Hey all: if there is already a thread for troubleshooting this please point me in the right direction; so far I can't find any.
Here's my story: 1997 Discovery died one night, headlights faded out and multiple dash lights flickering.
The tach hadn't been behaving and I had heard that the alternator can cause this so I figured I needed a new alt.
I put in a "known good" used one; seemed to help, but it would only put out 13.7 volts max. Tach still wouldn't behave, but vehicle was more driveable.
Decided to put in a brand new alternator with a higher amp rating (120A) and a new battery. Also cleaned up the main ground on the chassis.
This is where it now stands: with the engine off the battery sits at 12.2v. Start the engine and voltage goes up very slowly.
I have a voltmeter plugged into the cigar lighter and while driving the voltage gradually climbs. When it hits 14.1 - to 14.2 volts the tachometer starts working.
So, the alternator is slow to reach 14+ volts but eventually does get there.
What measurements at the alternator (or elsewhere) can I make to start narrowing down why the alternator is slow to start charging?
Fingers crossed someone has seen this before.

 

 

Hey, at this stage I'll try anything!
Thank you. I will try it and post back here on the results.

 

SOLVED! The serpentine belt was routed incorrectly. I can't take credit for that, as I've never had to take the belt off entirely. It must have been like that when I bought it because the rev counter (tach) never worked, or never worked properly.
Anyway, I read somewhere someone else had the same issue so I though I'd check and bang, there it was.
I re-routed the belt as per all the diagrams on the net (how could someone get it wrong??). Started it up, and immediately the rev counter is working and the alternator goes to 14.2v right off the bat.
Thanks to all who responded!