Voltmeter reading low

[MISFIT 1]

I'm new to this forum and have a weird problem! My Backdraft is 5 or so years old. The voltmeter reads 10 volts and when the head lights or fan are on it reads 8 volts. The weird thing is at the alternator I get a reading of 12-13 volts.

Has anyone seen or have any opinion of something to look into?

Thanks in advance!!!

[Tommy]

12-13 volts is what you'd expect from a fully charged battery with the engine not running. 12-13 volts at the alternator with the engine running sounds like it is not charging. If the voltage on the meter is different from the voltage at the battery on a multimeter, then perhaps the meter is bad. If the voltage at the battery (using a multimeter) does not increase above 13 volts when the engine is running at a fast idle, the charging circuit is not working.

[spdbrake]

Tommy's right. You're pretty low. 12.40-12.60 is normal on a charged battery no load.
The Alt should be approx 13-14.7 running and loaded (lights and fans)above 13.

You could have a blown rectifier or bad voltage regulator. (Some easy tests linked below.)
Least likely: The battery could be getting old and not up to cap, Best have it load tested (free at Autoparts Stores) since a meter really won't tell you the internal health of connections.

Alternator & Charging System Checks (Alternator Testing)

[ame]https://www.youtube.com/watch?v=11jbHS4XXXY[/ame]

Doubtful this is the issue (full diode pack blown) since you'd have the Red Alt charging light on in the Dash.
[ame]https://www.youtube.com/watch?v=xgikeXt91vM[/ame]

[Kobura]

If the battery is as old as the car, it probably should be replaced anyway.

[MISFIT 1]

The battery is new. Could it be a grounding issue?

[twobjshelbys]

Originally Posted by MISFIT 1

The battery is new. Could it be a grounding issue?

Yes a good place to check. Remove the grounding cable from the chassis and be sure to scrape away any paint from underneath the stud so the cable makes a good contact. Don't trust "star" washers to make contact through the paint.

[Tommy]

My favorite way to check for a bad ground is to run a temporary jumper cable from the negative pole of the battery to the frame or whatever piece is your central ground. If the symptoms go away, it was because you have a bad ground. If the symptoms remain, it is something else.

[1795]

Not sure what your electrical set up is, but some alternators require a feedback loop to charge. I had to wire in an idiot light on one car for the alternator to start charging the battery.

[MISFIT 1]

Originally Posted by Tommy

My favorite way to check for a bad ground is to run a temporary jumper cable from the negative pole of the battery to the frame or whatever piece is your central ground. If the symptoms go away, it was because you have a bad ground. If the symptoms remain, it is something else.

Good idea! I will try this weekend and advise you guy

[Bartruff1]

Just feel the alternator, if it is hot, it could be as simple as a loose belt...

[dcdoug]

Originally Posted by 1795

Not sure what your electrical set up is, but some alternators require a feedback loop to charge. I had to wire in an idiot light on one car for the alternator to start charging the battery.

And if the bulb burns out, you lose the excite circuit. Took my a while to figure that out last year. Now I have a resistor wired in parallel just in case.

[Grey 65]

There is a 50amp to 80amp fuse buried in a harness just about the right footbox inside the engine compartment. check to see if that fuse is burnt.

Kevin....

[MISFIT 1]

Originally Posted by Grey 65

There is a 50amp to 80amp fuse buried in a harness just about the right footbox inside the engine compartment. check to see if that fuse is burnt.

Kevin....

What does it do? Can you elaborate on "buried"?

[Grey 65]

Originally Posted by MISFIT 1

What does it do? Can you elaborate on "buried"?

Fuse is inline from alternator to the harness under the dash and splits off and goes to the battery cutoff switch which in turn goes to the battery. If it blows it protects the harness from frying. If it blows the alternator charges nothing as it is cutoff from the rest of the system.

Buried means it is all wrapped up in the harness. Follow the red hot wire from the alternator and it will go to this maxi fuse prior to leaving the engine compartment.

Kevin.....

[MISFIT 1]

Originally Posted by Grey 65

Fuse is inline from alternator to the harness under the dash and splits off and goes to the battery cutoff switch which in turn goes to the battery. If it blows it protects the harness from frying. If it blows the alternator charges nothing as it is cutoff from the rest of the system.

Buried means it is all wrapped up in the harness. Follow the red hot wire from the alternator and it will go to this maxi fuse prior to leaving the engine compartment.

Kevin.....

Thanks Kevin! I will look tonight

[dcdoug]

If you get the alternator tested and it's good, my money would be on the voltage regulator. The vibration and heat in a cobra engine bay is a fairly hostile environment for them and I've had several of them go bad. Anyway, my $.02.

[MISFIT 1]

I messed with it yesterday and things are strange. When I start the motor the voltmeter reads 11V if I switch off the fuel pump it reads 13.5V. With everything on (headlights, fan, fuel pump) I am around 8V. New battery installed last week

[jimbo01]

According to what your blog states. Looks like you need a new alternator. The alternator is not putting out. These alternator's have a built in regulator. In most cases the regulator goes out and they are through away alternator's.

[Gaz64]

Are those voltage readings with a multimeter at the battery, or at the dash voltmeter?

Sounds like new alternator to me.

I had one fail that gave no warning light.

[MISFIT 1]

New alternator on the way