1987/05/01 Caterpillar

All Caterpillar Engines

Reference: "Testing The Starter On The Engine", in this issue.

Servicemen often need a procedure to follow to determine if an alternator or generator is working, or if it needs to be replaced. This general procedure is valid for any type of system, voltage, or charge rating.

... 6V7070 Digital Multimeter (or 6V7800 or 6V3030 or equivalent)

... 8T900 AC/DC Clamp-on Ammeter or equivalent

1. Put the multimeter positive (+) lead on the B+ terminal of the alternator. Put the negative (-) lead on the negative (-) terminal of the alternator. Put the clamp-on ammeter around the positive output wire of the alternator.

2. Turn off all electrical accessories. With the fuel off, crank the engine for 30 seconds. Wait two minutes to let the starter cool. If the system appears to be up to specifications, crank the engine again for 30 seconds.

NOTE: Cranking the engine for 30 seconds partially discharges the batteries in order to do a charging test. If batteries are already low in charge, skip this step. Jump-start engine or charge batteries as required.

3. Start the engine and run at approximately 1/2throttle.

4. Immediately check output current. This initial charging current should be equal to or greater than the full output current shown in the Service Manual. The most common alternators are:

6N9294 - 35A

5S9088 - 50A

8N999 - 75A

6T1395 - 35A

7T2095 - 35A

7N4784 - 40A

6T1396 - 55A

7T2096 - 55A

5. Within approximately 10 minutes at 1/2 throttle (possibly longer, depending upon battery size, condition and alternator rating), the alternator output voltage should be 27.5 plus or minus 1 Volt^* indicating the alternator is performing within spec. See Fault Condition Chart.

6. The charging current during this period should taper off to less than approximately 10 Amps, depending again upon battery and alternator capacities. See chart.