Battery drain troubleshooting

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Battery drain troubleshooting

Post by Milktruck » Fri Feb 14, 2020 1:18 pm

First off, my apologies for starting a new thread. I see this is pretty common, but I haven't been able to figure out my issue. Here is what's happening:

-'88 GW has a slow drain if left without starting for awhile.
- When started with a good charge, the voltage reads around 12.4-12.6, then moves to 13 in a minute or so. If no other drain is applied to the system, the voltage runs up to 14.6-14.7 and generally stays in that range.
- if I turn on the headlights, or the heater, the voltage drops down to around 13 and then gradually drains the battery until it won't run.

My system:

- 360 with Holley Sniper
- Newish starter
- New alternator - high output
- Newish battery (recently tested good)
- New voltage regulator
- New battery cable set

I've checked the engine and frame ground and both are solid and clean. I also fixed all the previous owner slice/dice on the wiring harness from the alternator to the voltage regulator. I have NOT replaced the fusible links in the runs to the voltage regulator, but have inspected them and they look pretty clean. I have tried to isolate the drain through fuse box, but have not been successful.

Any thoughts or places that I've missed? I'm getting pretty frustrated that I can find this. Seems like it should be simple.
1988 Jeep Grand Wagoneer

Posts: 41
Joined: Fri Sep 13, 2019 11:13 pm
Location: Hillsboro, OR

Re: Battery drain troubleshooting

Post by sirrus » Fri Feb 14, 2020 2:06 pm

Not sure that’s your case, but I thought I should mention it anyway - I was having issues with charging and voltage going up and down.

Was ready to look for new alternator, but it turned out that alternator pulley bolt became a little loose and pulley/belt was slipping. Tightened that bolt and everything back to normal, steady voltage all the time
1988 Grand Wagoneer, AMC 360, TF727, NP229

Posts: 27
Joined: Thu Aug 11, 2016 2:30 pm

Re: Battery drain troubleshooting

Post by PBG928 » Fri Feb 14, 2020 3:38 pm

Is tailgate working correctly? If not unplug connecter on drivers side rear before loom enters tailgate under car near bumper.. Easier than pulling tailgate apart. If voltage drop stops then problem is in tailgate. There are only a couple of circuits that are hot with ignition off. Tailgate in rear, lights , brake and emergency lights, dome light and exciter wire to alternator is always hot. Do lights all work? You could pull engine wiring harness from fire wall by removing 5/16 bolt in middle and unplugging it. This isolates engine from interior and if still dropping it is in your engine wiring that is pretty easy to chase. this all assumes there was no aftermarket wiring like a brake controller. Check hot wire going to starter. Pull fusible linked wires off starter relay on wheel well one at a time and check if voltage keeps dropping. Do you still have a door chime? Blue box about size of cigarettes plugged into fuse panel. Pull it as you do not need it. A few thoughts.

Posts: 27
Joined: Thu Aug 11, 2016 2:30 pm

Re: Battery drain troubleshooting

Post by PBG928 » Fri Feb 14, 2020 3:45 pm

Heater uses only fan on firewall. disconnect orange wire feeding fan and try with lights only. With car off turn on lights and check wiring feeding each bulb. I had a license plate light that was grounded and it took out the dash lights. Check headlight wiring and front side markers by pulling them out and looking at them. Could be crimped.

Posts: 27
Joined: Thu Aug 11, 2016 2:30 pm

Re: Battery drain troubleshooting

Post by PBG928 » Fri Feb 14, 2020 3:47 pm

As a Sniper owner is wire feeding sniper relay switched from ignition?

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Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: Battery drain troubleshooting

Post by letank » Fri Feb 14, 2020 10:09 pm

dome light fuse removal seems to be a constant on most FSJ to stop parasitic looses
74 wag (349 Kmiles... parked, next step is a rust free body)
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See Engine bits and Fuel and brake lines, and Body work

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