Battery only showing 10 volts

[Mythic]

Hey everybody, I have a 1990 c1500. It's 2 wheel drive, 1/2 ton 5.7 TBI.

Last thursday, I left the lights on and killed the battery. I tried jumping it but it would not jump, even after letting it charge off the other car for 10 minutes. Eventually grabbed the battery out of my 1978 C10 and it fired right up.

Then Saturday, I was driving down the road and the whole truck died and would not start back up. It has over a 1/4 tank of fuel in it. After letting it sit over night, it fired up yesterday morning. But it was only registering 10 volts or less. I put it on the charger overnight and the charger said it was fully charged when I put it in today. It fires up, but is still only showing 10 volts or less and if I turn on any accesory, it drops below 9 volts and will barely idle.

Today I purchased a brand new battery thinking that maybe the one I took out of the C10 was just no good anymore. Wrong. The new batterry registers the same voltage as the old one. I am lost. All the connections seem to be good. The ground is attached to the intake manifold.

Any and all advice would be very very much appreciated. Even if it's the most basic advice, please throw it out there. This is my daily driver and I need it running for work.

Also, there was a wire coming off the alternator, it is green with a gray connector thar matches up to another connector under the air filter. This was dissconnected. I reconnected it. I have not tried to start it since then so I don't know if it has affected anything yet. Being a single green wire, I assume it is a ground, but being connected to something under the air filter threw me off.

Thank you in advance for any replies.

 

[jdyates]

The gauge doesnt show how much voltage is in the battery, it shows how much power the alternator is pumping back into the battery to recharge it, if i remember correctly. Mine usually sits at around 10-12 with my lights and radio on. Then it drops a bit when i turn my Off-road bumper lights on because of the alternator now powering something else.

 

[MOBS]

It's bad on the alts to have to charge a battery from near dead and to keep your engine running........especially for the stock alts that have such finicky regulators. It seems you've burned out your regulator, or it's so weak it's not outputting anything. What happens is the current draw overheats the diodes in the regulator, so you technically just need a regulator, but since it's technically an internal(non-replaceable unless you can solder very well)regulator, you need a new alternator. I'd take this chance to upgrade to an optioned alt if I were you, look for one from a c3500 with a 454. The stock alt's series is most likely a C130(105amp), so look for a C144(144amp).

 

[MOBS]

I didn't notice your comment about the unplugged green wire, go do a test with it idling. Immediately after it starts, use a volt meter and test the voltage at battery....after it's run about 5min, test again. It shouldn't be any more than .5% loss or you have an alternator issue. At cold, the alts put out a tad bit more voltage(and alot more amperage), but the voltage you should be aiming for is at the very LEAST 13.5+.

Do the above tests with all lights, radio, a/c, heat, fan turned off. Also, if you get more than 10% voltage loss with all accessories turned on, your cabling isn't large enough and needs to be enlarged to handle more current draw. If it still drops more than .5% after upgrading cabling, you need a bigger alt.

 

[bowtie-till-i-die]

my thoughts exactly, alternater

 

[Mythic]

Thanks for the replies guys. I really appreciate it.

Mobs, is the alternator off the c3500 with the 454 a direct fit for the existing bracket on my truck?

Also, what years c3500 should I be looking at?

 

[MOBS]

I'm actually checking now, the bolt up would be direct fit, just double-checking power plug.

 

[MOBS]

What side of the motor is your's installed on, driver's or passenger's?

 

[MOBS]

Also, when looking at the front of alternator, imagine a clock's face. The lower mounting point is the pivot joint, that should at at the 6:00 position. I need to know if your upper mount point is around the 10:00 position. Also, is your electrical plug in the back around the 5-6:00 position when looking at the front of alt?

Thanks, I should have you a vehicle from which to order an alternator from here soon. Just wanna make sure you get the right replacement. The CS130 alts like your's came in 3 different configurations, so wanted to make sure I chose the right one.

 

[Mythic]

Mobs, thanks for burning so many calories for me, I really appreciate it.

The mounting points for the alternator are at 4 and 7. The mount at 4 appears to be the pivot. As for the wiring, there are two seperate wires, the black attaches at 3 and the red attaches at 11.

These are all in refernce to a clock face looking at the alternator from the front of the truck.