how to replace a alternator in a 1998 mercury sable

What engine size?
Before you replace it it might be helpful if you give us a little history. What kind of problem are you having. Please post back here. We will be glad to help.

Engine size doesn't matter.
One of those few times when size doesn't matter :-)

2 cylinders or 22 cylinders :-), a charging problem is a charging problem. What exactly is the problem, and why do you want to replace the alternator?

Mac,

Big Block is correct on the diagnostic end but on the replacement procedure (which is what you asked about) it can have everything to do with it. If you wish to replace the alternator we'd be glad to help but I think most of us would agree. It would be helpful to know what your problem is. Again, please post back here.

Mac,

Here are some basic and fundermental tests. Do them and post the results.

Battery Test

Connect a voltmeter directly across the "Battery Terminals". (Not to the Cable Clamps).

While watching the meter have a helper start the car.

The battery voltage should never fall below about 10 volts. If it does the battery is either "Low in Charge". or the battery is "Low in Water". The water level in the battery should be about 1/4 of an inch above the plates. "DO NOT" fill it up full. The battery could also have a "High Internal Resistance”. This could be due to “Sulfated” Plates. In this case it’s time for a new battery.

Now repeat the test but:

Connect the voltmeter directly across the "Battery Clamps on the Battery Cables".

This reading should be within 0.1 volt maximum. The same as step 3 above. If not clean the battery terminals and clamps. If necessary repeat the test to verify it is now OK.

Now repeat The test again but:

Connect the voltmeter directly across the positive terminal of the starter and the negative to ground on the case of the starter. If the voltmeter reading is more than 1/2 volt lower than the previous test you probably have a cable or connector problem. Check the connections between the cable and the cable connectors on both ends.

Alternator Voltage Test

Start the car and let it idle at normal speed.

Connect a voltmeter directly across the battery terminals.

Turn the headlights on to high beam.

Check the battery Voltage.

The battery voltage should read between about 13.8 to 14.2 volts. (possibly up to 14.5 volts.)

Increase the motor speed to about 1500 RPM. The alternator should now produce a battery voltage between 14.2 and 14.6 Volts.

If it is lower than 14.2 volts, the alternator probably has one or more bad diodes and the alternator needs to be rebuilt or replaced.

Now maintain the 1500 RPM and turn off the headlights. Run this test for one full minute. The battery voltage should never exceed about 14.8 Volts. If it does the alternators voltage regulator is faulty and over charging the battery. Replace the regulator or rebuild the alternator.

Alternator Amperage Test

Connect an amp meter to the alternator and start the engine. Turn on all electrical accessories and increase engine idle to 1200 RPM. The alternator should be the maximum output that the alternator was designed to produce. (Example) a 90 amp alternator should output at about 88 amps. An alternator is not designed to maintain maximum output for long periods of time. If forced to operate at maximum output the alternator overheat and fail. An alternator is designed to max out amperage output only for a short period of time.

my engine is a 3.0 V6, HAD TO REPLACE THE BATTERY THAT I HAD IN MY CAR AND
PUT IN A NEW BATTERY AND IT WENT DEAD, ALL THE SYSTEM LIGHTS GAS GAUGE AND THE RPMS DO NTO EVEN REGISTER. HAVE TO USE JUMPER CABLES TO GET IT GOING

Mac,

Your last posting changes things a little. While you still have to post the results for the earlier mentioned tests, you now have to post the results for the the test that appears below as well.

http://www.wikihow.com/Find-a-Parasitic-Battery-Drain

Mac,

Hopefully this link will help. Register at autozone.com and use this link:

http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c1528025fe3d

I personally would recommend a more thorough alternator test. Thay can do this free at many parts stores. The link also gives you R&R should you choose to replace it. I would also check for a parasitic drain ( something draining the battery while you're in bed asleep) if the alternator tests out good. Be sure to also check all of your connections and make sure they are tight and free of corrosion.

Try www.autozone.com for the step-by step guide along with pictures for this procedure.
Once at the site identify you car under the heading "current Vehicle"
Next click "Repair Info" on the tabs below the AutoZone logo.
At the middle of the page go to "Your Ride, Your Guide" and click "Become a member" set you username and password
Log-in and click "Vehicle repair guide"
Select the category for the repair needed.(ENGINE ELECTRICAL)
Select (on the left) type of repair needed.( CHARGING SYSTEM)
Page opens to your repair info.

This is for the 3.0 & 3.4 liter engines.