Buy wp20420 - Genuine Cummins Belt Tensioner - wisentpartscenter.com

Description

Cummins 3937555 Belt Tensioner - Genuine Cummins Automatic Drive Belt Tensioner for B/C Series and ISB/ISC Engines

The Cummins 3937555 is the spring-loaded automatic tensioner on the accessory/serpentine drive. Its spring keeps constant load on the belt through a pulley on a swinging arm, taking up slack as the belt stretches and wears so it doesn't slip, squeal, or throw. Set it and the spring does the rest - no manual adjustment.

Fitment & Specifications:

- Application: Cummins B Series, C Series, ISB, QSB, ISC (5.9L / 6.7L / 8.3L)
- Type: Automatic spring-loaded accessory drive belt tensioner

Verify fitment with us before ordering to ensure the correct part for your application.

Purpose of the Part

It keeps the drive belt tight without anyone touching it. The spring loads the arm, the arm loads the belt, and that constant tension is what keeps the alternator, water pump, and the rest of the accessory drive turning without slip. When the spring weakens or the pulley bearing goes, tension drops and belt problems start.

Issue Symptoms / When to Replace

- Belt squeal or chirp, especially on start or under load
- Bearing whine, growl, or roughness from the pulley when spun
- Visible belt slip, flutter, or a tensioner arm that bounces
- Weak spring that no longer holds tension - belt goes loose

No fault codes on this one. It's a purely mechanical part - a bad tensioner shows up as noise, slip, or a wobbling pulley, not a code. Anyone pointing you at an SPN for a belt tensioner is guessing.

Brand Information

Cummins runs this same tensioner across its B/C Series and ISB/ISC engines, so one genuine 3937555 covers many builds with the correct spring rate and arm geometry for the drive. Cummins builds engines and components for trucks, buses, ag, construction, and power worldwide.

Installation Notes

Spring-loaded arm: put a wrench or breaker bar on it to relieve tension and pull the belt, then let it back slowly. Don't let it snap. Check belt routing and that the pulley sits square to the belt before final torque.

How to Identify an OEM Part

Genuine units are cast by Dayco, Cummins' OEM supplier - look for the Dayco logo, USA, and 3937555 cast into the body. The cast numbers are crisp and part of the casting, not laser-etched on after. OEM castings look duller and rougher than shiny aftermarket copies.

Why buy from Wisent Parts
  • ⏱️
    Fast shipping
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  • 🛡️
    180-day warranty
    All parts covered from delivery date.
  • ↩️
    Easy returns
    30-day returns.
  • ✅
    Verify fitment
    Send VIN/ESN and we’ll confirm.

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Warranty & Return Information
Warranty Information

Wisent Parts offers a 180-day warranty on all parts from the date of delivery. This warranty covers orders placed directly through our website. If a part fails during this period, we’re here to help - whether it’s a replacement, repair, or refund. For full details, refer to the Warranty page.

Return Information

Wisent Parts accepts returns within 30 days of delivery. All returns are subject to a restocking fee. To qualify, parts must be unused and in their original condition. For full return instructions and restocking fee details, please refer to the Returns page.