Before I give you my thoughts try one more test. Disconnect all alternator leads and plugs (be careful to insulate the red lead) and repeat your tests.

The 100 amps during the load test and the 2 amps from the bulb test are both in the correct range indicating that your amp clamp meter is close enough. Your battery load test is good. Your voltages and currents in the truck make no sense. The question is how can a battery draw 100 amps and hold 12v in a load test but later in the truck draw 8 amps and drop to 5V? You have done a good job confirming your meter function and you properly load tested the battery. Lets say for now that your readings are accurate, then there are only three explanations:

Intermittent bad battery
An unmeasured additional battery drain

A couple of thoughts. When you load tested the battery is it clamped in the Blazer battery tray or out of truck? IOW does the battery get moved or clamped down after the load test? Does the battery look good on the outside, no cracks or signs of overheating? Is the positive battery post near any other wires, cables or metal? Have you tried separating the red and black battery cables as far apart as possible for these tests? If it wasn't for the good load tests I would have said long ago that you need a new battery but I cant say that with certainty given all these test results. That's why I had hoped that you had access to a known good battery as a test so you don't have to spend over a hundred dollars and possibly find out that there is nothing wrong with the battery you already have. If the alternator test is a bust, I don't know what else to suggest. I don't get stumped often but I am almost there.


George

 

George,
I think you may be on to something. I have figured all along that it was probably something simple, something that I thought was insignificant. I read your post a 2am this morning and it made me think, what have I not told you?

Well when the battery is being pulled down I can smell sulfur so the battery must be getting hot.

The battery is being left in the truck during testing. Clamped in. No visible damage.

Now the big one, I think. When I went to get a battery they did not have a group 75 with just side posts, so I got one with both top and side. All my testing in the truck have been from the side posts, except the load test. Is it possible for a battery to be bad on one set of posts and not the other? I quickly got up and went out to check. Load tested using side posts and it was a complete failure, pulled the needle all the way down as far as it would go.

I think I need another battery, but before I put another brand new battery in is this something that the truck caused? Or is just simply a bad battery?

Let me know your thoughts.

Thanx,
Karl

 

My guess would be a bad battery but I can’t say that with 100% certainty. I don’t know what a dual post battery looks like inside. That’s said I would have the amp clamp meter and wrench ready so that as soon as you install the new one you can test both cables in all modes and disconnect quickly if needed. Start with that black cable disconnected from the starter as it’s the only one that’s unfused and capable of 100 amps. Go slow, a shorted battery can explode. Happened to me once in the 70’s.


George

 

Two sets of battery posts. Now I can finally get some sleep. lol. Then we get back to your original problem after the new battery and get to work looking for excessive power draw.


George

 

What is considered excessive?

 

I'll take some measurements of my 2002 start up sequence in the morning. Your standby parasitic draw is good.


George

 

Key off. Put battery in. No problems.
Key off. Hooked up starter. Hooked battery back up. Engine starts cranking.

 

Pulled crank fuse and starter relay still cranks.

 

The starter cranks with the ignition in the off position?


George

 

Assuming yes one of two things is happening, Either there is a short to power on the purple wire going into the starter which energizes it or the starter is stuck engaged regardless of the status of the purple energizing wire at the starter. Two things to try:

Disconnect the purple wire at the starter, reconnect the battery and see if the behavior stops

Wrap the starter a couple of times to see if it returns to the resting position then reconnect the battery

If the starter is always on after trying both of those things then either it is defective or the bendix is binding on the flywheel and it wont disengage when power is removed.


George