Alternator or bad cable - you be the judge

[rgpinder]

Here are the symptoms:
- Battery was bench tested last month and deemed good.
- 2001 suburban wouldn't start the other day. My wife jostled the battery cables at the terminals and it fired up
- Driven around today with no problem, but after greasing the steering shaft it wouldn't start. No headlights and no crank.
- Cleaned the terminals and tightened the connections. Now at least I have power but still won't turn over.
- Voltage tested at 12.5v at the battery terminals while not running. A bit weak but still should be enough to at least hear a crank or clicking, right?
- Voltage tested at 12.8v at the battery terminals while running and with headlights, dome lights on. Hmmmm. Makes it appear to be an alternator problem, although I just put a rebuilt unit in a month ago.
- Alternator tests at 14.8v from the alternator to the negative battery terminal. Good alternator? Weird. Why do I get 12.8 at the terminals but 14.8 between the alt. and the battery? Bad cable?
- Jumped the car and it fired up but died a few times. Finally got it running and let it idle for a few. Took if for a 5 minute drive. No problems. Ran as normal but battery needle kept fluctuating erratically up and down. Another indicator of a bad alternator?
- When I parked it and shut it off, I tried to immediately restart but it wouldn't even crank. Not even a clicking from the starter motor.
Thoughts?

 

[Spriteman]

Battery voltage does not equal cranking amps. An almost completely bad battery can still read 12v but there is no way that it's going to crank over a vehicle - let alone a big 5.7L or larger in a burb.

You said the battery was bench tested - did they test the battery under load? In other words drawing the same load it would take to crank an engine and measure the volts during that test.

14.8 volts from the alternator sounds reasonable. It still could be a bad cable but you can easily test that at the battery end of the alternator cable. If you see 14.8v there or just below the cable should be good. Have you cleaned the ground cables where they attach to the frame or body. If you don't have a good ground to complete the starting/charging circuit you will not have enough energy to turn the engine over and the alternator will not be able to charge the battery.

 

[Chrisman889]

I'm thinking the alternator doesn't have anything to do with lights or starting. I took my alternator out completely last night and still started and cranked had lights and stereo, just very low voltage. I would say battery or cable. I've been toying around with getting some 2 gauge myself just because of all the recent problems I've had.

Is it very cold where you are? That affects the battery a lot where I live.

 

[VanGo]

You can test the battery ground cable by bypassing it. Just hook a jumper cable from the neg post to a good chassis ground. Does that make any difference?

 

[rgpinder]

SOLVED!
Thanks for the suggestions. Here's how I resolved this today. I went to do a voltage drop on the positive cable but the way the 01 Suburban was designed made it impossible to get to the connections on the solenoid with my mammoth manly hands. So I pulled the starter. Starter fired up. But the voltage drop on the cable indicated a major drop. Pulled all the positive cable connections (starter trigger, starter amp wire, battery junction box, battery connection and the fuse box) and cleaned everything with a wire brush. Put some anti-corrosion gel on the things and reassembled. Then did the same with the negative. No issues there but just wanted to make sure the grounds were solid (engine block and frame). Charged the battery while doing all this. Fired right up.

 

[zora]

Great to hear it was a simple fix!

 

[Spriteman]

It's crazy how many things are affected by bad grounds.

Now you should be good to go for a while.

And - you didn't have to spend big $$$ throwing parts at it.