I *think* I'm having trouble with the alternator on my 2002 Navigator with the 5.4L DOHC engine.


Back Story:


A year ago in Feb 2014 the alternator quit charging. I thought the headlights were a little dim, but when the radio turned itself off and then the heater fan did the same, I noticed that the alt light was on. Then the engine shut down about 1.5 miles from home.


I'd heard of war stories about rebuilt alternators so I sprang for a brandy new one. I also got a voltmeter that fits in the cigarette lighter so I can monitor the system voltage. All seemed good and the new alternator put out 14.2 to 14.5 volts. Except that the battery never seems to be fully charged. If I check the battery voltage with the engine off it typically reads 11.9 to 12.2 volts. I had Interstate Batteries check the battery. They thought it was okay, but offered to put a new battery in to keep me happy.


Six months and 8,000 miles later I'm up in Ontario back country (on vacation - no tools in the truck) and I notice the engine idles at 900 RPM instead of the usual 650 +/- RPM. Oh-oh, I also notice the volt meter is reading 11.9 volts and drops a tenth about every 10 miles.


Found a shop that was open and they installed a rebuilt alternator and it was putting out 14.2 to 14.5 volts. All was good (except for my wallet!).


Now, here we are six months and 5,000 miles later and the rebuilt alternator is struggling to put out 12.5 volts this afternoon. At a steady 50 MPH with the cruise Control on, the radio, and the heater fan on low, the voltage is bouncing between 12.2 volts and 12.7 volts.


Now to my questions:


1. Shouldn't an alternator put out 14.2 to 14.5 volts all the time? Not just when the alternator is new?


And


2. Is there anything on my truck that could be killing these alternators? Or am I just unlucky in the alternator lottery?

 

Yes, 14 + all the time unless you have EVERYTHING running at once. That would be headlights on, A/C on, blower on high, etc. It may drop some then.

The next time it happens, take the alternator off and put it on one of those machines in the store to check it and see if it's good. Changing the alternator fixes your problem each time, and the only other thing I think it could be is a loose connection somewhere near the alternator, and just changing the alternator disturbs it enough to make it start working.

 

While it's good to monitor the system voltage on the fly you're going to need to do some more in depth troubleshooting, so you'll want a DVM or multimeter. Checking for a parasitic load that may be draining battery on a continual basis for example.

Not saying this is the problem but it's one of the things to look for. A battery that is continuously discharged will sulfate and eventually fail and presents a heavy load on the alternator. Be sure to charge battery thoroughly before installation, this is important.

Also check for voltage drop across cables, connections and alternator itself, while under load. Voltage drop is a very simple test and no disassembly required, but is often misunderstood on how it's done. It will quickly determine a bad connection regardless of how it "looks". I'd start here:

http://www.aa1car.com/library/charging_checks.htm

http://www.aa1car.com/library/voltage_drop_testing.htm

There are several good troubleshooting procedures and techniques at this site, btw. Hope this helps.

 

So, did Interstate actually replace the battery or not? You didn't say...

 

Thanks for the reply, Dave. Looks like I'm gonna have to go for another alternator then as the low voltage seems to be pretty much all the time now. Its just a pain to change it out - when I did the first one I ended up having to drain the radiator, remove the upper radiator hose, pull the radiator shroud off to remove the fan and the thermostat housing in order to get the alternator out.


When I changed out the first alternator, I did take all the grounds off and shine them up with emery cloth as well as the alternator mounts to be sure it had a good ground. All the connections and pins on the back of the alternator appeared clean and tight.

 

Thanks for the reply Tedster9. I forgot to mention that I checked for parasitic draw with my graphing multimeter. I can't recall the exact numbers, but it end up at < 50 milliamps after about 40 minutes, same as the shop manual called for.


I also checked voltage drops with the system loaded, except for the alternator output wire as I couldn't get to the alternator output terminal with the engine running - back of the alternator is pretty much under the intake manifold on the Intech engine.

 

Yes, they did. They first recommended that I charge the battery with a slow battery charger but it would only charge up to about 12.4 volts, so I asked them to replace it and they agreed.


With a new alternator and a new battery, I figured I was good to go. Then six months later I noticed the PCM had bumped the idle up to 900 RPMs because the alt light was on!

 

First off, I want to thank Dave, Tedster9, and projectSHO89 for there responses.


I had some time (and better weather) to check out the alternator today. I put one of my voltmeters across the battery while I checked for a voltage drop between the alternator frame and the negative battery post with my other voltmeter. Bingo! 1.2 volts dropped with just the engine running at idle and the battery was reading 13.5 volts!


I hooked up a light weight jumper wire with alligator clips between the alternator frame and the negative battery post and the voltage drop dropped to 0.8 volts. Added a second jumper wire from alternator frame to negative battery post and the voltage drop dropped to 0.4 volts and the battery was now reading 14.3 volts!


Battery terminals are both clean and tight. Followed the battery ground cable and it is about ~6 feet long and snakes its way down to a bolt/stud securing the starter motor. I remove the cable end and cleaned the contact area with emery cloth and removed the bolt/stud and cleaned it all up on the wire wheel (looked kinda rusty), put it all back together and (without the jumper wires) the voltage drop was still 1.2 volts.


Those first test wires I used were pretty light weight and felt lukewarm when I took them off, so I dug up a pair of heavier test wires - (maybe 16 gauge?). With them connected the voltage drop was 0.2 volts and the battery was showing 14.4 volts!


Next I loaded the system with both the front and rear HVAC blowers on high, the lights and high beams on and the voltage drop rose to 0.8 volts and the battery voltage dropped to 14.0 volts.


I had to run an errand so I drove it with the two test wires connected between the alternator frame and the negative battery cable. Wow! It seemed like a different truck to me. Engine felt much "perkier" (trademark) and lights seemed brighter. I don't think it was my imagination.


So now we're "digging where the 'taters are"


Next, I wanna figure out why the factory ground isn't cutting it. The starter seems to spin the engine over, no problem, so I'm thinking the original cable is good. But I wanna re-run the voltage drops by cutting the circuit in half.
I wanna see the voltage drop from the alternator frame to the bolt/stud on the starter motor, and then from the bolt/stud to the battery negative terminal.


And I just may end up making up a 4 gauge(?) ground cable to run from the alternator to the battery negative terminal if I can't find where the voltage is getting dropped.


Stay tuned !

 

I have done that before. I wil make some effort to trace it down, but then after awhile I will give up or it's just not practical to take all the bracketry off to find. One particular truck I was working on was very rusty. Someone had already broken one of the bolts off trying to take the alternator off before me. I found the same problem as you, and I just ran a new ground wire to the battery from the alternator bracket.

 

"Diggin' where the taters are" I love it.

Thanks for the update, very interesting. You could test the cable junction crimps themselves? Or replace them. It may be internal corrosion inside. Sounds like you did a great job of troubleshooting.

 

Since the starter turns and cranks the engine, I suspect he has a good ground to the starter/engine block, but a poor ground from the alternator to the block somewhere.

 

Okay. Got some time (and weather) to get back on this issue. Checked for voltage drops at various places and found all the voltage is being dropped between the alternator frame and the engine block. With all my temporary jumper wire grounds off, it is dropping 1.2 volts between the alternator frame and the block. (I think maybe that rebuilt alternator was painted silver by the rebuilder and that's prolly why it doesn't have a good ground to the engine block.)


Is there a way to calculate how big a wire I'd need to run from the alternator to the engine block to get rid of that 1.2 volts being dropped?

 

Let's assume you have the big 130 amp alternator. I would go to the store and get one of those universal battery cables, hopefully they sell one that is black with ring terminals on both ends, and that is long enough. They will be #4 or #6 gauge wire, which should be plenty big enough.

 

Thanks, Dave, I have a few feet of 4 gauge stranded copper black cable and some #4 copper lugs. I'm gonna learn how to make up a custom cable.

 

Just wanted to wrap this project up and report my results:


I made up a 4 ga stranded copper cable and added copper ring terminals to each end. Mounted one end to one of the alternator bolts that threads into the alternator housing and the other end to as easily accessible bolt on the upper intake manifold.


I hooked up the DVOM between the alternator housing and the negative battery post. Started the engine and its dropping 0.05 volts ! Turned the front and rear heater fans on high and the headlights on high beam and it drops 0.09 volts !


Put the volt meter across the battery terminals and the battery is seeing 14.8 volts with no load on the alternator ! That's about 1.75 volts more than before.


I'm a very happy camper ............Thank You to all who responded to my request for assistance.