2000 bonnie won't crank, voltage loss, No Codes!

[joeharley]

My 2000 Bonneville will not crank over. It was asimptomatic before yesterday.
The battery is new and holds a charge overnight when ground is disconnected.
BUT, 10 min after connecting ground the voltage reads 10.5. (ign off, doors closed) and car will not crank.
It cranks easily, then runs well if jump started. After running down hwy for 10 miles..shut it off and it will not crank.
Battery still reads 10.5(actaually 10.45) after 3 hours.
I cleaned and inspected as many connections as I could find including the starter. All looked good.
What/how do I check next. MY mech says he can't diagnose with out a code.

[Mickey#1]

Time to find a new mechanic!

Don't rule out the possibility of the new battery being bad.
What's the voltage reading while the engine is running?
Do you have a multimeter with a 10 amp or higher range?

[joeharley]

The voltage reading while engine running is 14.73. The battery holds a charge when NOT connected to vehicle. Yes, I have a good multi-meter. Just moved to this state , and don't know a good mechanic yet. Only wrenched my '68 and '69 LeMans and a '55 HD . So, my mech experience with modern electrics is limited..But, I am willing to learn.

[Mickey#1]

1st thing is to get the battery charged & load tested. Many auto parts stores will do this for free. Just because a battery is showing 12 or more volts doesn't mean it has the amps to turn over the engine.

If the battery checks out then you need to see how much drain is on the battery. Set the meter up to check DC amps on the 10 amp range. Connect the positive terminal and put the meter leads on the negative terminal & negative battery cable. The meter will be connected in series with the battery. Wait about 1/2 minute and check the reading. It should be drawing less than 1/10 amp. When doing this test the keys should be out of the ignition & all doors closed.

[joeharley]

Mickey#1, Thanks for the reply/help.
I'll stop by the auto store tomorrow and get the bat load tested... and do the amp test per your instructions.
The car will start easily with a jump from another battery.

[joeharley]

The amp reading is 1.07...then 1/2 min later it is .26

[Mickey#1]

I would have expected the .26 reading to be quite a bit lower. More like .026 or even lower. Even .26 isn't going to drain the battery in 10 minutes, it could drain it after a couple days. Did you get the battery checked?

If the battery checks OK then I'd drive over & have them check the alt. while on the car.

[joeharley]

...been out of town working for a few days...
I had the battery checked out at a 'box' store. they said it was OK...But, when I found a "Interstate' battery dealer, they checked the battery and said it was defective and agave me a new one 'NO Charge".
Now, the air-con works. It never did before!
I should have had them check the altenator, but i'll do that next.
Thanks for the help, Joe Harley