Electrical '89 GT Fast Drain of Battery - Battery Won't Charge When Connected

aroorda

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May 15, 2022
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I have an 89 GT 5.0 vert that to this point has had very few problems. I recently drove the car, then parked it for 20 minutes and the battery was completely dead. I replaced the battery and found it ran great again until I parked it overnight and the battery was dead again the next morning. I've tested and the alternator is getting plenty of voltage and the car runs great if the battery has enough juice to turn it over. Here are some other symptoms:

  1. When I attempt to charge the battery using a Battery Tender with the positive and negative cables connected, the battery does not charge at all. If I remove the positive and negative cables, the battery charges fine.
  2. I left the charger on overnight, but because the battery won't charge while it is hooked up, it took about 12 hours to take the battery from 12.5v to 0.5V
  3. I accidentally cut the line that went to the ground and the light in the hood as well. Wondering if that could create an open circuit that would rapidly drain a battery?
  4. I removed the starter relay cover to inspect and found that one of the wires had its cover worn off (it was one of the wires that was at the bottom of the stack - it goes down and forward). I wrapped that up - but now the gauges on the left half of the cluster - tach and battery - don't work...and the gas light is stuck on. The gauges on the right hand side - speed, temp and fuel seem to be fine.
  5. The aftermarket stereo stopped working (the least of my concerns).

Any thoughts on a process to start troubleshooting this? Thanks in advance, everyone.
 
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Pinellas50

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Aug 5, 2020
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It sounds like you have a significant amperage draw that shouldn't be there when the car is off. You need to do an amp draw test on all sysyems. You basically put a meter between the battery and a battery cable to measure amp draw. Then begin pulling fuses one at a time until you find the circuit that has the problem.

I had this same problem when I first got my car. It ended up being the alternator. There is a circuit within the alt that prevents it from draining the battery when not in use. That circuit can fail causing a drain but the alt still charges normally when in operation.

Anyway, you should be able to find videos on youtube showing how to do the amp draw test.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto-range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit.Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

See http://assets.fluke.com/appnotes/automotive/beatbook.pdf. You will need the Adobe Acrobat viewer which is also a free download – http://www.adobe.com/products/acrobat/readstep2.html

If you can’t find the current drain by pulling fuses, then the alternator is suspect or any aftermarket sound or lighting equipment that may be on the car. Disconnect the alternator output power plug, then disconnect any aftermarket sound or lighting equipment while watching the current on the DVM
stolen from jrichker
Yeah I don't have much original material
 
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aroorda

Member
May 15, 2022
6
1
13
It sounds like you have a significant amperage draw that shouldn't be there when the car is off. You need to do an amp draw test on all sysyems. You basically put a meter between the battery and a battery cable to measure amp draw. Then begin pulling fuses one at a time until you find the circuit that has the problem.

I had this same problem when I first got my car. It ended up being the alternator. There is a circuit within the alt that prevents it from draining the battery when not in use. That circuit can fail causing a drain but the alt still charges normally when in operation.

Anyway, you should be able to find videos on youtube showing how to do the amp draw test.
Thanks will give it a shot!
 

aroorda

Member
May 15, 2022
6
1
13
Thanks all, I've tested all the fuses and didn't find the drain there or when disconnecting the aftermarket stereo.

When I reconnected everything, I got the tach and other gouges to start working again. However, the voltage gauge now fluctuates rapidly in relation to idle and accelerating. So I assume that it's either the alternator and/or a bad ground.

There is a remanufactured o'reillys alternator installed and the plug positions make them literally impossible to remove as opposed to the stock setup on the top and sides. It looks like I'll have to remove the alternator to test it.

Any other ideas on tests for ground or to isolate the alternator without major work are appreciated. Thanks!
 

Monday

Member
Feb 8, 2022
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I have an 89 GT 5.0 vert that to this point has had very few problems. I recently drove the car, then parked it for 20 minutes and the battery was completely dead. I replaced the battery and found it ran great again until I parked it overnight and the battery was dead again the next morning. I've tested and the alternator is getting plenty of voltage and the car runs great if the battery has enough juice to turn it over. Here are some other symptoms:

  1. When I attempt to charge the battery using a Battery Tender with the positive and negative cables connected, the battery does not charge at all. If I remove the positive and negative cables, the battery charges fine.
  2. I left the charger on overnight, but because the battery won't charge while it is hooked up, it took about 12 hours to take the battery from 12.5v to 0.5V
  3. I accidentally cut the line that went to the ground and the light in the hood as well. Wondering if that could create an open circuit that would rapidly drain a battery?
  4. I removed the starter relay cover to inspect and found that one of the wires had its cover worn off (it was one of the wires that was at the bottom of the stack - it goes down and forward). I wrapped that up - but now the gauges on the left half of the cluster - tach and battery - don't work...and the gas light is stuck on. The gauges on the right hand side - speed, temp and fuel seem to be fine.
  5. The aftermarket stereo stopped working (the least of my concerns).

Any thoughts on a process to start troubleshooting this? Thanks in advance, everyone.
This is from an earlier post:

 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Cheap Chinese rebuilt alternator from O'reillys that a previous owner had installed. Replaced it with the LMR updgrade kit and we're rolling.

Hate to break it, but the LMR alt is likely the same cheap Chinese alternator. They source stuff like this from the same place as your local autoparts store.
 
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