need help tracking down electrical problem

[svasica]

Whenever my electric fan or headlights (or for that matter any high draw electrical accessory) turns on, my volt meter goes down about 2 or 3 volts and stays there until that accessory turns off, at which point the volt meter goes back up to about 13 or 14 volts. I have installed a 130amp altenator. How do I go about tracing this problem to its root? Or possibly several roots?

 

[ECU5.0]

check all the grounds. battery ground, alternator ground, block ground. add another ground from the negative terminal to the block somewhere and see what happens

 

[HISSIN50]

Are you using the stock voltmeter to determine this? If so, use a DMM and see what it says. Also check the alternator voltage vs the battery voltage, vs interior voltage to check for drops.

Upgrading the motor ground is good on a 3G equipped car.

Good luck.

 

[bynummustang]

I wish I read this first, I could say the same thing, and people would see that I AM A SMART FELLOW. lol

:I'm with stupid sign:

 

[Daggar]

After completing what is mentioned above:

Start looking at how your acessories are wired. Your fan for instance, should not be wired directly to the battery. Instead it should be wired to a junction somewhere between the battery and the alternator feed. It should also not be wired directly to the alternator. Elactrical current will take the path of least resistance. It sounds like (and this is only suspicion at this point) that when your accessory is kicking on that it's drawing enough current from your charging system that it's pulling voltage away from the alternator stator circuit. Normally, current from the battery during surges will make up for it but if your wiring is (for lack of a better term) questionable, then your high amp consuming accessories may be cycling your alternator on an off. There is a simple method to test this theory but I'm leary about putting it out there until you're sure that the wiring you've got in there now is up to the task of handling the current draw you're asking of it.

Hissin -- You thinking what I'm thinking?

 

[HISSIN50]

Andy, I am hypnotized by the pulsing red eyes. I am scared to think what you think.

 

[mustang-a-holic]

I don't agree with the ground theory.

Here's why:
A poor ground will act as a resistive termination. This restricts current draw, causing a voltage drop on the load, not the voltage meter. Unless your meter is on the load side of the equation ?

The fact the meter is dropping is an indication you have a large load that is SUCESSFULLY dropping large quantaties of current. In fact this current "hogging" will cause a drop in system voltage if the source can not provide sufficient amperage.

Just because you have a 3G does not mean your free from problems. What loads are you running. Standard (head lights, high beams, brakes, blower at max, turn signals will draw 50-60 amps a 14.7 vdc).

1. What is your battery terminal voltage with car on at idle?
2. see #1 above but at 3000 RPM.

I bet your terminal voltage is dropping below 14.7vdc.

Also for you advanced users. Get a clamp on amp meter. Measure the current exiting the alt. It should be larger than the current on the + battery wire. The battery wire should have only a few amps on it (for re-charging)

I recently diagnosed a mustang with a diminishing charg potiental.

at startup the alt read 14.7vdc. after 15 minutes it was down to 12.6 vdc with no load changes (all measurements at idle). The current load shifted from the alt to the battery. This causes subsequent slow starts and a weak battery (which actually stranded them several times).

my previous ID was IDWITHELD. Try a search...I have a decent history of electrcial how-to's on the stangnet....

 

[HISSIN50]

ID, I sent you an email quite some time ago - did you get it?

So you dont think an upgraded (higher output) alternator needs an upgraded motor ground? If the stock wiring was designed for a 65 amp alt and you double that, it would seem that one would want to upgrade the power ground path too......

In any case, glad to see you back in here bud.

 

[svasica]

Actually, I haven't had the time to go check what the voltage is at the battery, alt, or interior. What i'm seeing right now is what i'm bringing to you guys. BTW, i'm not having a charging problem because as soon as the lights are turned off or the fan goes off the voltmeter will jump back up.

What I'm seeing is the stock voltmeter in the instrument cluster jump down a couple of volts when the lights are turned on or when the fan turns on. I would think looking at the fans wiring woud be a good place to start however this is also occurring when the stock headlights are turned on. Checking the grounds may be a good start. Other than that, where else could the wiring be going bad? Also, the fan is NOT wired directly to either the battery or alt, it is wired to a relay through a temperature controlled switch mounted in the thermostat housing.

I don't think this is an alt issue since the volt jumps right back up after the accessory turns off, plus, when I rev up the engine the volts will go up as well.

 

[mustang-a-holic]

I don't think this is an alt issue since the volt jumps right back up after the accessory turns off, plus, when I rev up the engine the volts will go up as well.

When your accessories turn off (load reduced) the voltage rises because your alternator can not supply the required ampacity.

If your alternator had infinite ampacity your volts would not drop. Unfortunatly, your limited in ampacity. Once you exceed your maximum ampacity voltage begins dropping.


A super simple test on the ground wire theory is to add a ground directly onto the alt housing run directly to the frame / batt neg.. It's either going to improve or not change the symptoms.

Hey JT. Glad to be back.

 

[jrichker]

Voltage drop testing of grounds.
A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection: more than .5 volts across a connection indicates a problem.
See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103 for help

Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 18-20 gauge wire for the test leads if you have to lengthen them.
1.) Most grounds use the negative battery post as their starting point.
2.) The voltage will be small if the ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) To measure grounds, place one DVM lead on the battery negative post and the other on the wire or connector that goes to ground. Place the other DVM lead on the wire or connection you want to test.
5.) Read the voltage drop: Light duty circuits (1-5 amps) should show .1-.25 volts. Medium duty circuits (6-25 amps) should show .25-.5 volts. Heavy duty circuits (26 amps up) should show .5-.75 volt drop. A voltage drop lower that spec is always acceptable.

 

[svasica]

Where would I directly connect a wire to the alt housing and how?

 

[HISSIN50]

Where would I directly connect a wire to the alt housing and how?

A jumper cable with an aligator clip works for diagnostics. A cable with an eyelet around a bolt works otherwise.

Good luck.

 

[svasica]

Ok, I hooked up a jumper cable from the negative battery post to a bolt holding the altenator in place and no change, the voltage still drops at the stock guage in the instrument cluster. The same still happened when I turned on the headlights and kept the jumper cable in place. Sooo, does this dismiss the ground as a culprit? If so, heres another idea, I wired a 130amp alt into the car several years ago and it seems to charge up just the battery just fine since when cruising along at highway speeds....even when the fan is on at highway speeds the alt keeps the voltage up pretty well. What else could be making my voltage drop so big?

 

[jrichker]

Go back and study the voltage drop diagram I posted. If the alt is putting out good voltage, then there is an excessive voltage drop somewhere in the circuit.

Start the car and turn on the radio, A/C, lights, etc. that you are running when the problem occurs. All the tests will require the normal load to be applied and the engine running at 1000 RPM or a little more.

Remember that with voltage drop testing, lower voltage readings are better.

Measure the voltage output at the alternator. Connect the + meter probe to the alt output terminal and the other probe to the negative battery terminal. At 1000 RPM you should see 13.8-14.2 volts on a warm engine.

Measure the voltage drop with one meter lead on the alternator body and the other on the negative terminal of the battery. You should see less than .5 volt. More than that means a bad connection or bad wiring.

Next measure the voltage drop across the alt output wiring. Leave the + probe attached to the alt output. Connect the - probe to the starter solenoid stud that is connected directly to the battery. You should see less than .5 volt. More than that means a bad connection or bad wiring.

If these measurements are good, the problem is in the wiring for the fuse panel or ignition switch. Use the same technique to measure from starter solenoid stud that is connected directly to the battery to the fuse panel and then to the ignition switch. Continue to measure the drop across each part of the circuit. At someplace you will find an excessive voltage drop that is causing the problem.

 

[svasica]

I'll definentaly go over the electrical system like you specified and try and find the gremlin.