My '00 Disco 2 has started an interesting problem, need some help troubleshooting it. Some background, everything until now is great! AB long block installed 20K ago, all good, runs great. Daily driver. All belts/hoses new at the install, Extinct inline thermostat.

A few days ago running errands and the red battery light comes on. I check my Scan gage II on the dash and the VLT is reading 11.6 volts, Whoa! Problem! I assume the alternator is toast (installed new in 2/18, O'Reilly's Ultima-rebuilt Bosch with a lifetime warranty). Then the light goes out and the VLT reads 13.9-14.0. But it keeps going from 11.5 to 13.8, then goes to 11.5 and stays there. OK, I'm about 4 miles from home, with a new Interstate battery (installed 7/23), so I shut off the wipers, heater, headlights and head for home. Made it and figured I had a bad alternator. Pulled it off, borrowed the wife's Tiguan (thank you dear!) and headed for the O'reilly's store. Where they tested the "bad" alternator and it read 14.4 volts consistently on a bench test 3 times straight. So, took it home, reinstalled and started the truck, 14.1 volts on my voltmeter. Yet, I went to drive it around the block, with lights on, all good, until about a minute into the ride, the lights dimmed and the Scan gage reads 12.4, then 13.8, then back to 12.6. Came back home and parked it.

So, any ideas from you all? I use the Disco as a daily driver and I really need 2 working vehicles in my household.

Thoughts and suggestions as to a course of action would be greatly appreciated.

Thanks!

 

The first place I’d go is cleaning all the main power connection points, positive and negative. On my P38 I added a large power wire directly from the alternator to the battery positive terminal. A poor ground connection could be just as bad. Even poor connections at the battery. This may not be your problem but it’s where I’d start.

 

You can check the alternator output directly -

• Multimeter to positive then negative to the alternator mount that should give you between 13.8 to 14.4
• Multimeter negative to engine side of negative cable should be the same - positive stays on alternator
• Multimeter negative to battery negative - positive stays on alternator
• Finally check directly at the battery

Ideally check as you see the voltage drop - It could be a couple of things but start by cleaning all the primary cable connections from the alternator.

Also you have to do the check above, your scangauge picks up at the OBD port.
Mine floats between 14 and 13.3 at the OBD port as the disco warms up, stabilizing at 13.3 - 13.4, but I always 14.2 to 13.8 at the battery. I have an issue somewhere else I have to track down, but it is not critical as the battery is charging and voltage is good for the computers.

 

Will do those checks. Also, what voltage should be at the small exciter wire on the alternator.

Looking at all possibilities.

 

It should be battery voltage when not running and alternator voltage when running.

 

In the DII, what you refer to as the exciter wire actually just goes directly to the charging system warning light in the instrument pack. The voltage at that terminal should be the same as the alternator output measured as Richard describes in his first bullet above.

The charging system warning light is a very simple, elegant design. I got curious about how it works and looked it up a couple years ago. Details are found in DII Electrical Library and the DII Electrical Circuit Diagrams. The charging system warning light is maybe the only thing in the instrument pack that's not fed from the BCU or ECM.

As I said, the small wire from the alternator runs directly to one of the two terminals on the charging system warning light. The wire from the other terminal gets power from the battery via the ignition switch and the passenger compartment fuse box. There is no ground/earth terminal in the traditional sense.

Think of that as the voltage from the battery pushing against the voltage coming from the alternator. So long as the alternator is putting out voltage, the light is not illuminated. But if the alternator is not putting out voltage (or too little voltage, I think) a path to earth is created via the alternator and the light is illuminated by battery voltage.

I am not an electrical engineer, so I may have misstated something here, but I what I have described is essentially correct.

 

OK, terminals were clean and almost shiny, 12.2 volts without the engine on at the battery terminals. Starts fine, but the battery light is on. 11.7 volts at all the points suggested by Richard and Havir. Also, even lower voltage when going from negative post on the battery to the small warning light terminal in the back with just the key on. I understand now that the small wire is only a warning light wire in the instrument gauge pak and not part of the actual charging system; Thanks mln01.

With all the tests done, it sure points to a dead alternator. But as I said, when tested on the bench at O'Reilly's, mine tested 14.4 volts. Unless it is now totally crapped out.

I plan to go again to O'reilly's tomorrow (Saturday) and pick up another one. I just want to make sure I've covered all the bases before going back. I can't think of anything else to check or test.

More thoughts if anybody has some would be great!!!

 

Check your ground wires off the battery and into the chassis. I had some burn or corrode.
Also those voltages sound about right. 11.5-13.4 is about right I think. I’ve only had one alternator that was consistently above 14 volts. Rest are 12-13 volts. Reading at ultraguage

 

And ground wire engine to chassis.

 

FIXED!!!!

Apparently with some of the rebuilds, the voltage regulator on its way out, will work sporadically, on then off, which would cause my random readings. Changed out with a new rebuilt from O'Reilly's (Bosch unit) and we're good to go. 14.3 volts at the battery, 14.3-14.4 volts showing on the Scan Gage II. No charge $$ from O'Reillys' as it has a lifetime warranty. If I had to buy it, it would have been $319.00 USD.

So save those receipts. Though the manager of the store said as long as they can read their part # on it, they will make good the exchange.

Thanks to all who helped out. Once again, great group of people on this forum!!!