Hunting a voltage leak

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Jun 16, 2021
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My Car
1973 Convertible, Q code, 4 speed, 3.25, A/C, P/S
I'm trying to find a voltage leak that's killing my battery and could use some help.

I have a test light that I connected between the battery negative post and the negative battery cable (while disconnected). In this configuration, with the key off, there is 12 volts running across the test light.

I have pulled every fuse and disconnected every wiring harness that I could get my hands on without taking the dash apart. The harnesses under the hood as well. I cannot stop the 12v running across the test light.

I've disconnected the underhood light, all of the headlights and interior lights, but it persists.

I'm assuming this has to be alternator or voltage regulator, but I don't know how to know where to test. My voltage regulator is 5 years old, but I bought the best I could (Blue Streak I think?). No idea how old the alternator is.

I disconnected the test light and reconnected the battery negative cable and post. I then looked for 12 volts on wires connected to the voltage regulator and alternator. If you see my attached pictures, I have 12 volts with the key off on the wires that my test probe is on.

Is that correct? Should I have a constant 12v to those wires, with the key off?
 

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Test lights for parasitic drain are worthless except for finding excessive drain. Best to use a DVM that can measure DC amperage. You can expect anywhere from 25-50 milliamps of current in a well-maintained system. You're on the right track by removing fuses, unplugging things, but you didn't unplug the voltage regulator or the lead to the alternator. By measuring the current difference when you remove devices, you can more quickly isolate where the problem is.
 
midlife,

Where am I connecting the DVM? Between the positive battery post and the disconnected positive cable?
 
No no no! Always disconnect the battery on the negative side, then measure current from the negative battery post to the disconnected negative battery terminal.

You never want to remove the positive battery lead first, because if you slip, the wrench could contact the body and you will have in essence a welding experience.
 
No no no! Always disconnect the battery on the negative side, then measure current from the negative battery post to the disconnected negative battery terminal.

You never want to remove the positive battery lead first, because if you slip, the wrench could contact the body and you will have in essence a welding experience.
uhm..... and the battery connection at the starter relay, but that has never happened to me:whistle:
 
Do you have any known electrical issues with the car when it is running? A good place to start is to ensure everything that should work properly when the car is running is working. If there is a light, buzzer, warning light or other feature that doesn't work when the car is on then it could have an open power wire grounding out.

It is not unusual to have voltage across your negative battery terminal to the negative battery cable. There are several circuits that need power even when the ignition is not on; cigarette lighter, clock, power to your light switch so you can turn on interior lights, headlights, roof light, trunk light...

These cars have a few known bad actors like the wire that goes to the license plate light, the wires under the door plate, the wires under the seat to the seatbelt warning light and the alternator warning light in the dash as well as the alternator charging gauge.

I would start with these things then test your solenoid to see if it is leaking power across the terminals with the key in the off position. A greasy old dirty battery can also discharge itself by completing a current through debris on the battery, just wash it off with baking soda if you haven't already.

Good luck, make sure you tell us what you found. You may save someone else their sanity one day.

PS. One last thing, are you sure your battery isn't cracked around the base somewhere and has good fluid level? Battery test it to see that it has enough acid! When I removed the battery stand to paint the interior I found an ugly surprise. Battery acid had eaten right through the holder and into the apron, grrr, more welding!
 
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To add to what midlife recommended, I like to use a block of wood to depress the little buttons on the door jambs so the car thinks the doors are closed, but you can still get in and out easily. On cars with computers, you want to let the car sit for a few minutes after turning it off and blocking the door buttons. Let the computer transition to low power mode. This shouldn't be needed on our cars unless you have aftermarket stuff (efi, stereo, dash camera) hooked up.

When shopping for a meter to use, get one with the highest fused amp rating you can find. A lot of the ones in the store are like 10a, which is kind of easy to blow. Look for one 15a or 20a. Then make sure you're using the fused lead when you have the load running through the meter to look for draw.
 
Talk about a timely thread. I have been dealing with this exact issue for years and haven’t been able to locate the culprit. So I’m following this with great interest. Because I haven’t been able to find it, I put a battery disconnect switch on the negative terminal. After every drive I open it, that way I can count on the battery being ready for the next drive. I really appreciate this forum!!!
 
This is my method that I provide to every customer for ensuring no shorts or excessive parasitic current.

First, always install electrical wiring with the battery disconnected.

Physically install the underdash harness with fuses. Connect all underhood and taillight harnesses.

When ready to check out the harness, close all doors, turn off all lights, accessories, etc. Make sure both battery connectors are disconnected. Connect the positive battery cable to the starter solenoid. Leave the negative terminal disconnected from the battery. Then use a digital volt meter to measure the current draw across the battery. Place one probe of the DVM on the negative battery cable terminal, and the other probe on the negative battery post. Be sure that the DVM is set to amps, DC. If there are no shorts in the wiring connections, the reading should be on the order of 30 milliamps or less. Anything above 1 amp indicates some accessory is on; readings above 10 amps (may blow the fuse on the DVM) indicates a dead short.

If you have a short, disconnect all other wiring from the starter solenoid post where the battery connects and re-measure the current. If current drops, the source is the underdash wiring harness or the harness from the starter solenoid to the firewall. Reconnect the wiring to the starter post and then disconnect the underhood harness from the firewall and test again to isolate the source of the short.

If the short is not in the underhood or underdash harness, the problem lies in the alternator or voltage regulator. Disconnect each of these in turn to isolate the source.

If the short or high current comes from the underdash region, keep the battery negative side disconnected and remove one fuse at a time. Measure the current as above. If you now see low current, there is either a short or an item on that particular fuse circuit is on. Turn off that item and continue checking.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. At that time, and only at that time, is it safe to connect the negative battery cable to the battery.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. This series of tests checks all of the battery directly powered systems. To check the Accessory systems, now repeat all of the tests above with the ignition key in the ACC position. The current with the key in the ACC position may be up to 1 amp or so. When all of the tests are done and no anomalous current is noted, at that time, and only at that time, is it safe to connect the negative battery cable to the battery.
 
This is my method that I provide to every customer for ensuring no shorts or excessive parasitic current.

First, always install electrical wiring with the battery disconnected.

Physically install the underdash harness with fuses. Connect all underhood and taillight harnesses.

When ready to check out the harness, close all doors, turn off all lights, accessories, etc. Make sure both battery connectors are disconnected. Connect the positive battery cable to the starter solenoid. Leave the negative terminal disconnected from the battery. Then use a digital volt meter to measure the current draw across the battery. Place one probe of the DVM on the negative battery cable terminal, and the other probe on the negative battery post. Be sure that the DVM is set to amps, DC. If there are no shorts in the wiring connections, the reading should be on the order of 30 milliamps or less. Anything above 1 amp indicates some accessory is on; readings above 10 amps (may blow the fuse on the DVM) indicates a dead short.

If you have a short, disconnect all other wiring from the starter solenoid post where the battery connects and re-measure the current. If current drops, the source is the underdash wiring harness or the harness from the starter solenoid to the firewall. Reconnect the wiring to the starter post and then disconnect the underhood harness from the firewall and test again to isolate the source of the short.

If the short is not in the underhood or underdash harness, the problem lies in the alternator or voltage regulator. Disconnect each of these in turn to isolate the source.

If the short or high current comes from the underdash region, keep the battery negative side disconnected and remove one fuse at a time. Measure the current as above. If you now see low current, there is either a short or an item on that particular fuse circuit is on. Turn off that item and continue checking.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. At that time, and only at that time, is it safe to connect the negative battery cable to the battery.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. This series of tests checks all of the battery directly powered systems. To check the Accessory systems, now repeat all of the tests above with the ignition key in the ACC position. The current with the key in the ACC position may be up to 1 amp or so. When all of the tests are done and no anomalous current is noted, at that time, and only at that time, is it safe to connect the negative battery cable to the battery.
Excellent writeup. I will save this one for later.
 
No no no! Always disconnect the battery on the negative side, then measure current from the negative battery post to the disconnected negative battery terminal.

You never want to remove the positive battery lead first, because if you slip, the wrench could contact the body and you will have in essence a welding experience.
When I was a kid, the wrench slipped and it welded the wrench to a ring I was wearing. Of course the ring got instantly crazy hot and I couldn't get it off quickly as it had a wrench welded to it at a weird angle. Left a nice burn where the ring was for several months. Needless to say, I take off the negative side first these days and think twice when wrenching with my wedding ring on.
 
Two of the sneakier drains I have found are the Alternator diodes where one or more of the diodes is leaking. The other is the top of the battery itself. To test for the leaking diodes you can use an Amp Clamp DVOM around the red cable between the battery and the alternator output terminal (the largest alternator terminal) with the gine off and all devices turned off. If you show any amperage you can bet a faulty diode or two is at fault. In my case I actually use an old school Sun VAT-40 that looks for any indication of voltage leakage by looking for an AC current running on the system. But, using a Amp Clamp will work well, if you use a lower amperage scale. There may be an acceptable milliamp level before there is enough of a problem to be concerned, but I look for no measurable current flow between the battery and alternator with the engine and all circuits turned off.

The 2nd sneaky area of leakage is less of an issue with modern vent-less batteries, but still worth checking out. In the day when we had batteries with vent caps, and had to add distilled water to top off cell levels in the batteries, I found when the vapors mixed with dust/dirt on top of a battery there would be, at times, just the right (wrong) combination of moist particles to allow a current to flow between the positive and negative terminals across that mix of moist dirt/dust. To test for it I would simply touch the positive lead of a VOM (pre-DVOM days) to the positive post of the battery. Then near the negative post I would touch the negative rtest lead to the top of the battery in different areas and look for any "significant" voltage. It was not unusual to see a few volts, but if I saw more than 10 volts it was time to clean off the top of the battery. I never tested for amp/milliamp current levels, but I am sure I could have using the electrodes, but only valid if I were to have removed the negative termianl from the battery negative post. If you only have an amperage tester with an AmpClamp, this is one of the few times it will not be a benefit to use without making a circuit using a wire between the battery positive post (with the negative terminal disconnected, and touching the other end of the wire on various parts of the battery top while using the AmpClamp around the test wire. I would not want to see any significant level of milliamp current across the top of the battery, especially when modern batteries are less prone to getting a conductive grime buildup on top of a battery. yet, despite the low likelihood of a current leak across the top of a battery, I feel it is worth checking out.

As for the more traditional method of testing, after checking for leaking diodes and battery surface current, I would (also) start pulling fuses one at a time, and keeping the pulled fuses out in case more than one circuit is causing grief. If all fuses are out and there is still a significant current draw using the AmpClamp on the positive cable of the battery, I would begin to remove wires from the starter relay positive terminal fed by the battery positive cable - while keeping the large battery cable connected to the starter relay terminal of course.

There are several circuits that are hot when the ignition key is off, to include:
  • Interior courtesy, map, glove box, and under-hood/under-trunk lights,
  • seat back latch relay/actuator (aka solenoid),
  • head/parking lights,
  • brake lights,
  • emergency flasher,
  • parking brake warning light,
  • cigarette lighter,
  • electric clock,
  • Circuit # 36 (Yellow with White Stripe) feeding the charging system Voltage regulator and the Noise Suppression Capacitor at the regulator,
  • heated back-light (stuck relay)

If by chance your Seatbelt Interlock is still active (1973 only, and nearly all were disconnected decades ago), it may cause a problem - I need to look at that archaic system to see if anything was constantly hot in that circuit. --->> I looked at the Seatbelt Interlock circuitry schematic and do not see any "always hot" circuitry involved with it.

I hope all that is helpful, along with the other excellent suggestions offered to you by our other fellow enthusiasts...
 
Two of the sneakier drains I have found are the Alternator diodes where one or more of the diodes is leaking. The other is the top of the battery itself. To test for the leaking diodes you can use an Amp Clamp DVOM around the red cable between the battery and the alternator output terminal (the largest alternator terminal) with the gine off and all devices turned off. If you show any amperage you can bet a faulty diode or two is at fault. In my case I actually use an old school Sun VAT-40 that looks for any indication of voltage leakage by looking for an AC current running on the system. But, using a Amp Clamp will work well, if you use a lower amperage scale. There may be an acceptable milliamp level before there is enough of a problem to be concerned, but I look for no measurable current flow between the battery and alternator with the engine and all circuits turned off.

The 2nd sneaky area of leakage is less of an issue with modern vent-less batteries, but still worth checking out. In the day when we had batteries with vent caps, and had to add distilled water to top off cell levels in the batteries, I found when the vapors mixed with dust/dirt on top of a battery there would be, at times, just the right (wrong) combination of moist particles to allow a current to flow between the positive and negative terminals across that mix of moist dirt/dust. To test for it I would simply touch the positive lead of a VOM (pre-DVOM days) to the positive post of the battery. Then near the negative post I would touch the negative rtest lead to the top of the battery in different areas and look for any "significant" voltage. It was not unusual to see a few volts, but if I saw more than 10 volts it was time to clean off the top of the battery. I never tested for amp/milliamp current levels, but I am sure I could have using the electrodes, but only valid if I were to have removed the negative termianl from the battery negative post. If you only have an amperage tester with an AmpClamp, this is one of the few times it will not be a benefit to use without making a circuit using a wire between the battery positive post (with the negative terminal disconnected, and touching the other end of the wire on various parts of the battery top while using the AmpClamp around the test wire. I would not want to see any significant level of milliamp current across the top of the battery, especially when modern batteries are less prone to getting a conductive grime buildup on top of a battery. yet, despite the low likelihood of a current leak across the top of a battery, I feel it is worth checking out.

As for the more traditional method of testing, after checking for leaking diodes and battery surface current, I would (also) start pulling fuses one at a time, and keeping the pulled fuses out in case more than one circuit is causing grief. If all fuses are out and there is still a significant current draw using the AmpClamp on the positive cable of the battery, I would begin to remove wires from the starter relay positive terminal fed by the battery positive cable - while keeping the large battery cable connected to the starter relay terminal of course.

There are several circuits that are hot when the ignition key is off, to include:
  • Interior courtesy, map, glove box, and under-hood/under-trunk lights,
  • seat back latch relay/actuator (aka solenoid),
  • head/parking lights,
  • brake lights,
  • emergency flasher,
  • parking brake warning light,
  • cigarette lighter,
  • electric clock,
  • Circuit # 36 (Yellow with White Stripe) feeding the charging system Voltage regulator and the Noise Suppression Capacitor at the regulator,
  • heated back-light (stuck relay)

If by chance your Seatbelt Interlock is still active (1973 only, and nearly all were disconnected decades ago), it may cause a problem - I need to look at that archaic system to see if anything was constantly hot in that circuit. --->> I looked at the Seatbelt Interlock circuitry schematic and do not see any "always hot" circuitry involved with it.

I hope all that is helpful, along with the other excellent suggestions offered to you by our other fellow enthusiasts...
An aside, I have several nice/decent handheld digital volt/ohm meters that include an AmpClamp inductive ammeter. I got the following from Amazon:

KAIWEETS HT206D Digital Clamp Meter T-RMS 6000 Counts, Multimeter Voltage Tester Auto-ranging, Measures Current Voltage Temperature Capacitance Resistance Diodes Continuity Duty-Cycle (AC/DC)​


AC-DC Current Digital Clamp-Meter - 100A 6000-Counts Low Impedance Voltage 1 mA Reading Auto Range Multimeter Tester with Temperature,True RMS​


ACM91 Digital Clamp Meter AC/DC Current True RMS Auto-Ranging 6000 Counts Volt Meter Low Impedance Voltage Tester Portable Digital Multimeter​


Two of the three units have done a nice job for me, one was purchased as a gift for ShelbyFlyer (Facebook name),
and has apparently been a useful piece of equipment for him.
 
There are several circuits that are hot when the ignition key is off, to include:
  • Interior courtesy, map, glove box, and under-hood/under-trunk lights,
  • seat back latch relay/actuator (aka solenoid),
  • head/parking lights,
  • brake lights,
  • emergency flasher,
  • parking brake warning light,
  • cigarette lighter,
  • electric clock,
  • Circuit # 36 (Yellow with White Stripe) feeding the charging system Voltage regulator and the Noise Suppression Capacitor at the regulator,
  • heated back-light (stuck relay)
Not accurate. The lines that are BATT powered without fuses are the underhood lamp; seat back latch relay; headlights, parking lights. brake light. e-flasher. All the others are fused. My recommendation is that when you still have a parasitic loss with all fuses pulled, start removing wires one by one off of the BATT post of the starter solenoid: that will eliminate the alternator, voltage regulator, and any relays you might have.
 
When I was a kid, the wrench slipped and it welded the wrench to a ring I was wearing. Of course the ring got instantly crazy hot and I couldn't get it off quickly as it had a wrench welded to it at a weird angle. Left a nice burn where the ring was for several months. Needless to say, I take off the negative side first these days and think twice when wrenching with my wedding ring on.
I chose to listen to and heed the advice is more experienced folks re: removing my ring when working on cars, whether it was electrically oriented work or not. I am glad you did not end up losing your finger.
 
This is my method that I provide to every customer for ensuring no shorts or excessive parasitic current.

First, always install electrical wiring with the battery disconnected.

Physically install the underdash harness with fuses. Connect all underhood and taillight harnesses.

When ready to check out the harness, close all doors, turn off all lights, accessories, etc. Make sure both battery connectors are disconnected. Connect the positive battery cable to the starter solenoid. Leave the negative terminal disconnected from the battery. Then use a digital volt meter to measure the current draw across the battery. Place one probe of the DVM on the negative battery cable terminal, and the other probe on the negative battery post. Be sure that the DVM is set to amps, DC. If there are no shorts in the wiring connections, the reading should be on the order of 30 milliamps or less. Anything above 1 amp indicates some accessory is on; readings above 10 amps (may blow the fuse on the DVM) indicates a dead short.

If you have a short, disconnect all other wiring from the starter solenoid post where the battery connects and re-measure the current. If current drops, the source is the underdash wiring harness or the harness from the starter solenoid to the firewall. Reconnect the wiring to the starter post and then disconnect the underhood harness from the firewall and test again to isolate the source of the short.

If the short is not in the underhood or underdash harness, the problem lies in the alternator or voltage regulator. Disconnect each of these in turn to isolate the source.

If the short or high current comes from the underdash region, keep the battery negative side disconnected and remove one fuse at a time. Measure the current as above. If you now see low current, there is either a short or an item on that particular fuse circuit is on. Turn off that item and continue checking.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. At that time, and only at that time, is it safe to connect the negative battery cable to the battery.

Eventually, you’ll have low current readings with all fuses installed and all wiring connected. This series of tests checks all of the battery directly powered systems. To check the Accessory systems, now repeat all of the tests above with the ignition key in the ACC position. The current with the key in the ACC position may be up to 1 amp or so. When all of the tests are done and no anomalous current is noted, at that time, and only at that time, is it safe to connect the negative battery cable to the battery.
Am I doing this wrong? I have a basic home depot volt, amp, ohm meter. I disconnect the negative cable and put my meter between the battery negative post and the negative (disconnected) cable. I see 1.38 ma. Does that mean I had no draw at that time? Maybe my issue is sporadic?
 
Am I doing this wrong? I have a basic home depot volt, amp, ohm meter. I disconnect the negative cable and put my meter between the battery negative post and the negative (disconnected) cable. I see 1.38 ma. Does that mean I had no draw at that time? Maybe my issue is sporadic?
Depending on your meter, you may need to explicitly switch it to DC. It may not be smart enough to know what you want, and default to AC.
 
Ok, So I'm still hunting this down. I bought a new meter, my old meter was not reading DC current correctly.

The new meter works well and I can tell that I have a draw of 2.6 amps with everything off. It kills the battery in about a day or less.

I've gone through the motions of removing all of the fuses one by one and the drawer remained. I also disconnected some main wiring harnesses and the draw remains.

However, I did manage to remove one wire from the voltage regulator and that stopped the draw.

Please see my picture below. I am holding a yellow wire against a post on the side of the voltage regulator. If the wire is attached (as it has been for years) the 2.6 amp draw remains. If I remove that single wire, the draw is gone.

I did replace the voltage regulator a few years ago, but I understand that it could be bad again. Does this wire point to my voltage regulator being bad? Or something else?
 

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