So I'm driving down the freeway at about 70 and my radio starts cutting out. I look down at the dash and the tach is sitting at 0 as if the engine wasn't running but it was running and I still had power. The charge light was also on. I checked voltage at the alternator with the car running and got 12 volts, which is what i get with just the battery. But since the battery has a strong charge it seems ok. I drove it for about 45 minutes after the problem started. I parked it and turned it off and it started right back up strong. I have had it off and on a number of times since and the alternator seems to be charging the battery. Do you think it's just the accessories that are the problem... like the radio and instruments?

 

Your alternator is dead or a wire has come unhooked from the back of the alternator.
Keep driving and it WILL leave you stranded.

 

Went thru several alternators - several which displayed the "dead tach" syndrome. Original was bad, had a bad-outa-box from Auto Zone, then a second Auto Zone with a loose nut inside it, 3rd time was the charm and is OK. 12 volts is not enough to charge the battery, it needs to be in the mid-high 13's or low 14's, on a digital meter. Measure when vehicle is off, but something is "ON", like head lights. Then crank vehicle and see if the voltage increases.

 

I went through 2 AutoZone alts and 2 Advance Auto alts...buy one off of e-bay or have yours rebuilt.
Savannah is dead on about the voltage.

 

Ive had my speedo drop off to zero for a few seconds and come back up to speed but its been about 6 months since its happened...wrote it off as one of those Rover things.

 

Another simple electrical test of the alternator while in the vehicle - put digital meter across battery, and observe charge voltage with engine running. Turn on head lights, wipers, heater fan, etc. As load increases you should still be charging the battery. If charge voltage drops off with basic loads, the alternator could have a problem with electronic parts inside it, like the diodes or regulator. This can allow a "weak" alternator to seem OK, but allow battery to go dead under normal driving conditions (like night time, with rain, and with AC or defrost running). It can actually go dead while you are driving down the freeway, then come back to life as some loads are switched off. If using a salvage yard alternator, stop by auto parts store and they can usually test them for you.

 

I figure out how to relieve the tension on the belt in order to get the alternator off. Any help?

 

Put your socket on the nut in the middle of the belt tensioner, loosen the tension.
DO NOT loosen the nut.
Once the tension is loose remove the belt and then SLOWLY release the tensioner.

 

Savannah, I can make you a good deal on a new alternator that benches at 140amp. It has a slightly smaller pulley so it will put out higher amps at lower RPM, which is good if you winch or run accessories at low speeds. Downside is your tach will be a little off.

 

Here are some battery volts readings on my 97 Disco, at 201 degrees coolant temp, 90 outside air, Fluke model 189 meter, battery about six months old:

12.61 - engine off, headlights on low beam
13.13 - everything off
13.91 - engine on, head lights off, at idle
13.75 - engine on, A/C on high fan speed, electric condenser fans running
13.64 - engine on, at idle, A/C on full, front and rear wipers on, head lights on, radio on (50 watt Sony at normal volume, no bass amp, etc.). Note that car radio set ups can draw more amps, limit is based on your available ATM balance...

You will note that battery volts drop if engine is not running, but should come back up to a charging level even at idle with most normal loads. Now six "flamethrower" landing lights on the brush guard is not a "stock" load.

A weak alternator will exhibit more dramatic drop as load of electrical devices increases. I had a Ford that would kill the battery if driven in the rain for more than an hour. A bad "regulator" usually shows up as no output at all, you just get what is left in the battery. Batteries have a reserve minutes rating, how long they will hold up under a 25 amp load. Well, you are already running the fuel pump and ECM, so extra lights eat that up quick. I once made it home by unplugging one head light and one tail light, engine died when I pulled in the driveway and stepped on the brakes. Was a $7 regulator (Audi 5000 - a fine driving German "made-outa-margarine" model).