Alternator: 5 Checks Before You Order
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An alternator can match your old one in size and shape and still be wrong: the voltage, the pulley, the clock position or the regulator can all differ. On machinery, marine engines and motorcycles, that means a unit that won't fit or won't charge. This guide covers the checks we recommend before you order, using real examples from our own range.
Prefer to read? The full guide is below.
First: is it really the alternator?
- Check the basics: belt condition and tension, the battery, the main cables, the earth strap and the fuses.
- Measure the battery voltage with the engine off, then with it running. On a 12 V system, a healthy charging system typically reads around 13.5–14.5 V at the battery with the engine running.
- If the voltage doesn't rise above the resting battery voltage, the alternator (or its regulator) is the likely cause.
Check 1: Match the part number
Read the label on your old unit and find the number in the cross-reference table on the listing. For example, our 24 V 50 A alternator for the Caterpillar 320C / 320D cross-references to Caterpillar 10R-7561 and 212-8561, Mitsubishi A4TU3599 and Lester 20111.
Check 2: Voltage and amps
12 V and 24 V alternators are not interchangeable. Machines from the same brand can use either:
| Machine | Alternator | Caterpillar ref. |
|---|---|---|
| Caterpillar 302.5C / 303 CR mini excavator | 12 V, 90 A | 0R9699, 144-9963 |
| Caterpillar 305.5–308 mini excavators | 24 V, 35 A | 139-7850 |
Then check the amps. For the Honda GL1500 Gold Wing we list the standard 40 A alternator and a 90 A upgrade. More amps is fine, but inspect the battery cables and main fuse when you upgrade.
Check 3: The pulley
- V1, single V-groove: Mitsubishi S4S / S6S
- V2, twin V-groove: Caterpillar 308B
- S6, 6-rib serpentine: Mercury Verado
- S12, 12-rib serpentine: Caterpillar 320C
Also check whether the pulley is included, and compare the pulley diameter if the listing gives one.
Check 4: Mounting and clock position
The clock position describes where the terminals sit relative to the mounting ear. Units that look the same can have different clock positions:
| Alternator | Clock |
|---|---|
| Caterpillar 308B | 12 |
| Mitsubishi S4S / S6S | 11 |
| Mercury Verado | 6 |
| Caterpillar 320C | 4 |
Heavy trucks use specific mounts too, such as the J180 hinge mount on our 160 A Kenworth W900 alternator.
Check 5: Regulator and terminals
- Internal (IR) or external (ER) regulator: most of our units are internally regulated. The Ford Courier / Mazda B1600 alternator uses an external regulator.
- Terminals: the Alfa Romeo 33 / 75 / 164 alternator uses B+ and D+. The Vetus marine alternator has a W terminal for the tachometer.
Quick checklist before you order
- Alternator at fault: belt, battery, cables and fuses checked, voltage measured?
- Part number: is it in the cross-reference table?
- Voltage and amps: 12 V or 24 V? Same or higher amps?
- Pulley: type, grooves, diameter, included?
- Mounting and clock: mount type and clock position?
- Regulator and terminals: internal or external? B+, D+, W?
FAQ
Can I fit a higher-amp alternator?
Usually yes, if the voltage, pulley, mounting and connections match. Check that your battery cables and main fuse can handle the extra current.
What does the "W" terminal do?
It provides a signal for the tachometer. If your engine's rev counter uses it, your replacement needs one too.
Is the pulley included?
It depends on the unit. Check the listing's technical specs and FAQ.
Not sure which alternator you need? Send us your part number or a photo of your old alternator before you order. Browse all alternators. More buying guides: stators, regulator rectifiers, starter motors and power trim & tilt motors.