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post #1 of 23 Old 06-12-2017, 10:10 AM Thread Starter
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Extremely low voltage at idle 1995 gt...PLEASE HELP

My 1995 gt has been having some voltage issues the past week or so. I noticed the factory gauge dipping down a little further than usual at idle with the ac set to max, blowing on the highest speed. So I wired in a temp digital gauge to the battery terminals to verify the stock gauge reading.

If I turn the headlights on at the same time the ac is on, the voltage will drop to 12.5 volts. If I rev the engine, the voltage will come back up, but then drop shortly after it idles back down. I went ahead and swapped the alternator for a new one, thinking for certain that was the problem…guess I was wrong….still having the same problem. However, with everything off and with the car running at idle, its reading 14.1 volts when warmed up.

The battery is a autocraft gold 58-3 from advance auto parts and has a 2014 date sticker, so it’s only about 3 years old. I plan to take it by advance auto this afternoon after work to have them test it.

Any ideas on what might be causing this? I’m thinking a bad battery, but I have been wrong before, hahahahahaha.


2000 Mustang GT
5 speed- bbk 70mm TB- JLT cold air intake -bassani o/r xpipe -dynomax ultra flow c/b- eibach pro kit- ram heavy duty clutch-steeda quadrant/firewall adjuster- 373 gears-k&n filter-mgw orange shifter


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post #2 of 23 Old 06-12-2017, 12:02 PM
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hook a volt meter on

engine running

one lead on batt post on alt and the other lead on battery positive post

if you have a clamp meter you can measure amp produced by alt vs amp draw of car

report volt drop you get

when doing alt batt post do both the stud and wire seperately

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post #3 of 23 Old 06-13-2017, 08:38 AM Thread Starter
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Originally Posted by indy2000 View Post
hook a volt meter on

engine running

one lead on batt post on alt and the other lead on battery positive post

if you have a clamp meter you can measure amp produced by alt vs amp draw of car

report volt drop you get

when doing alt batt post do both the stud and wire seperately
I really wasn't sure what you were asking me to do, but here is what I did.

I tested voltage with the car running at the positive post on the battery, with negative lead on negative post on the battery. I also checked the voltage at the post on the alternator, with negative lead on the negative post on the battery. Both read roughly the same at ~14.0 volts (car was warmed up) with the voltage on the battery, being just a touch lower. This was with all accessories off. I forgot to get a voltage reading on the wire itself, but I would think it would be about the same.

I have a temporary voltage gauge inside the car, tied to the battery posts, to help me keep an eye on the true voltage. At idle, it reads around 14 volts with everything off. If I turn on the ac full blast, it will drop to 13.4. Then if I turn the headlights on, it will drop to 12.9. If I let it sit at idle long enough, the voltage will start dropping slowly. The worse I have seen was 12.2 and that was last night, after letting it sit for 3 minutes or so, with the brake lights on. These voltage numbers vary from test to test, but these are the worst numbers I have seen.

I am getting a new belt today, since the current belt has been squealing/squeaking on occasion. I think it could possibly be slipping, but I have no way to test that. I am also going to try cleaning the battery terminals really good and see if that helps in. They don’t look that bad, but it won’t hurt to clean them, and its free to do, LOL.

EDIT- I also had the battery checked, and it tested "ok".

2000 Mustang GT
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post #4 of 23 Old 06-13-2017, 09:35 AM
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you need to have one lead on the battery positive terminal

the other lead going to the alt positive post, and the wire

we are measuring for a voltage drop, you are measuring across the SAME wire.

then we will do the SAME on the ground side

redo.

Do NOT clean anything at this time

we are trying to find the problem

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #5 of 23 Old 06-13-2017, 11:58 AM Thread Starter
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like this? (copied from link below)

Voltage Drop Testing



ELECTRICAL CHECKS ON THE CHARGING CIRCUIT

To check the alternator connections on the positive side for excessive resistance:

A. Set DVM on 2 volt DC scale.

B. Connect the meter positive lead to the alternator output stud (B+ terminal).

C. Connect the meter negative lead to the positive (+) battery post.

D. With the engine running at 1,800 to 2,000 rpm with all lights and accessories on (except the rear electric defroster), check the voltage drop reading. It should be 0.5 volts or less. If higher, the connections between the alternator output stud and battery need to be cleaned. Also, look for loose connections or undersized cables.

To check the alternator connections on the negative side for excessive resistance:

A. Set DVM on 2 volt DC scale.

B. Connect meter negative lead to alternator case.

C. Connect meter positive lead to battery negative (-) post.

D. With engine running at 1,800 to 2,000 rpm with all lights and accessories on (except rear defogger), check the voltage drop reading. On the negative side, it should be 0.2 volts or less. If excessive, the connections need cleaning or the negative cable needs to be replaced. Some alternators are mounted in rubber bushings and have a separate ground strap. If so equipped, be sure to check the voltage drop across this strap, too.
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post #6 of 23 Old 06-13-2017, 07:38 PM
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like I said

both leads on the same wire

what is the voltage

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #7 of 23 Old 06-13-2017, 08:04 PM Thread Starter
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Voltage was .014 on the positive wire. and was .051 on the negative wire. This was with ALL accessories off, at idle. DVM was set to 2 on DC voltage. I have a picture of both readings if needed.
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post #8 of 23 Old 06-13-2017, 08:08 PM
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did you do at the post?

now, turn on all the loads, fan on hi, everything and get the voltages

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #9 of 23 Old 06-13-2017, 08:13 PM Thread Starter
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Yes, that was at the posts. I will check and post back tomorrow.

Thanks for your help.
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post #10 of 23 Old 06-13-2017, 09:33 PM
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alt has a post and wire

IT is VERY important you do both!

give a post and wire voltage

2 readings


89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #11 of 23 Old 06-14-2017, 09:08 AM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
alt has a post and wire

IT is VERY important you do both!

give a post and wire voltage

2 readings
What wire do I need to test voltage on? The post is obvious, since it has the main wire leading to the battery attached to it. But there are 2 plugs on it as well...I'm assuming is one of the wires in either of the plugs?

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post #12 of 23 Old 06-14-2017, 09:20 AM
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the main wire on the post

its connected with an eyelet lug

stick your probe on the wire lug get a reading

then stick the probe on the post get a reading

the other probe is on battery positive

we are looking for a voltage drop between the post and wire
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post #13 of 23 Old 06-14-2017, 09:38 AM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
the main wire on the post

its connected with an eyelet lug

stick your probe on the wire lug get a reading

then stick the probe on the post get a reading

the other probe is on battery positive

we are looking for a voltage drop between the post and wire
I checked both and got the same reading or within .001 volts. I even poked the wire and still got the same voltage or within .001.
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post #14 of 23 Old 06-14-2017, 09:43 AM
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okay

if you got the same on the ground side, alt case to battery neg, then we go to the alt

try full fielding the alt while measuring battery voltage

on the alt reg you will see a label 'test' screw, carefully jump battery positive to that screw

Does the alt produce over 14 volts to the battery?

only test for a few seconds, you should hear the idle drop during the test

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #15 of 23 Old 06-14-2017, 09:55 AM Thread Starter
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Originally Posted by indy2000 View Post
okay

if you got the same on the ground side, alt case to battery neg, then we go to the alt

try full fielding the alt while measuring battery voltage

on the alt reg you will see a label 'test' screw, carefully jump battery positive to that screw

Does the alt produce over 14 volts to the battery?

only test for a few seconds, you should hear the idle drop during the test
The alternator is brand new. NEW...not a reman unit. No reason to test it, I don't suppose. The alt puts out 14.2 volts after warmed up, I have verified this. My problem is that the voltage drops at idle when accessories are turned on....it drops a little with each, and then continues to slowly drop if I leave them on at idle. If I tap the gas, the voltage goes back to 14+ and then as the car idles down the voltage begins to drop again.

The battery also tested good as well...forgot to post that, I think. No issues with the battery at all.
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post #16 of 23 Old 06-14-2017, 10:35 AM Thread Starter
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I just don't see the purpose of that full yielding the alt using the jumper to the test on the regulator. Its already putting out over 14 volts to the battery at idle...the test to check for 14+ volts at the battery is redundant imo.

It puts out 14 volts at idle....what's the next step?

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post #17 of 23 Old 06-14-2017, 11:01 AM
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what was the voltage at the battery when you full field?

i am not concerned about your opinion or whether something is new

this is called diagnosing

you follow steps and elimate direction!

you have a better way?

enlighten me
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post #18 of 23 Old 06-14-2017, 11:29 AM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
what was the voltage at the battery when you full field?

i am not concerned about your opinion or whether something is new

this is called diagnosing

you follow steps and elimate direction!

you have a better way?

enlighten me

I have not done the full field test yet. Which, I'm glad I didn't...especially following your instructions. The screw should have a jumper to GROUND not to the positive post on the battery.

Quote:
Originally Posted by indy2000 View Post
...on the alt reg you will see a label 'test' screw, carefully jump battery positive to that screw
Your attitude is worse the a menstruating woman going thru menopause. I don't mind following instruction (which i'm honestly glad I didn't based on your statement above), but I know what the car is doing and isn't doing. Its not the alternator...has nothing to do with it.

Now...lets move on. Assume I did the full field test and it put out 15 volts. I know it puts out 14.2. And that is .2 volts more than you said to verify it put out during the full field test. I am 110% sure it will put out 14+ during the full field test...considering it does 14+ without the jumper

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post #19 of 23 Old 06-14-2017, 02:42 PM Thread Starter
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appreciate your help!

I'll be sure to clean those battery terminals as soon as I can. I'll report back to update the thread for anyone else who may come across this thread. Hopefully they won't have to deal with your "all the way around the world philosophy"

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post #20 of 23 Old 06-18-2017, 12:51 PM
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Quote:
Originally Posted by white2kgt View Post
My 1995 gt has been having some voltage issues the past week or so. I noticed the factory gauge dipping down a little further than usual at idle with the ac set to max, blowing on the highest speed. So I wired in a temp digital gauge to the battery terminals to verify the stock gauge reading.

If I turn the headlights on at the same time the ac is on, the voltage will drop to 12.5 volts. If I rev the engine, the voltage will come back up, but then drop shortly after it idles back down. I went ahead and swapped the alternator for a new one, thinking for certain that was the problem…guess I was wrong….still having the same problem. However, with everything off and with the car running at idle, its reading 14.1 volts when warmed up.

The battery is a autocraft gold 58-3 from advance auto parts and has a 2014 date sticker, so it’s only about 3 years old. I plan to take it by advance auto this afternoon after work to have them test it.

Any ideas on what might be causing this? I’m thinking a bad battery, but I have been wrong before, hahahahahaha.
Does this only happen when the A/C is on? How about when the radiator fan is on?

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post #21 of 23 Old 06-19-2017, 03:05 AM
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You have made this WAY too complicated. This has every symptom of the alternator shaft speed being too slow to keep up with the electrical load or a bad regulator. Measure between the alternator output post and the battery negative.

If you are measuring low voltage at idle and slowly creeping down but everything is normal at cruise engine speeds the problem is the alternator system.

A bad connection would show at all speeds as good alternator but low battery.

A regulator issue, an open diode, or some of those stupid under drive pulley kits will do exactly what this is doing.

I just changed the alternator on my truck. It was 12.3 and creeping down at idle. It was 13.0 volts when at 2000 RPM.
The new alternator is 14 volts at idle and 14.4 volts at fast idle.

If you have a under drive pulley kit, you are also causing electrical issues.


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post #22 of 23 Old 06-19-2017, 09:48 AM Thread Starter
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Does this only happen when the A/C is on? How about when the radiator fan is on?
Only happens when I turn the ac on. It does it with just the headlights on as well, but drops at a much much slower rate. From what I can tell, with the ac off and the engine warm, when the fan kicks on, it drops the voltage a little, but doesn't seem to continue to drop as fast as when the ac is on...similar voltage drop like with the headlights.

Quote:
Originally Posted by TomR View Post
You have made this WAY too complicated. This has every symptom of the alternator shaft speed being too slow to keep up with the electrical load or a bad regulator. Measure between the alternator output post and the battery negative.

If you are measuring low voltage at idle and slowly creeping down but everything is normal at cruise engine speeds the problem is the alternator system.

A bad connection would show at all speeds as good alternator but low battery.

A regulator issue, an open diode, or some of those stupid under drive pulley kits will do exactly what this is doing.

I just changed the alternator on my truck. It was 12.3 and creeping down at idle. It was 13.0 volts when at 2000 RPM.
The new alternator is 14 volts at idle and 14.4 volts at fast idle.

If you have a under drive pulley kit, you are also causing electrical issues.
It does have underdrive pullies, but this problem just started about 2 weeks or so ago. It did not do this before then, and I haven't changed or touched anything. The day it started, I hit a puddle of water after some heavy rain and the battery light came on for a split second, lost power steering, and the car almost stalled (was at 10mph or less). A few seconds later, it cleared up and drove fine...battery light went off as well. Thought nothing of it other than water must have hit the belt causing it to slip.

Ever since then, the stock gauge has been dropping much lower than it use to (at idle) before the water puddle incident. The car registers 14.2 volts at idle at the alt post and battery post, with negative lead on the battery neg post. I highly doubt its the ud pullies...they have been on the car since I bought it in 2009. I have never had a voltage issue before now.

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post #23 of 23 Old 06-19-2017, 09:13 PM
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wont put a dc clamp on to measure

alt output compared to system consumption

rather 'its been fine all this time'

cant be this cant be that....,,

take it to a qualified mech
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