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Cummins 3907749 is the 12 volt 127 amp alternator: the cast housing with the mounting feet, the cooling fan and the single V belt pulley, and the output terminal on the back. It is used on Cummins 4BT3.9 and 6BT5.9 engines. This aftermarket version has the same mounting and electrical connector as the original, so it plugs in and bolts on where the old one came off.
Sold as one unit. Check the number on your old part, or in the parts manual for your machine, before ordering.
| Part Type | Alternator |
| Replaces | Cummins 3907749 |
| Voltage | 12 V |
| Output | 127 A |
| Engine | Cummins 4BT3.9, 6BT5.9 |
| Fits | machines, trucks and gen sets with the Cummins 4BT3.9 and 6BT5.9 engines |
| Condition | New, aftermarket |
| Stat. HS-Code | 851150 |
| Country of Manufacture | China |
Machines
Check the part number on your old unit before ordering.
| Cummins Part No.(SPM Part No.) | Fits |
|---|---|
| 3907749(SPM-HEP-5226) | machines, trucks and gen sets with the Cummins 4BT3.9 and 6BT5.9 engines |
Battery not charging
A charge warning light or flat batteries every morning point to the alternator once the belt and wiring check out.
Noise
A worn bearing whines or grinds.
Low voltage at idle
Weak output shows as dim lights and slow electrics at idle.
Fit with a new belt
Change the belt and check the tensioner when fitting the alternator.
We check what other aftermarket sellers charge for this same part. Their prices on the date we checked:
| Seller | Price | Checked |
|---|---|---|
| eBay (4 aftermarket sellers) | USD 184.29 to 284.86 | 3 Nov 2026 |
| SINOCMP | USD 309.00 | 3 Nov 2026 |
| Spare Part Mart | USD 147.43 | Today |
Our price of USD 147.43 is 20% to 52% less than these sellers.
Found this part cheaper somewhere else? Send us the listing and we will beat it by 5% under our Price Beat Guarantee. Every order is also covered by our 30 day money back guarantee.
Yes, it is a direct replacement for the Cummins 3907749 with the same fitting.
Typically 5,000 to 10,000 hours, depending on how often the machine is started and how well the batteries are kept.
A charge warning, low battery voltage with the engine running, flickering lights or bearing noise.
Check the old unit before you send it away. Pulleys and fans are often fitted to suit the machine and have to be moved across.
Disconnect the battery earth before removing the old unit. The main cable is live at full battery current.
No. It is an aftermarket alternator made to replace the Cummins 3907749. It is not made by the original manufacturer; the number only shows what it replaces.
Live rates for the quantity selected, delivered to your door. Confirmed at checkout.
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