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Old 05-10-2015, 10:44 PM
Cigar Havana's Avatar
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Location: London Ontario Canada
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1989 420SEL Dead Battery

1989 420SEL with 300k km has dead battery, after sitting off for 2 days.

Charged the battery with charger for 20 hours. Battery reads 13.4v with key off. Put an amp meter around the neg. lead at the battery, shows 1 amp drain, when key off. With engine running, shows 14.1V at the battery, 15 amp on the neg. battery lead. Alternator belt is not loose.

Battery was new in Dec. 2013, alternator (bosch re-built)was replaced 6,000 km ago. The car did sit throughout the winter Nov. 2014 to April 2015 in an unheated garage, never turned on. The battery was disconnected for the period, put on a timer, on an automatic intelligent battery charger, and turned on for 30 min. every 24 hours during the winter. This is in Toronto Canada so there were some nights as cold as -30c. When the battery was reconnected and the car started immediately and the charger showed only 1.7 amp to charge the battery.

Where should I start the diagnosis?

1989 420 SEL
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Old 05-10-2015, 11:09 PM
TnBob's Avatar
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Location: Shelbyville, Tn
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" Put an amp meter around the neg. lead at the battery, shows 1 amp drain, when key off."

THERE is your issue. Even the early 300's only drew about 50ma with the key out and that was from the clock running. Key out, amp meter connected, Id start popping fuses until the 1 amp draw goes away. What ever circuit that is .. is your problem area.
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Old 05-13-2015, 09:43 PM
Hit Man X's Avatar
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Posts: 9,087
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Agreed on pulling fuses.

I have had alternators cause big enough drains to kill a battery over the night too.
I'm not a doctor, but I'll have a look.

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Old 05-15-2015, 01:55 AM
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Join Date: May 2015
Location: Utah
Posts: 28
Agreed. 1A is way too much draw for being off. I would start by double checking lights inside the vehicle. Check the glovebox, courtesy lights on the doors (my wife has killed 2 batteries with that one), trunk lights, etc. 1A draw seems about right for a medium sized light.

If there's no lights on, pulling fuses, while somewhat tedious, will be quite effective.

I wonder, do you have an aftermarket stereo? Is it wired properly? If not, it may have its own fuse that doesn't sit inside the fuse box that you may want to check out.
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Old 05-15-2015, 05:22 PM
Cigar Havana's Avatar
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Location: London Ontario Canada
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The amp meter jumps around a lot, it's a clamping meter, not inline, and the minimum scale is 400amps. I better get an inline meter on it to get a more stable reading.

No after market radio, but it is a refurbished original radio, and it does not turn on at this time.

Can't see any lights on during daylight, but I'll check tonight, in a completely dark garage.

Today when I checked the voltage at the battery terminals, with engine one, prior to the first drive, it was 14.1v. Made 2 drives of 10-15 minutes each, at the end of 2nd drive, while engine on, showed 13.86v at the battery terminals.
1989 420 SEL
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Old 05-16-2015, 12:16 PM
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Location: PNW
Posts: 242
That amp meter does have a "DC" setting I hope?

Many amp meters are AC only. For a car you need a DC amp meter (Typically amp meters which have AC and DC).

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