How to Test Alternator - PeachParts Mercedes-Benz Forum

ejsharp · #1 06-30-2003, 11:32 AM

How do I test to see if the alternator is putting out current without reading the current from the battery? What must I disconnect? I have been told that the alternator must be under load in order for it to operate. If I disconnect the battery... will I not then lose the load for the alternator?

TX76513 · #2 06-30-2003, 11:57 AM

This maybe what you are looking for and they are cheap - but I use this for testing the battery or alternator with/without a load

TESTER

Duke2.6 · #3 06-30-2003, 12:16 PM

It's very easy and nothing needs to be disconnected if you are sure the battery is okay. With the engine/ignition off, measure the battery voltage. A good battery should be about 12.5 volts at room temperature. If the voltage is near 12 the battery is significantly discharged. If voltage is below 12 the battery probably has a shorted cell.

Now start the engine and read battery voltage at idle with accessories off. It should read 13.5-14.5 volts. If the voltage reading is 12.5 or less the alternator may not be functioning. If the voltage reading is below 13.5 bring up the revs to see if it increases. If so the alternator output is weak or the battery has a shorted cell. Check for a shorted cell by disconnecting the negative battery cable. Let the revs down, and if the engine keeps running, the alternator is okay.

The regulator should keep system voltage in the 13.5 to 14.5 range under all conditions except possibly a very heavy current load at idle, such as what may occur if virtually every accessory is on or a severely discharged or shorted battery, so this simple comparison test of system voltage with the engine off and on is how to test an alternator on the car.

The higher than battery voltage with the engine on places a "back voltage" on the battery, which is what charges it.

Duke

tunk · #4 06-30-2003, 12:23 PM

Quote:

If so the alternator output is weak or the battery has a shorted cell. Check for a shorted cell by disconnecting the negative battery cable. Let the revs down, and if the engine keeps running, the alternator is okay.

hey there Duke2.6, can you add a little more to this post? My question is what exactly are you checking for when you run the car and disconnect the neg term (alt or batt)? And why?

Thanks,
cdt

Duke2.6 · #5 06-30-2003, 04:00 PM

A shorted battery may draw so much current that the alternator cannot maintain the minimum 13.5 volts, so disconnecting the battery will tell you if the battery is shorted. If voltage goes up after disconnecting the battery, a cell is probably shorted.

Duke

ejsharp · #6 06-30-2003, 08:21 PM

Duke ...

I want to thank you for a brilliant test. The charge on the battery was 13 volts with car not running and the output at the altenator with the engine running was 14.25.

It looks like the new voltage regulator did the trick.

Thanks a million.

Car Nut · #7 06-30-2003, 09:16 PM

Is it okay to have 15V output on the Alternator? My 1987 Pontiac 6000LE V6 battery light is on intermittently. I changed the battery already, but the light is still on intermittently.

Thanks

Car Nut

haasman · #8 06-30-2003, 09:50 PM

Duke-

Am I wrong, but I always understood that one should never disconnect the battery while the engine is running and the alternator.

Haasman

ejsharp · #9 06-30-2003, 10:27 PM

Car Nut...

Take this as an interim reply untill someone more authorative answers your post.

My guess is that your voltage regulator (which is mounted on the back of your altenator) is starting to go bad... in that it is overcharging your battery by allowing too high a voltage to pass from your altenator. Simply replace the voltage regulator... and not the altenator.

haasman · #10 06-30-2003, 11:56 PM

Battery and alternator testing

The specifications for alternator testing by MB require that the voltage output be read after the engine has run for at least two minutes and all non-required electrical consumers are turned off (ie: dome lights, etc

..

-The allowable voltage is 13.0 to 14.5 volts.
-If the voltage is over 14.5 volts the regulator should be replaced.
-If the voltage is under 13.0 volts, the diode's should be tested along with the voltage regulator.

The connecting point for the voltmeter is the positive and negative terminals of the battery.

Charging voltage depends on temperature. At freezing, around 1.8 volts above battery voltage is normal. At 100F, this should drop about a half a volt. While the regulator has temp compensation, it's difficult to tell what its correction is except on a cold start.

Haasman

Duke2.6 · #11 07-01-2003, 01:19 AM

Quote:

Originally posted by haasman
Duke-

Am I wrong, but I always understood that one should never disconnect the battery while the engine is running and the alternator.

Haasman

Were'd ya hear that?

Now here's a question for all you stick shift guys. Can you "bump start" a car with the battery disconnected? Explain your answer.

Hint: "field excitation"

Duke

G-Benz · #12 07-01-2003, 11:57 AM

Quote:

Originally posted by Car Nut
Is it okay to have 15V output on the Alternator? My 1987 Pontiac 6000LE V6 battery light is on intermittently. I changed the battery already, but the light is still on intermittently.

Thanks

Car Nut

15V seems a bit high (14.4 or so is nominal, as previously posted).

I had my regulator fail on my W124 once, overcharging so badly the battery was gassing! A quick voltmeter measurement showed 17.5V at the battery terminals!!!