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Matt, Engineer
Category: Cadillac
Satisfied Customers: 21857
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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I have a 99 STS. Battery drained to 8 volts a 2 days after

Customer Question

I have a 99 STS . Battery drained to 8 volts a 2 days after driving it on a 300 mile trip. I removed the battery and it had recovered on its own to about 10 volts. I believe the problem is in the car. Can you suggest some common causes?
JA: Have you checked all the fuses? Do you have a wiring diagram?
Customer: I have not checked the fuses. I assume a drain would go through and active fuse
JA: What year is your STS?
Customer: 99
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: I am hoping to fix this myself. I have checked the ohms resistance across the battery leads, And found it shows 8 ohms resistance.
JA: Anything else you think the mechanic should know?
Customer: The indicated voltage while I was on my trip was within normal range. I actually checked it periodically as I was running the AC a lot
Submitted: 1 year ago.
Category: Cadillac
Expert:  Matt replied 1 year ago.


I'd first check the easy things like is the boot light or any other interior light staying on then check that the main battery connections are tight and in good condition also check the earth strap to the engine / gearbox.Next I'd check the current drain aspect, the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.

A typical current drain with everything off and un-alarmed should be around 0.05Amp and if its higher than this remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.

Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

Also check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.

if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement

And one last thing is – how old is the battery? – As even good quality ones need replacing every 5 years and faulty battery just won’t hold a charge

If its not then check the charge light bulb in the dash which provides the load to the diode pack in the back of the alternator, if this bulb has blown then the diode pack will never switch over to charging when its running. If the bulb is OK then check the wiring to earth for the bulb.

Customer: replied 1 year ago.
Battery is a premium quality, about 14 months old. I was looking for an answer that might list common components that tend to fail in this model car. It has 110 thousand miles. Would leaving a seatbelt for right front passenger seat buckled do anything to cause a drain?
Expert:  Matt replied 1 year ago.


as the caddy has a lot of small lights I'd try approaching the car after dark to see if you can spot any 'glimmer' of light anywhere

Have you checked the sun visors? They have a lighted mirror, with a dimmer, and if they aren't closed completely they can stay lit and you won't see it.

also they can be prone to the brake pedal switch freezing up and leaving the brake lights on

and plugging in the seat belt shouldn't drain the battery