Had the batteries checked, both good and less than 36 months old. Alternator with a load (headlights and A/C on) 13.99V. Removed both neg terminals, placed a test light between one neg batt post with the other end of the test light connected to one neg cable. The test light is on but dim. Removed fuses one at a time, light never went out. I am now clueless as to where to go next. Any ideas?

David

PS Would a dead cell cause the test light to stay on? Also, would a dead cell still allow the battery to load test good?

 

I use a multi meter and set it for amps. I remove 1 positive cable and put the meter inbetween the post and the cable. Remove the fuses one at a time and see if the "load" drops.
A dead cell if the battery has just been charged may test out as good.

 

If I understand what you are doing, the test lamp is in series with the neg terminal of the battery and the negative cable to the truck. In that condition you will have a slight current drain to the truck electrical system and will be normal. You need a clamp on ammeter to test the system to get a good test. I would suggest that you reconnect the batteries to the truck. measure the voltage at the battery positive terminal, then disconnect the large lead at the alternator. If the the voltage increases even slightly you have bad diodes in the alternator which will drain your batteries. When the truck is not running the normal condition on the charge lead of the alternator is that there will no current in ether direction.

 

I just went out to do as you suggested. The batteries charged all night. After pulling one pos term to check as you suggested I decided to check across the pos and neg post of each battery. Batt 1 had 12.66V, batt 2 had 10.24V......

Bad battery I think. Assuming I need to replace them what would you suggest. Technically they are within Fords free replacement period (0-36 months) but I am not sure I want the OEM batteries.

Also this still leaves the test light still burning indicating a current draw. What do you think?

David

 

OK, I going to check this now, I'll advise in just a few minutes.

David

 

There are some draws in the system, Keep alive memory, hood light stuff like that. Like ick said the alternator may be the culpret. They have been known from time to time to quietly take out the batteries.
Here is a write up that I did a while ago on the batteries in our trucks.
https://www.ford-trucks.com/forums/7...y-q-and-a.html

 

the battery with 10.24 is probably bad and will draw the other one down

 

I would leave the bad battery disconnected and let Ford replace the bad one for free and then buy the second battery. The lower voltage on the one battery indicates a shorted cell. You never want to replace only one battery, they need to be a closely matched as possible! They actually change internal resistance with age and will not perform well if not changed out together.
Nick

 

With the meter across the pos and neg terminals I removed the charging wire from the alternator. No detectable change in the voltage reading. After connecting both batteries back up I checked the voltage on the battery that showed weak. It was up to 12.5, I then disconnected the weak battery and watched the reading on the meter slowly drop from about 12v to 10.2 in about 2 minutes.

 

Definitely a shorted cell in that second battery. Time to replace.

 

Thanks a lot all! I will replace both batteries. As I mentioned one battery is in warranty but I am not sure I really want the Ford batteries again. I'll have to but two I know but what batteries can you guys suggest?

Thanks a lot, David

 

the Ford battey is as good as you can buy

 

I would buy the highest cold cranking amp batteries that will fit. That said if Ford only warranties they batteries for 36months then I would look for a better source. I use both Walmart and Costco because they stand behind their longer warranties.
Nick

 

Batteries replaced! Thanks guys for all your help. I really appreciate it.

David

 

NP Dave Glad it worked out for you.