For some reason my truck won't start with the truck battery in it. With the battery in the lights come on in the dash and the interior lights come on but when I crank it nothing happens no noise or anything. The battery is reading about 12.79 volts. If I put my car battery in it the truck starts right up. With it running the alternator is showing a little over 14 volts so I think the alternator is working fine. I changed the starter solenoid and cleaned up the connections. I would think 12.79 volts would be enough to start the truck or at least make it click. Any suggestions??

 

How are you checking the 12.79 volts? If you are just touching the meter to the battery terminals while it's just sitting there, then that is not a good test. You cannot check a battery unless it's under load. Put your truck battery back in, and get someone to try and start it while you are holding the meter leads on the battery. You may find your 12.79 will quickly drop very low when someone tries to start the truck.

 

Yeah that is how is was checking it. So if it drops below 12 under load then the battery is probably bad? The battery is almost 5 years old so it's probably about time to replace it anyway.

 

They can drop to around 10v or so when powering the starter, but it will still crank the truck. Do you have a small battery charger? It may just need charging. If you can check the water level, check that also.

 

I can't find any of my chargers of course because I need it. Can you add water to these batteries I seem to remember my dad putting water and aspirin in batteries when I was a kid but maybe I'm imagining things

 

I just checked it while trying to start and was getting 12.58 still no start just a click

 

It can't have 12.58 volts while the key is in the Start position and the engine not turn over since you've proven that the solenoid, starter, and connections are good by using the other battery. Make sure you check the voltage, preferably at the solenoid, with the switch in Start.

Or, if it truly did have 12.58 volts in Start you somehow have a bad connection at the battery on this battery and not the other one. And, that would mean you are checking the voltage at the battery post and not on the cable's connector.

 

I had 12.58 volts at the battery post when turning the key to start I'm pretty good at reading numbers.

 

Sorry, didn't mean to say you aren't good at reading numbers. Was just thinking as I typed.

If it was at the battery post and stayed at 12.58 while the solenoid clicked then you have a bad connection at the battery post. Either the positive or negative battery cable is not making good connection with the post. But we know the cable is good as the other battery cranks it. Is the post smaller and the cable doesn't tighten up? Is the post very dirty?

Edit: Check the voltage at the solenoid on the positive side and the engine block on the negative. I'll bet the voltage drops to or close to zero.

 

Yep you are right, I just checked it at the solenoid, with the key off it is 12.63 and when I turn it to start it goes to between 3 and 5, what does that mean a bad wire or bad battery? Thanks for the help.

Also the battery posts are spotless and the cable terminals are clean (soaked them in coke for a few hours)

 

I think the cables and other connections are fine if the other battery cranks it well. The only difference is the post the cable is connecting to, and it must be smaller such that the terminal can't clamp it. So check across the connector, meaning from the post to the connector, first on the positive side and then on the negative. When you hit Start you will see ~10 volts at the bad connection.

 

I think the cables are probably good also, I'm kind of confused what you mean by checking the post and the connector. If I put the positive on the post and the negative on the terminal cable I get 0. Maybe I'm just reading that wrong. I could probably put one of those battery post shims on the terminal ends are tight but they are stretched past normal.

 

I think either the positive or negative battery cable isn't making a good connection with the battery post. If so, you will see voltage across the connection when you try to start it. Put your positive lead on the battery post and negative on the connector and try to start it.

 

Think of your battery cables as a garden hose. Your cable has a bad connection between the starter and the solenoid. This bad connection is just like stepping on your water hose. The kink in the hose will let a little water through(that's what your meter is reading) but when you really go to use the hose full force(like trying to use the starter) it fizzles out because the kink(or the bad connection in the cable) is too restrictive for the full flow. The starter needs full flow of electricity through those large cables to work correctly.

 

This is a common problem with top post batteries in that they are tight, but the connection doesn't actually get made due to corrosion or such. Typically when you disturb it, the connection isn't as troubling, but not always true. Make sure the battery posts and the inside of the cable terminals are clean. Use the terminal cleaning brushes to verify that any oxidation or corrosion is removed. Most times this will fix the issue.