Ok, so here's the deal. I've had this problem with my Disco for some time now. First what happened is the alernator started dropping out under a load. I would turn my heater and head lights off and the tach would come back online. I know the alternator runs the tach, so I had the alernator tested. It came back bad so I replaced it. Then went out, hadn't driven it in a couple weeks,and the battery was dead again.. I had the battery tested and it would hold a charge. It was an old battery so I replaced it.

Then the tach dropped out again. I have gone through 3 alternators, 2 batteries, and 3 EMCU's on the vehicle. So, I've done a little more troubleshooting on it and here is something that is blowing my mind. When I disconnect the main wire that feeds the fuse box off the back of the alternator the tach reads and there is 14.67 volts coming out. As soon as I hook that wir back up to the back of the alternator the voltage drops down to battery voltage. No matter what I do I can not get the alternator to have the appropriate voltage when it is hooked up to the fuse box. I replaced all fuses as well.. Any ideas? Please help, I'm in dire need!!!

 

3 wires on back of alternator.

Big one is + volts to underhood fuse box, front lugs. Another front lug carries fuse box over to (+) battery on a short wire. All needs to be kissable clean and tight.
This wire on alternator should read 13.6 - 14.4 to ground at idle, with bare minimun turned on. With everrything stock turned on my D1 draws about 85 amps, and volts drops to 13.28. 13.2 is about the minimum under load for any charging. If can'rt make that volts, alternator not putting out. Could also check frame ground on (-) battery lead, follow it to a point down near bottom of radiator.

Smaller wire on alternator is (+) volts from the check battery light, mine reads about 13.6 when engine running.

Tiny wire that plugs onto alternator is the tach lead.

Be sure serpantine belt is routed correctly and tight, not greasy, etc. One guy went through three alternators before finding bad tensioner and slipping (but quiet) belt after using vey strong degreaser.

 

All of the wires are hooked up properly on the back of the alternator. Ground is good. Positive leads are good. I even tried using a set of jumper cables from ground to ground and hot from the alternator directly to the battery. Still indicates battery voltage until you disconnect the D+ wire from the back of the alternator. I checked the routing of the belt and it is correct as well.. I am serious, I have no idea what to do next.

 

Jumper battery (-) to engine bock or serious metal?

What volts do you have on brown & yellow wire on back of alternator (not the brown only big wire)? If not (+) volts, alternator won't get too excited.

 

I jumped it from the battery to the exhaust manifold. I went back home for the day and the rover is at my father in laws. Should I disconnect the wire from the alternator and test the wire? The main thing I don't understand is that I can disconnect the big wire from the back of the alternator and the alternator puts out 14 volts and the tach comes back on, but as soon as I touch that wire back to the post on the alternator it drops down to battery voltage..

 

That would indicate a load that the alternator can't keep up with. Can you try a different battery, even if just with jumper cables?

 

I just replaced the battery before this round of troubleshooting and i put it on a trickle charge prior to working on it.. It has no problems starting up and running, well until the battery goes dead of course.. But I do not know what to do. I even went as far as pulling all fuses and installing them 1 at a time to see if there was an excessive load being put on it, even with the bare essntial fuses in place it has been the same outcome.. I know the alternator is getting the required field flash with the main power cabke disconnected because the out put is over 14 volts with it disconnected and then drops to battery voltage, which has been around 12.05 volts at idle speed, when the cable is hooked back up. Thisone just has me stumped. Everything points to the alternator, but it has good output.

 

It only has a good output with minimum or no load. Alternators have three sets of windings, and each of these phases has diodes to convert the AC into direct current, which is sent thru the regulator to charge the battery and operate the truck.

When one or more of these diodes is bad, it can reduce the output of the alternator from 100 amp (stock) to 66 amp or 33 amp. When that happens, it will show normal volts at minimum load, but add AC, fan, head lights; and it drops back to battery volts, and does not charge the battery. Instead it drains the battery as you drive, and eventually you can't crank the truck.

Since this is a replacement alternator, suggest you return it for test and warranty. There is a high failure rate of rebuilt units when rebuilt by the big chain store vendors. Lots of cheap parts made in main land China. Bean counters rule. My indy alternator shop says Tiawan, Korea, Japan - those used to be the cheap parts. Now they are good and China is the problem.

 

Thanks Savannah.. I will try to get a good enough charge on the battery to get it to the autozone and have them test it in the vehicle. That's about the best option I have right now.. Thank you so much for all the help.. I've noticed you assist so many people on here and if you don't hear it enough, THANKS!!!!

 

I fully agree with Savannah, either the new alternator has an internal short or the vehicle has a significant short somewhere. Or possibly some of your grounds have extremely high resistance.