Huge voltage drop across battery while running

[salami340]

Hopefully someone can help me out. I've had my 71 Demon for about 3 years now and I'm trying to wrap up my new instrument cluster swap. I also installed a Bluetooth amp which sounded great until I started the car and it began to cut in and out. After futzing around with the wiring I checked the voltage on the battery and noticed what you can see in the video below. When connecting the meter from negative and positive I'm getting a huge voltage drop while the car is running. It'll go down to close to 0 and back up to 13 which would explain why the amp is cutting out otherwise everything else electrical seems to work okay. When not running it sits solid at 12.7 or so. I don't have the instrument cluster in currently.

Here are some things I have tried and I am by no means proficient at electrical work. None of these things had any affect on the voltage dropping while running.

* replaced battery negative cable and cleaned connection to head
* cleaned and replaced ground from neg terminal to radiator support
* added ground strap on passenger side head bolt to firewall (this actually increased the max voltage at idle from 12.8 to 13.5 but voltage drop still occurs at battery)
* tried a different new battery
* new VR
* disconnected alternator
* basically disconnected everything behind the dash and pulled fuses and even pulled the bulkhead connectors (shorted the starter relay to start the engine) I assume this would isolate the issue to under the hood but I'm not sure...

That's all I can think of at the moment but I'm gonna go at it again tomorrow. I'm open to any suggestions like I said I'm not great at the electrical stuff. Thanks in advance!

 

[Bewy]

Sounds like the Blue Tooth is faulty & putting a huge drain on the battery.

 

[halifaxhops]

Also how many amps is it drawing and whats the alt rated at? Some are 43 amps.

 

[salami340]

Sounds like the Blue Tooth is faulty & putting a huge drain on the battery.

It still does it with the amp disconnected fuse out etc.

 

[MV8]

What model amp?
Your video is limited to people willing to share their info with google.

 

[salami340]

What model amp?
Your video is limited to people willing to share their info with google.

I'm 99.9 % sure it is not related to the amp. I'm assuming I was having these issues before I even took the dash apart. The amp allowed me to notice the problem. The amp is now completely out of the circuit and the voltage drop at the battery is still happening.

 

[MV8]

Battery negative disconnected, Coil positive disconnected, key to RUN, positive batt cable ohms to ground?

 

[67Dart273]

I would go clear back to step 1 so to speak the very very basics. You may have a major connection problem, or you may be missing something

You say you disconnected the alternator. How did you do that? If you simply disconnected the output connection, that can damage the alternator, as you must disconnect the field so it does not try to charge. Otherwise, with only the output connected the output V at the stud will go way way up.

Do you have a way of testing the battery, AKA load?

Otherwise, disable the ignition. Measure battery terminal voltage, and make sure it is good and fully charged. 12.6 is a good figure

Now, while monitoring battery voltage FIRST at the posts themselves, and second at the starter and the engine block, crank the engine a few seconds and read the meter while cranking. This is easy if you jumper the start relay. Bare minimum is 10. to 10.5, higher is better.

Not knowing, is your wiring factory? Modified? Generic harness? Anyhow, you need to trace the main path from the battery, usually to the start relay, and on into the vehicle interior to see if there is a bad connection there

What did you do about the ammeter connections? They MUST be bolted together when the dash is out

iF you get nowhere, pull and clean the battery clamps and recheck voltage right at the batt posts and then right at the clamps.

Assuming the starter is cranking OK, what you are telling us does not make sense. The starter and main cables are the largest possible load. For some other problem to cause the battery to fall as you claim is not logical.

MAKE CERTAIN your meter connections are not intermittent, or that you don't have a faulty test lead

 

[salami340]

I would go clear back to step 1 so to speak the very very basics. You may have a major connection problem, or you may be missing something

You say you disconnected the alternator. How did you do that? If you simply disconnected the output connection, that can damage the alternator, as you must disconnect the field so it does not try to charge. Otherwise, with only the output connected the output V at the stud will go way way up.
Disconnected both field wires and the power wire. I also took the belt off so it wouldn't even spin

Do you have a way of testing the battery, AKA load?
So far have just been testing with a standard voltmeter with the engine running across the battery terminals and from positive to various points

Otherwise, disable the ignition. Measure battery terminal voltage, and make sure it is good and fully charged. 12.6 is a good figure
Battery is at 12.4

Now, while monitoring battery voltage FIRST at the posts themselves, and second at the starter and the engine block, crank the engine a few seconds and read the meter while cranking. This is easy if you jumper the start relay. Bare minimum is 10. to 10.5, higher is better.
10.8 volts while cranking at the battery and the engine block. When I hooked to the terminal at the started the Brown wire is connected to it dropped to 1.5 volts... is that bad?

Not knowing, is your wiring factory? Modified? Generic harness? Anyhow, you need to trace the main path from the battery, usually to the start relay, and on into the vehicle interior to see if there is a bad connection there
Wire harness appears mostly stock and in good condition. I tried to isolate the issue to the engine compartment. I unhooked the bulkhead connectors and ran a clip from the ignition side of the ballast to the battery and jumped the starter relay to run it.

What did you do about the ammeter connections? They MUST be bolted together when the dash is out
Yes I have them bolted together

iF you get nowhere, pull and clean the battery clamps and recheck voltage right at the batt posts and then right at the clamps.

Assuming the starter is cranking OK, what you are telling us does not make sense. The starter and main cables are the largest possible load. For some other problem to cause the battery to fall as you claim is not logical.

MAKE CERTAIN your meter connections are not intermittent, or that you don't have a faulty test lead I tested the voltmeter on my truck battery it looks fine (voltage doesn't drop) with the engine running and not running

See my responses in red above. Thanks for your feedback!

 

[salami340]

Battery negative disconnected, Coil positive disconnected, key to RUN, positive batt cable ohms to ground?

I tried this and unless I was doing something wrong I was not getting any ohm reading on my meter.

 

[salami340]

So yesterday I tried a couple things. Replaced the starter relay with a new one which had no affect. I also tried bypassing the ignition ballast also with no affect. I will keep going tonight if it cools down a bit so if anyway has any suggestions I can try that would be great.

One thing I did notice is that after the car has been sitting over night on the first time I start it the voltage is stable for maybe 30 seconds before it starts dropping again. My dad was thinking it could have something to do with the time it takes the ignition ballast to heat up and drop the voltage to the coil however I bypassed the ballast and it was still doing it so I'm still at a loss...

 

[67Dart273]

Now, while monitoring battery voltage FIRST at the posts themselves, and second at the starter and the engine block, crank the engine a few seconds and read the meter while cranking. This is easy if you jumper the start relay. Bare minimum is 10. to 10.5, higher is better.
10.8 volts while cranking at the battery and the engine block. When I hooked to the terminal at the started the Brown wire is connected to it dropped to 1.5 volts... is that bad?

Are you saying the starter solenoid small terminal? That should be about the same as "cranking" voltage when cranking. A reading like that, IF it is cranking, shows maybe a bad lead or bad connection/ something wrong in your meter. The engine would never crank, with that low voltage at the start relay

 

[67Dart273]

Battery negative disconnected, Coil positive disconnected, key to RUN, positive batt cable ohms to ground?

Please do not do this. Do not try and troubleshoot high current circuits with an ohmeter. Hook them up and use voltage drop and voltage measurements.

The reason is that when a circuit is working or trying to work, the current draw in the circuit will help show up loose connections. An ohmenter uses almost no current and will not be much help in that case.

 

[MV8]

Thank you for your interest, but I also have extensive electrical experience and I disagree with much of your assessment. I agree it's unconventional (like this problem that doesn't make sense) but not useless.

I suggest that you focus on the original poster and I will not post further in this topic to help prevent any confusion. Sounds reasonable to me.

 

[salami340]

Now, while monitoring battery voltage FIRST at the posts themselves, and second at the starter and the engine block, crank the engine a few seconds and read the meter while cranking. This is easy if you jumper the start relay. Bare minimum is 10. to 10.5, higher is better.
10.8 volts while cranking at the battery and the engine block. When I hooked to the terminal at the started the Brown wire is connected to it dropped to 1.5 volts... is that bad?

Are you saying the starter solenoid small terminal? That should be about the same as "cranking" voltage when cranking. A reading like that, IF it is cranking, shows maybe a bad lead or bad connection/ something wrong in your meter. The engine would never crank, with that low voltage at the start relay

I had my multimeter lead alligator clip to the small terminal brown wire on the starter (solenoid) and the other lead on the battery positive and this showed 12.5 then dropped to 1.5v when cranking. When connected to the battery negative or the engine block drops to 10.8 while cranking. I went out and did the same thing just now and got the same result.