The Cat 3126 and 3126B represent two closely related generations of Caterpillar’s mid-range 7.2-liter diesel platform used widely between 1995 and 2003 in on-highway trucks, industrial equipment, construction machinery, and standby power generators.
Although both engines share the same basic block architecture, displacement, and HEUI fuel system concept, the transition from the early 3126 (1995–1998) design to the 3126B (1998–2003) introduced a series of electronic, emissions, and durability refinements driven by changing EPA standards and expanding electronic integration requirements in commercial vehicles.
Cat 3126 / 3126B Engine Specs
| Engine | Cat 3126 | Cat 3126B |
|---|---|---|
| Years of production | 1995 - 1998 | 1998 - 2003 |
| Number of Cylinders | 6 | 6 |
| Displacement | 442 cu in (7.2 L) | 442 cu in (7.2 L) |
| Bore | 4.33 in (110 mm) | 4.33 in (110 mm) |
| Stroke | 5.00 in (127 mm) | 5.00 in (127 mm) |
| Max Power | 175 - 300 hp (131 - 224 kW) | 175 - 330 hp (131 - 246 kW) |
| Power Measured | 2200 - 2400 rpm | 2200 - 2400 rpm |
| Peak Torque | 420 - 860 lb ft (570 - 1166 Nm) | 420 - 860 lb ft (569 - 1166 Nm) |
| Torque Measured | 1440 rpm | 1440 rpm |
| Operating Range | 1440 - 2500 rpm | 1440 - 2500 rpm |
| Firing Order | 1-5-3-6-2-4 | 1-5-3-6-2-4 |
| Compression Ratio | 16:1 | 16:1 |
| Flywheel and Housing | SAE No. 2 | SAE No. 1 or SAE No. 2 |
| Starter | 12 V or 24 V | 12 V or 24 V |
| Alternator | 35 - 70 amps | 35 - 70 amps |
| Rotation (from flywheel end) | CCW | CCW |
| Air Cleaner | Air intake precleaner filter, dry, centrifugal | Air intake precleaner filter, dry, centrifugal |
| Fuel System | HEUI (first gen) | HEUI, improved |
| Governor | Mechanical / Electronic | Fully Electronic |
| Electtronics / ECM | ADEM (first gen) | ADEM II |
| Diagnostic Capability | Limited | Improved diagnostics |
| Aspiration | Turbocharged and Aftercooled or Air-to-Air Aftercooled (DITA / DITAAC) | Turbocharger and Air-to-Air Aftercooled (DITAAC) |
| Emission Standards | Pre-EPA | US EPA Tier 1 / Tier 2 |
Weight and Dimensions
| Engine | Cat 3126 (1995-1998) | Cat 3126B (1998-2003) |
|---|---|---|
| Length | 42.8 in (1086 mm) | 42.5 in (1078 mm) |
| Width | 26.5 in (674 mm) | 31.4 in (798 mm) |
| Height | 36.5 in (927 mm) | 34.0 in (862 mm) |
| Dry Weight | 1250 lb (568 kg) | 1295 lb (588 kg) |
Service Refill Capacities
| Engine | Cat 3126 (1995-1998) | Cat 3126B (1998-2003) |
|---|---|---|
| Cooling System (engine only) | 3.5 gal (13.2 L) | 3.5 gal (13.2 L) |
| Lube System Capacity (Shallow Sump, 7 in / 178 mm) | - | 6.1 gal (23 L) |
| Oil Sump Capacity (Shallow Sump, 7 in / 178 mm) | - | 5.5 gal (21 L) |
| Lube System Capacity (Deep Sump, 9.5 in / 241 mm) | 8.5 gal (32 L) | 7.9 gal (30 L) |
| Oil Sump Capacity (Deep Sump, 9.5 in / 241 mm) | - | 7.5 gal (28 L) |
| Oil Change Interval | Every 250 Hours or 1500 gal (6000 L) of Fuel | Every 250 Service Hours |
Recommended oil: CAT DEO 10W30 or 15W40
Cat 3126 Ratings (1995-1998)
| Horsepower / Governed Speed | Peak Torque @ 1440 rpm | Torque Rise |
|---|---|---|
| 175 hp (130.5 kW) @ 2400 | 420 lb ft (570 Nm) | 19% |
| 190 hp (141.7 kW) @ 2200 | 520 lb ft (705 Nm) | 40% |
| 210 hp (156.6 kW) @ 2400 | 520 lb ft (705 Nm) | 23% |
| 210 hp (156.6 kW) @ 2200 | 605 lb ft (821 Nm) | 37% |
| 230 hp (171.6 kW) @ 2200 | 660 lb ft (895 Nm) | 37% |
| 250 hp (186.5 kW) @ 2200 | 660 / 800 lb ft (895 / 1085 Nm) | 26 / 52% |
| 275 hp (205.2 kW) @ 2200 | 800 / 860 lb ft (1085 / 1166 Nm) | 39 / 49% |
| 300 hp (223.7 kW) @ 2200 | 800 / 860 lb ft (1085 / 1166 Nm) | 27 / 37% |
Cat 3126B Ratings (1998-2003)
| Horsepower / Governed Speed | Peak Torque @ 1440 rpm | Torque Rise |
|---|---|---|
| 175 hp (131 kW) @ 2500 (DM3276) | 420 lb ft (569 Nm) | 25% |
| 190 hp (142 kW) @ 2500 (DM4201) | 520 lb ft (705 Nm) | 27% |
| 210 hp (156 kW) @ 2500 (DM4202) | 520 lb ft (705 Nm) | 21% |
| 210 hp (156 kW) @ 2500 (DM3278 / DM3279) | 520 lb ft (705 Nm) / 605 lb ft (820 Nm) | 23% / 45% |
| 230 hp (172 kW) @ 2400 (DM3280) | 660 lb ft (895 Nm) | 37% |
| 250 hp (186 kW) @ 2400 (DM3281 / DM3282) | 660 lb ft (895 Nm) / 800 lb ft (1085 Nm) | 26% / 52% |
| 275 hp (205 kW) @ 2400 (DM3283 / DM3284) | 800 lb ft (1085 Nm) / 860 lb ft (1166 Nm) | 39% / 49% |
| 300 hp (224 kW) @ 2400 (DM3285 / DM3286) | 800 lb ft (1085 Nm) / 860 lb ft (1166 Nm) | 27% / 37% |
| 330 hp (246 kW) @ 2400 (DM3287) | 860 lb ft (1166 Nm) | 19% |
Cat 3126B Performance Curves (9 options)
⚙️ 175 hp (131 kW) Cat 3126B DM3276 ⬇️
⚙️ 190 hp (142 kW) Cat 3126B DM4201 ⬇️
⚙️ 210 hp (156 kW) Cat 3126B DM4202 ⬇️
⚙️ 210 hp (156 kW) 3126B DM3278 / DM3279 ⬇️
⚙️ 230 hp (172 kW) Cat 3126B DM3280 ⬇️
⚙️ 250 hp (186 kW) 3126B DM3281 / DM3282 ⬇️
⚙️ 275 hp (205 kW) 3126B DM3283 / DM3284 ⬇️
⚙️ 300 hp (224 kW) 3126B DM3285 / DM3286 ⬇️
⚙️ 330 hp (246 kW) Cat 3126B DM3287 ⬇️
In terms of hardware, the 3126B is not a “new” engine but an evolution of the original 3126—retaining most mechanical systems while incorporating updated ECM logic, improved HEUI injector calibration, enhanced monitoring functions, expanded PTO controls, and broader compatibility with OEM electronic systems (J1939, ABS modules, and transmission networks such as Allison WT).
Cat 3126 / 3126B Engine Equipment
| Cat 3126 (1995-1998) | Cat 3126B (1998-2003) |
|---|---|
| Air inlet manifold heater | Air inlet manifold heater |
| Cooling: belt-driven jacket water pump, oil cooler | Cooling: belt-driven jacket water pump, oil cooler |
| Crankcase breather | Crankcase breather |
| Electronic Control Module (earlier ECM version) | Electronic Control Module (ECM) |
| Electronic Data Link (J1922 / J1939 early support) | Electronic Data Link, SAE J1922, J1939, ATA J1587 |
| HEUI Fuel System (first-generation HEUI) | HEUI Fuel System (Hydraulically actuated, Electronically controlled Unit Injector) |
| Flywheel and SAE No. 1 or SAE No. 2 housing | Flywheel and SAE No. 1 or SAE No. 2 housing |
| Fuel: spin-on secondary filter, transfer pump, hand priming pump | Fuel: spin-on secondary filter, transfer pump, hand priming pump |
| Governor: full-range (mechanical, electronically assisted) | Governor: full-range, electronically controlled |
| Lifting eyes | Lifting eyes |
| Lubrication: gear-driven pump, front or rear sump, full-flow spin-on filter, left-hand dipstick | Lubrication: gear-driven pump, front or rear sump, full flow spin-on filter, left-hand side oil level gauge (dipstick) |
| Turbocharger | Turbocharger |
| Vibration damper | Vibration damper |
Electronic Features
| Electronics | Cat 3126 (1995-1998) | Cat 3126B (1998-2003) |
|---|---|---|
| Core Electronic Control | Electronic Control Module (ECM) | ECM (updated generation) |
| Diagnostics | Self-diagnostics | Electronic self-diagnostics (expanded) |
| Service Tool Compatibility | Compatible with Cat ET, ECAP, and MPSI Pro-Link | Compatible with Cat ET, ECAP, and MPSI Pro-Link |
| Electronic Data Link | Electronic communications (early) | J1939 compatible - ABS, Allison WT |
| Cold Start / Idle Control | Cold-weather startup strategy & electronic idle control | Cold-weather startup strategy & electronic idle control |
| ECM Data Storage | Stores operational, maintenance & diagnostic data | Stores operational, maintenance & diagnostic data |
| Program. Parameters (General) | Customer-selectable, re-programmable operational parameters | Customer-selectable, re-programmable operational parameters |
| Engine Monitoring System | Engine monitoring system | Engine Monitoring System - off, warning, derate, or shutdown |
| Cruise Control | Cruise control with Cat SoftCruise | Cruise control with exclusive SoftCruise |
| Vehicle Speed Control | Vehicle speed limiting | Vehicle speed limiting & protection |
| Idle Shutdown | Idle shutdown timer | Idle shutdown timer & override |
| Fast Idle | Adjustable low idle | 2-speed fast idle + adjustable low idle rpm |
| Maintenance Monitor | Maintenance monitor | Maintenance monitor (miles/km or hours) + enhanced options |
| Cooling Fan Control | Not listed | Cooling fan control added |
| Password / Security | Password protection | Customer password protection + OEM parameter lockout |
| Exhaust Brake | Exhaust brake control | Exhaust brake operational modes (expanded) |
| Theft Deterrent / OEM Lockout | Not listed | Theft deterrent; OEM parameter lockout |
| PTO Functions (general) | Programmable PTO functions | Programmable PTO functions (expanded / more adjustable) |
| PTO Speed / RPM | Speed; Max rpm; Min rpm | Adjustable speed control (mph/km/h); adjustable max/min rpm |
| PTO Torque Limiting | Torque limiting | Limit engine torque to driven equipment |
| PTO Ramp Rate | RPM ramp rate (rate of acceleration) | Adjustable ramp rate up or down between PTO set speed(s) |
| PTO "Bump" Function | "Bump" rpm control | Adjustable rpm "bump" intervals (expanded logic) |
| PTO Control Location | In-cab or remote PTO operation | Selectable PTO configuration for in-cab or station/remote operation |
For fleets and equipment owners, the practical difference is that 3126 and 3126B engines generally interchange mechanically, but the later engines deliver better electronic functionality, more diagnostic capability, cleaner emissions behavior, and stronger integration with modern vehicle controls. This makes the 3126B more attractive for electronically managed chassis and generator controls, while earlier 3126 engines remain appreciated for straightforward integration and simpler calibration demands.
Together, these two models form the foundation of Caterpillar’s mid-range HEUI era bridging the gap between traditional mechanical diesel engines and the more advanced emissions platforms that followed, culminating in the CAT C7 beginning around 2003–2004.
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