decided to steal the original alt back from my other truck. The original was working fine in the other truck, and the new one in my truck worked for three days, then quit. Its on a stock 90 GMC Burb, so I think its the CS130. Anyways, I checked for voltage on the plug. The brown one is wired to the stock harness, but the red (or is supposed to be pink, cuz thats what it is now) one had issues and is now run out of the fuse panel. That one didnt have power, come to find out it had gotten knocked loose. Plugged it in, but still no charge.
So I go ahead and put the original back in and fire up the truck. But even with a known working alt, the voltage is only about 12.5, so the charge cycle isnt coming on. Checked all the connections, and everything is tight. Could something else be causing the alt not to cycle? I didnt unhook the batt to see if the alt was supplying power as I assumed at 12.5 volts, its not even on.
 
The CS130 alternator requires a sense wire to make them charge. I'm not familiar enough with the alternator wiring to tell you what to be looking for but i'm pretty sure Dorian can help you.
 
The 2 wires that are together should be hot with the key on . One is hot all the time and the other is set to come on when the key is turned on . They are both hot with the key on remove the wire that is all alone and with the vehicle running check that post and it about 14.7 volts if it does then the problem is the wire to the batteries is not working .
 
Both are hot with the key on, and with the engine running I'm only getting 12.5 volts. I've rad and re-read alot of dorians stuff, and the plug seems to be the usual problem that everyone has. I checked the plug first for that reason. The plug has voltage where it needs it, but something is still amiss. I'll double check the battery cables today sometime.
 
So the charge lug is also only at 12.5V? Not real clear from your other posts, I think I interpret you did indeed check that.

How are you checking voltage in the first place? Voltmeter?

Don't unplug the battery, that's a good way to make a good alternator a bad alternator. :)
 
Voltmeter and dash gauge. I have checked on the alt post, the plug terminals and the battery. At most is 12.5 volts. I'm wondering if the cable coming from the alt post to the battery has perhaps gone bad, but that still wouldnt explain why I have low voltage. Unless this is what is frying the alternators.
 
If you watch voltage while it's idling (off the alternator lug) and then turn the heater on high, I assume there is no voltage compensation?

If the wires to the alternator itself are good (you've tested with key on AND key off to ensure both are good?) then the alternator should start charging based on the system voltage I think. That's how the alternator "knows" (input) that its time to start charging, while the lug is the output.

I may have to look up diagnostics in my '91 service manual tonight for you.

On the "retrofit" setups like mine I have to run a resistor inline, I'm sure you saw that. But my understanding of the factory setups is the light does that function. If your re-wiring removed resistance, I don't know what happens, but I know it wouldn't work right.
 
Everything was working fine prior to taking the original alt out. It was used as a known working unit to aid in fixing my wifes truck. Once it was in, I just left it and got a new one for my truck. The first worked for a day and went bad. AZ tested said its bad gave me another. That one worked for three days then quit. Now the original doesnt work, so I am assuming something in the truck is causing the issues, not just bad alternators.

I havent tested with the key off on the plug, but if the key is off, the voltage shouldnt matter (duh, the engine isnt running). I havent tried placing a load on the system (lights, blower) and checking voltage, but there isnt a difference in voltage with the engine running or just in "on". At least unitil the battery starts to run down.:doah:
 
You wouldn't think the constant vs. ignition switched would matter, but I assume it's a "source" deal. Pull it from a different location to get a better reference of overall system voltage. If for some reason that wire is bad, should show up with the key off.

I expect the alternator wouldn't "backfeed" on the constant 12V wire or it wouldn't be a good reference, but I don't have my book here in front of me.

Does the truck have a "charge" light that is supposed to come on with the key in run, engine off?
 
If there is, it never works. I have the gauges, no idiot lights. My check engine light comes on periodically, but its been doing that for two years now, so I dont think its related.
 
Let me see what my service manual says for tests on these things, or where to look for a no charge condition.

It'll be a few hours though, sorry.
 
It's all good. I appreciate the help so its not my place to complain:D

Besides I'll be swapping a clutch slave this evening anyways.
 
To get started with, the manual states that the wire going to terminal "L" on the alternator plug MUST be hooked up through an indicator lamp or suitable resistor. It states that the charge light will come on if ANY charging problem occurs, or if the system voltage is too high or too low.

Forgot that my dad has my wiring diagrams for the truck, however the Camaro one is also brown on "L".

In-depth testing is going to be near impossible I suspect, as they call for a carbon pile.

However, up to the point where they call for that, here is the procedure, in order (abbreviated so I don't have to type as much):

1 Check belt for tension
2 Go to step 5 for vehicles without charge lamp
3 With engine control on, engine off, lamp should be on. If not, detach the wiring harness at the generator and ground "L".
If lamp lights, replace generator
If lamp does not light, locate open circuit between ground lead and engine control switch. Lamp may be open.
4 With switch on, engine running at moderate speed, lamp should be off. If not, detach harness at generator
If lamp goes out, replace generator
If lamp stays on, check for grounded "L" terminal wire in harness
5 If battery is undercharged or overcharged
Detach harness from generator
Key on, engine off, connect voltmeter from ground to "L", and to "I" if used. Wiring harness may connect to either "I" or both.
A zero reading indicates open between terminal and battery. Correct as required
Measure voltage across the battery. If above 16V, replace generator.

From here it starts testing alternator output with the carbon pile.

I don't know as that helps much, but that's all they've got up to the carbon pile. But if the voltage isn't changing, you know you don't have to dig that deep.
 
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If by indicator light they mean idiot light, I dont have one for the battery, just the gauge. But my check engine comes on, then turns off when the engine starts. I'll give it a shot tomorrow and let you guys know what happens. I'm going to take both alternators down to AZ to get them tested. Who knows, maybe they both went bad on a fluke.:(
 
Have you heard of the big 3 upgrade. search it on google. you replace our wimpy power wire.ground wire and something else. it really helps with dimming headlights running winch's etc. just something i thought i could throw in there since your messin with your altern.
 
Yeah, it would be an idiot light, on the older trucks I think they actually said "charge" or something to that effect.

Check engine light is separate, but it should be lit with key on/engine off.
 
Not sure how the exciter wiring works but my 89 blazer has a volt gauge and no idiot light at all other than the SES light which like you say is independant of the charging circuit so i don't know where it would get a resistance reference from.
 
On the wiring diagram I have, it showed either a light, or a resistor. So I expect it was one way or the other. I'd be interested to see where they hid the resistor.
 
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