I have a new battery and I just had the alternator checked today. It turned out ok. The volt meter in the dash never reads above 11.5, but seems to keep the charge around there. When I test the battery with the multi tester when off it reads 12.56. When I test it when the car is on, it reads 12.12. The harness type cable that connects to the alternator reads 12.6, but the one that gets bolted to the side only reads about 6. I just replaced the battery because the previous one seemed to be bad (even though I just bought it). It has stranded me two different times now. The hanes manual says it should read about 15 or so with the engine running. What do you think?

 

I think it is time to chuck the Haynes manual. You possibly changed a battery for nothing. It sounds like your battery electrical connections need to be cleaned and tightened. Some cables, maybe, replaced. Check the alternator output, directly from the alternator to a GOOD ground. Your battery voltage with the engine off, reads about right. The alternator should be putting out 13 to 14.5 volts. Running, the battery should read the same voltage, or close, to the alternator voltage. I suspect a bad battery cable and/or connection.

RACE ON!!!

 

How do I check the output of the alternator?

The battery cables themselves look good. They are tight and have no visible corrosion and the nuts that hold them on are new. There are a couple of wires connecting to the positive terminal that aren't real great, but they are connecting and tight.

The battery was bad when tested. It was new, so it didn't cost me anything to replace it.

 

I just had the exact same problem on my 88 and the altinator checked ok when I took it to the electrical place, put it back in still the same problem so changed the altinator and all is well now.

 

Cables can look good outside, but be bad inside...follow CFI's recommendations, clean, tighten and/or replace...

 

Double check the alt. Sometimes the people at the parts stores don't know what they are doing when it comes to using their testers.

See that thick red wire attached to the back of the alternator with a black rubber boot on the end? Pull the boot down, then touch one end of your voltmeter to the nut under there, and the other end to one of the master cylinder mounting bolts. If you're not getting an immediate jump in voltage to about 14 volts when engine starts there, your alt ain't charging. You can also just touch a flat blade screwdriver to the back center of it with motor on. If the screwdriver sticks like a magnet, the alt is generating a charge. If not, it's not charging. Check the alt harness wires.

 

With the engine running, use a voltmeter as described. You should have the 13-14.5v at the alternator. If you do, keep the engine running and use the same method to test the battery. The voltage should be the same. If not, you have a bad cable or connection inbetween. Don't necessarily rely on the dash gauge, use a voltmeter to be positive.

 

http://members.shaw.ca/agent86/Charging%20System.pdf

 

Alrighty. I cleaned the battery cables really good. Unfortunately, it is still the same.

As far as the big red wire is concerned, there is one set of wires that comes in via a nice harness. The only other wire is then bolted to the side. I retested this wire with the same results as the original post. Actually, the only way I got six volts off of it was to take it off of the alternator. When it is on the alternator and I try to test it, I only get like .062 volts, never anything more out of that particular wire weather the engine is running or not (regardless of where I put the ground).

If I were to replace wires, what exactly would I be replacing, the entire positive and negative wires?

Edit: I was driving today and the car's volt meter went from its low 11.5 to 13.5 and stayed like that for the time that I was driving. Why would it do that? Does that confirm the bad wires suspicion of earlier posts?

 

I recently had the same issue with my 85. My in-dash gauge would jump from 11.5 to 13.8. Using a DVM I checked it at the battery and saw the same results only .5 volts higher on the DVM (12.0 to 14.3). I just figured everything was OK, just a screwy in-dash gauge. Two days later the battery was dead.

I pulled the ALT off and took it to Checker Auto and had it tested. It tested good. Charged up the battery and had it tested. Checked out good. Put it all back together and drove it to Autozone and tested it in the car. Turned out the ALT had an internal short draining the battery.

Replaced the ALT and all is well. In-dash gauge now reads 13.8 to 14.2 (still have a half volt difference between the gauge and DVM)

 

You are doing something wrong. The large, single, red, wire that has a threaded stud connection on the alternator should read battery voltage (approx 12) with the engine off and alternator, charging voltage (13+) with the engine running.



That is what it should read. It could be bad wires, but a sudden jump like you describe sounds more like a lose connection somewhere that got jarred into place. Expect it to change back, off and on.

RACE ON!!!

 

Alright, I had it tested on the car this time. While he was testing it, it went back and forth between charging and not charging. So, with this, I replaced the alternator today. I started it and it maintained 11.8 v. So then I took out the ol' voltmeter and got 12.5 from the battery when off and 12.1 when it was on. I checked the voltage from the red wire described above and when it was off, it was exactly the same as the battery. Whoo hoo, a break through. The problem was that it registered 20.8 with the car on. Don't forget that before I was hardly able to get anything out of this wire when checked with the voltmeter. Now what does that mean?

 

you want to make sure ALL of the connections are clean and servicable. Check all ends of the wires to the alternator

Did you ever check the alternator brackets? I know this sounds odd, but remove the alternator bracket and clean all of the contact areas it has to the alternator and engine. Spray the areas with contact cleaner.

I know it sounds far fetched, but it worked for me on another car.

 

'86E, 13.6 volts while running.

 

The alternator puts out 14.7 volts cold, engine running, measured from the large output terminal on the rear of the alternator to the case of the alternator! When the alternator gets to its operating temperature its output voltage decreases with increasing temperature and reaches 13.7 volts output. The output wire on the alternator goes to the battery and has a fusible link (high amperage fuse made up of a short piece of copper wire) and connects to the jump start terminal behind the battery. The voltage on the alternator output wire should be identical to the voltage across the battery terminals. If the fusible link is burned out, the voltage on the alternator should still be 13.7 - 14.7 volts but the battery will measure 12.0 to 12.9 volts. If the alternator is not charging the battery, another indication would be the red battery symbol light being lit on the dash! Ignition off, measure the resistance from the positive battery terminal to the alternator output terminal, it should be the same as touching the ohmeter probes together. High resistance indicates a blown fusible link in the alternator's red output wire!
You can tell the state of charge of your car battery by measuring its terminal voltage with the ignition off. 12.0 volts and below, discharged. 12.9 volts and above, fully charged and linear in between [example, 12.5 volts, 50% charged].
The dash voltmeter measures the voltage after the ignition switch contacts which have a 0.3 volts drop across them, so the dash voltmeter will read 0.3 volts lower than your voltmeter reads across the battery terminals.

 

Ok. I obviously don't know much about this stuff, so I want to reiterate just to make sure that I am getting it.

I connected the oam reader to the positive terminal of the battery and the bolted red wire on the side of the alternator. I hope thats right. Anyways, when I touch the ends of the oam tester together it was somewhere in the neighborhood of .19 to .20 or so. When I put one on the positive terminal and one on the red wire from the alternator I got around 21 or so. Not .21, but a full 21. According to jfb's post am I to assume that the fusible link is bad? If it is bad, how do I find it? Is it an inline fuse that can just be replaced? Do you think that this is what would cause my previous alternator to charge one time and then not to charge a another moment? I ask, because I can still return the new alternator if you think the old one is good. Thanks for your help.

 

Follow the red wire from the alternator output terminal back to the jump start bolt behind the battery and close to the jump start bolt you will find a bulge in the red wire with black heat shrink tubing over it. The bulge is the fusible link. I would confirm the defective fusible link by connecting your ohmeter from the jump start bolt to the alternator output terminal. If your ohmeter reads 21 ohms again, then your fusible link is defective and needs to be replaced. The link could have been damaged if you put a wrench on the alternator output nut and the wrench hit ground and sparked. When removing the alternator, first disconnect the negative cable from the battery so that accidental grounding any positive wire doesn't short the battery to ground. When replacing the fusible link, also disconnect the negative cable from the battery. When the fusible link is replaced, the last thing you do is re-connect the negative cable to the battery terminal.

 

YES!!! Thanks for all of the help. It looks like it is finally working. I traced the red wire back like you suggested and found what looked like the fusible link. I cleaned up everything included where all of those wires connect, slapped it all together and it tested out perfect. Thanks a ton. Also, I tested the old alternator again just to make sure (at NAPA this time) and they said that it had a bad internal regulator. SO, new battery, new alternator and many clean connections later, we have a perfectly charging system. THANK YOU!!!

 

Another satisfied customer!