Land Rover Repair: Electrical Issues, gauge works, random intervals

Question
I recently bought my 2nd Classic LWB ('95 4.2 L). This one has 208k. It's been taken care of and the previous owner says he installed a rebuilt alternator before I bought it. I'm having similar troubles to that of Wes'. It seems as though my alternator charges the battery when it is idling but not when it is running down the road. I could be wrong about that but if I don't let it run for a good 15 minutes before taking it out it sometimes causes the stereo to fade in and out, the RPM gauge works about 20% of the time, the speedometer jumps from normal speed reading to zero whehn the batteru gets low, and when I press on the brakes the dash lights dim significantly. A few times, I have found that the rover loses power and comes close to dying until I let off of the gas at which point i can pull it over, let it idle and get it the rest of the way home after it seems to charge. This only happens if I don't allow it to idle after starting for a significant amount of time. I recently replaced my Interstate battery with a Optima Deep Cell Battery. No corrosion on the connections, clean. When I bought the vehicle I noticed the engine compartment was very clean and  noticed you mentioned it could be because the alternator got wet. I brought it in to get the battery and alternator tested. They told me both were charging and fine. Can alternators intermittenly work? Oh yeah, the serpentine belt is plenty tight and not slipping at all. I am stumped and damned frustrated. =/

Answer
Hi Ethan,

tricky problem.  Usually, if there's a problem with the alternator, it'll show itself if you connect a multimeter and continually measure the voltage output to the battery.  If it's faulty (even at random intervals) you should see obvious drops and spikes in the reading. If it's constant...it's ok.

To me, it sounds like the belt is not fed properly.  I've seen the belt installed wrong AND still it looks tight and no obvious slippage.  However, at high speed and with a full load (electrically) the tach begins to pop up and down like a puppet.  

It has to be charge related.  Dimming lights, hesitation, fading stereo.  If it's not the belt, something is drawing off too much 'juice' from the alt.  

You have to be absolutely sure about how the belt is fed.  If it's not the belt, you'll have to search with your multimeter to determine what is drawing away all that charging....maybe its the AC compressor.  I've seen that happen on my '95 Disco.  Maybe you can disconnect the AC compressor and run the vehicle as a test.

Best of luck,

JohnMc

NB:  let me know what happens. Thx