Howdy D2 owners! Wanted to share some unexpected results after recently replacing my alternator.

In early January my alternator failed a few days after doing a single pull with a new winch that I installed on the Disco. I replaced with a 150amp Range Rover P38 alternator, as suggested by many folks on the forum. The next morning driving to work early I noticed the following:
1. Headlights increased brightness - I had previously done a relay mod and thought they were already bright, but wow!
2. Dash lights and dome lights brighter - guess I never realized how dim they were, thought it was a Land Rover thing
3. The transmission is shifting like butter - I never really had any issues and am up to date on maintenance, but always had a double shift at 3-4-overdrive when in the throttle, it's gone now
4. The biggest surprise, my fuel gauge is working again! - it quit reading correctly about 2 years ago, a couple months after I bought the vehicle. Suddenly after a normal fill it starting reading half when full and went down slowly from there. I have now filled and emptied to the low fuel light 3 times now, backing up with the Ultra-Gauge and GPS miles and it is tracking accurately.

So I'm still two years new to the Disco and know that electrical issues can manifest in strange ways (my CJ7 and Dodge truck are notorious for electrical gremlins), but I had not expected this. I make a habit of testing my alternators regularly (with multimeter, diagnostic reader, and smart charger) and the old (possibly original) alternator always tested fine and the battery was always charged (new last fall). Could the alternator had been weak enough to cause low enough voltage under load so the fuel gauge read incorrectly? Did having the battery disconnected for an hour reset the ECM/fuel reading? Bad battery ground? I never read the resistance of the sending unit. That was on my spring do list and replace, if required. Maybe some of you smarter, experienced D2 owners can weigh in.

Sorry for the long winded post, but I wanted to share in case others are having fuel gauge, transmission shifting or suspected electrical issues. Maybe check your alternator......

 

Presumably you’ve checked your voltage on the new alternator, including at higher rpm’s. Hopefully the voltage isn’t too high. My guess would have been that the old alternator wasn’t putting out full voltage for some time, but you say that you checked it regularly.

 

Harv,

I did check the new alternator output after replacement and remember it being in the 14+, but couldn't remember actual numbers. Got home tonight and pulled out the multimeter to have accurate readings.
Cold start, cold battery (50F) at 12.6V after sitting for 3 days.
No Load/idle -14.54V initial on startup, then settled to 14.44V right away, 14.38V at full operating temp
Load/idle - 14.14V (headlights, fog lamps, radio, AC and blower on high, all interior lights on)
No Load/2000RPM - 14.38V
Load/2000RPM - 14.17V
I don't have my notes from the last time I check the old alternator before it smoked checked, but I remember it being more in the low 14.1-14.2 at idle no load and high 13.8 or so with full load at 2000RPM. Always passed the alternator checks, for what it's worth, but I agree that it was likely weak and starting to fail. Having owned for a little under two years, I had no baseline. Now I do.
Still scratching my head on the fuel gauge working again, but I'll take it!
Thanks for the feedback.

 

Yes, all good news.

 

You can fix a weak alternator with a new voltage regulator and slip ring. Most likely the bearings, stator coils and rectifier diodes are not defective. Latter two can be tested with a multimeter easily. Either they are dead or OK, they do not wear down over time, they just break. Worn out slip ring and brushes can deliver a wrong voltage to the rotor coil. The regulator applies a voltage to the rotor coils which in turn generates a magnetic field which determines the output voltage of the brushless stator coils (this is how the voltage regulation is done btw).

You need to remove the plastic cap on the back only and get access to the regulator, slip ring (by removing the regulator) coil and diode contacts in order to investigate. Rebuild kits are not expensive ($36 ebay) or buy the voltage regulator (which contains the brushes) and a slip ring separately.

 

Good info! When I have the chance I'll test the original with the multimeter and likely rebuild and keep for a spare. I rebuilt my truck alternator years ago and it lasted for many years until it the diodes kept going bad and throwing excessive AC voltage. Caused all kinds of torque converter lockup issues with the transmission.