Northern Lights · Generator

Northern Lights M673 / M773 / M843

Northern Lights of Seattle builds the M673, M773 and M843 as small marine diesel generator sets on three-cylinder, naturally aspirated engines that run at 1800 rpm for 60 Hz or 1500 rpm for 50 Hz; the number names the engine block (67, 77 or 84 mm bore), which the maker says it developed together with Ishikawajima-Shibaura. The current Tier III sets are the M673LD3 (5 kW) and M673L3 (6 kW, same engine and brush-type generator end, rated higher), the M773LW3 (9 kW, also as LW3G and as the US EPA version LW3GN) and the M843NW3 (12 kW); a G stands for the Gem base frame that takes the bolt-on sound enclosure. Before them came the Tier II sets M673L2/LD2, M773LW2 and M843NW2 and older ones such as the M643, M673L, M773LW and M843JK/NK, which this page includes wherever Northern Lights’ documents give their numbers. Filters and impellers often carry over between generations while thermostats, glow plugs and service intervals changed, so every part row names the sets it fits.

Manuals & documents

The links go straight to Northern Lights. We host no manuals or drawings — the manufacturer’s copy is the current one.

Wear parts & part numbers

PartOEM numberAlso fitsNote
Raw water pump impeller25-120091,2,3—M673 family with raw water pump 25-12007: M673L2, L3, LD2, LD3 (incl. G) and M673L, and the M643 from the serial number on which 25-12007 replaced its first pumps; set screw included. The earlier M643 pumps 145016820 (impeller 145116180) and 25-18305 (impeller 25-18325) are superseded by 25-12007. The on-board kits list it as an impeller kit with gasket.
Raw water pump impeller25-120604,5,6,2—M773 and M843 families with raw water pump 25-12057: M773LW2, LW3, LW3G, LW3GN and M773LK; M843NW2, NW3, NK and JK.
Engine oil filter24-080017,1,8,2—M673L2, L3, LD2, LD3 (incl. G) and M673L; the 2021 filter chart also gives it for the M643 and M673. The 1997 and 1999 operator’s manuals of the M643/M673 still print 24-02001.
Engine oil filter24-020029,4,5,6,8,10replaces 24-03100—M773L, LW2, LW3, LW3G, LW3GN and, per the filter chart, every M843 (NW2, NW3, JK, NK). The M843NW3 parts manual lists it as the successor of 24-03100, which the M843 operator’s manuals still print. The 2009 M773LW2 manual and parts list print 24-02001 without naming a supersession; the pre-Tier II M773LW used 140516190.
Fuel filter (engine)24-520207,9,10,8—Spin-on secondary filter on the engine; the same on every set from the M643 to the M843NW3.
Fuel pre-filter24-50012 · 24-5000210—24-50012 is the element, 24-50002 the complete filter/water separator shown in the M843NW3 operator’s manual. Northern Lights asks for a Racor-type primary filter in every installation, but the installer usually supplies it, so check which make is on your boat.
Air filter24-280037,1,8—Washable foam element for the M673L2, L3, LD2 and LD3 (incl. G). The filter chart gives 24-28002 for the M673L and 24-28300 for the M643/M673, although their 1997 manual lists no serviceable element.
Air filter24-286024,5,8,11—M773L, LK, LW2, LW3, LW3G and LW3GN. The M773LW3 operator’s manual prints the M673 element 24-28003 by mistake; the pre-Tier II M773LW used 24-22003.
Air filter24-2200910,6,8,2—M843NW2 and NW3 (incl. G). The older M843JK/NK take 24-23100 (operator’s manual) or 24-23102 (parts catalogue).
Alternator V-belt40-020021,12,3,2replaces 080109048replaces 080109035—M673L2, L3, LD2, LD3, M673L and M643. The M673L and M643 parts catalogues show it replacing 080109048, the M643 catalogue also 080109035.
Alternator V-belt40-08601 · 40-020034,5,6,13,2—40-08601: M773LW2, LW3 and LW3GN with the standard 12 V 40 A alternator 185046220 (or the 24 V 22-48603); the isolated-ground alternators 22-48206/22-48604 take 40-08604, medium-frame 24 V alternators 40-08603. 40-02003: M843NW2 and NW3 with the standard 12 V 40 A alternator; the optional alternator kit with DC regulator 22-40226 takes 40-02005.
Pump cover O-ring25-12031 · 25-120671,4,6,2,7—25-12031 for pump 25-12007 (M673, M643), 25-12067 for pump 25-12057 (M773, M843). The manuals advise keeping one on board with the spare impeller.
Water pump repair kit25-12010 · 25-120042,4,6—25-12010 for pump 25-12007 (the kit list names the M643D and M673L; the same pump sits on the M673L2/L3): impeller, gasket, face plate, bearings and seals. 25-12004 for pump 25-12057 (M773L, M773LW3, M843N, M843NW3): impeller, cam, wear plate, end cover, bearing, lip seal, mechanical seal and O-ring.
Raw water pump25-120071,3,2replaces 145016820replaces 25-18305—M673L2, L3, LD2, LD3, M673L and later M643; gear-driven. Mounting O-ring 052400400, drive gear 340460170. Replaces the early M643 pumps 145016820 and 25-18305.
Raw water pump25-120574,6,2—M773LW2, LW3, LW3G, LW3GN and M843NW2, NW3; gear-driven. Mounting O-ring 16-13101, drive gear 340460141.
Thermostat1452063201,12,2replaces 145206062—M673L2, L3, LD2 and LD3; 145206062 is the number in the M673L and M643 parts catalogues.
Thermostat145206022 · 1452062104,6,5,2—145206022 (opens at 82 °C / 180 °F): M773LW2, LW3, LW3G, M773LK and the M843NW2, NW3, JK and NK. 145206210: only the M773LW3GN.
Thermostat gasket11-18602 · 11-131014,6,2—11-18602 on the M773LW2/LW3, 11-13101 on the M843NW2/NW3.
Glow plug185366220 · 185366060 · 1853662501,4,5,6,13,12,3,2—Three per set. 185366220: M673L2, L3, LD2, LD3 and M773LW2, LW3, LW3G, LK. 185366060: every 12 V M843 (NW2, NW3, JK, NK), M673L and M643; 24 V M843: 185366100. 185366250: only the M773LW3GN.
Fuel injector131406480 · 131406470 · 131406560 · 1314063601,4,5,6,2—Three per set. 131406480: M673L2, L3, LD2, LD3. 131406470: M773LW2, LW3, LW3G. 131406560: M773LW3GN. 131406360: M843NW3 (incl. G). Tighten to 43–50 ft-lb (6–7 kgm) on new copper washers.
Exhaust mixing elbow27-380081,2replaces 135616570—Stainless elbow for the M673L2/L3; gasket 135996550.
Exhaust mixing elbow27-320146,4,2—Listed for the M843NW3 (as stainless) and the M773LW3 (there as cast iron, with 27-32003 as the stainless option); gasket 11-31902 on both.
Pressure cap21-120131,4,6replaces 310110220replaces 21-12007—M673L2/L3, M773LW3 and M843NW3; opens at about 0.5 bar (7 psi). The parts manuals give 310110220 as the M673’s former number and 21-12007 as the M843’s.
End cap1455363704,6—Rubber end caps of the heat exchanger (element 20-12002) on the M773LW3 and M843NW3, two per set; they come off for cleaning the tubes.
Valve cover gasket111996470 · 111996420 · 1119964101,4,6,2—Rocker cover gasket: 111996470 on the M673L2/L3, 111996420 on the M773LW2/LW3, 111996410 on the M843NW3.
Gasket set195907370 · 29-08602 · 29-031021,4,6—195907370: M673L2, L3, LD2, LD3. Marine gasket kits 29-08602 for the M773LW3/LW3GN and 29-03102 for the M843NW3.
Onboard spares kit38-08015 · 38-08605 · 38-031202,7,9,10—Standard kits for heat-exchanger sets, Tier II and III alike: 38-08015 for the M673L2/L3/LD2/LD3, 38-08605 for the M773LW2/LW3, 38-03120 for the M843NW2/NW3. Keel-cooled sets take 38-08016, 38-08606 and 38-03130, without raw water pump parts.
Onboard spares kit38-08019 · 38-08612 · 38-031262—World Class kits for the Tier III sets: 38-08019 (M673L3/LD3), 38-08612 (M773LW3), 38-03126 (M843NW3); keel-cooled 38-08020, 38-08613 and 38-03136. Tier II: 38-08017 (M673L2/LD2), 38-08607 (M773LW2), 38-03125 (M843NW2). The kit list shows four-cylinder parts as the contents of 38-08612 and 38-03126, so confirm what is inside with your dealer.
Voltage regulator22-680061,14—AC voltage regulator for the TF-276D generator end of the M673L2/L3; its 3 A protector is 22-68004. Older M673 sets with the DS-71-2 regulator are converted to the DST-61 with kit 22-68000 (regulator with integral fuse) or 22-68035 (regulator with copper slug).
Voltage regulator22-4207115,9—DST-100-2FAK for the brushless PX-309K2 (M773LW3) and PX-312K2 (M843NW3) generator ends; its circuit breaker is 22-42084, which replaces 22-42077.
Generator brush assembly22-683041,2,7—Brush assembly of the TF-276D generator end on the M673L2, L3, LD2 and LD3 (single brush GM405900); the M673 manual expects a new assembly within about 3000 h. The PX generator ends of the M773 and M843 are brushless.
Generator bearing6306ZZ · 6305ZZ15,1—Generator end bearing (sealed ball bearing): 6306ZZ in the PX-309K2/PX-312K2 of the M773 and M843, 6305ZZ in the TF-276D of the M673.
Circuit breaker22-68005 · 22-6801016replaces 22-68007—The 30 A two-pole AC output breaker 22-68007 of the M673 is no longer available. Kit 22-68005 replaces it on older M673 with a snap-in DC breaker, kit 22-68010 on newer M673 with a screw-mounted DC breaker (new junction box side plate included).

Numbers checked against the sources below on 12 October 2026. Always confirm against your serial number and exact type before ordering.

Service intervals

TaskInterval
Oil level checkDaily before starting: oil and coolant level, primary fuel filter bowl and sea strainer7,9,10
First serviceFirst 100 h with diesel break-in oil and at least 50 % load (75 % is better); change oil and filter after 50 h and again at 100 h7,9,10
Engine oil changeOil and filter every 200 h or 6 months; every 100 h or 6 weeks in intermittent cold-weather use7,9,10
V-belt checkCheck the tension daily by thumb and every 100 h; full visual inspection at least once a year7,9,10
Air filterCheck and wash every 200 h; new element every 1000 h or 2 years7,9,10
Fuel pre-filterElement every 500 h or as needed; at once, together with the secondary filter, if the bowl fills with water7,9,10
Fuel filterSecondary filter every 500 h or as needed7,9,10
Raw water impellerEvery 500 h or as needed; keep a spare impeller and cover gasket on board7,9,10
Exhaust elbow checkInspect every 500 h; replace if rusted or damaged7,9,10
Siphon breakSiphon break (if fitted): service and clean every 6 months or at every oil change, whichever comes first7,9,10
Valve clearanceAfter the first 50 h, then every 1000 h7,9,10
InjectorsCheck every 1000 h (dealer job); clean the nozzles every 1500 h; for the EPA certification nozzles and injection pump are serviced every 3000 h7,9,10
Fuel pumpInjection pump: check settings and smoke every 2500 h7,9,10
Cooling systemFlush the system and renew the coolant every 2500 h or 24 months7,9,10
Heat exchanger cleanClean the core every 2500 h or 24 months, or when needed7,9,10
BatteryElectrolyte level and terminals every 100 h; battery condition every 200 h in the chart, with a hydrometer every 500 h in the text7,9,10
Full serviceEverything on the chart at least every two years, even if the hours are not reached7,9,10

From the servicing charts of Northern Lights’ Tier III operator’s manuals OM673L3 (8/24), OM773LW3 (1/26) and OM843NW3 (1/25), which give the same schedule; hours or time, whichever comes first. The Tier II manuals (M673L2, M773LW2, M843NW2) and older ones differ — oil every 250 h and impeller every 1000 h on Tier II — so older sets follow their own manual.

Specifications

OutputM673LD3(G): 5 kW at 60 Hz / 4.5 kW at 50 Hz; M673L3(G): 6 / 5 kW; M773LW3, LW3G, LW3GN: 9 / 7 kW; M843NW3(G): 12 / 10 kW; single-phase at PF 1.0, three-phase (PF 0.8) optional on the M773 and M84317,18,19,20,21,22,23,24
VoltageM673LD3: 240 V / 20.8 A or 120 V / 41.6 A at 60 Hz, 220 V / 20.5 A at 50 Hz; M673L3: 240 V / 25 A or 120 V / 50 A, 220 V / 22.7 A; M773LW3: 120/240 V / 37.5 A or 120 V / 75 A, 220 V / 31.8 A; M843NW3: 240 V / 50 A or 120 V / 100 A, 220 V / 45.4 A. Voltage regulation ±5 % on the M673 (TF-276D generator end with brushes), ±1 % on the M773 and M843 (brushless PX-309K2 / PX-312K2)21,22,23,24,7,15
Rated speed1800 rpm (60 Hz) or 1500 rpm (50 Hz); four-pole generator coupled directly to the engine; inline injection pump with 3–5 % mechanical governor21,25,24
Cylinders3 in-line, four-stroke, naturally aspirated, overhead valves, swirl chambers and glow plugs21,23,24
DisplacementM673L3/LD3: 0.761 l (67 × 72 mm); M773LW3: 1.13 l (77 × 81 mm); M843NW3: 1.5 l (84 × 90 mm); older sets: M643 0.617 l (64 × 64 mm), M673/M673D/K/M 0.676 l (67 × 64 mm), M843JK 1.33 l (84 × 80 mm)7,9,10,26,27,28
Compression ratioM673L3/LD3: 23.5:1; M773LW3: 23:1; M843NW3: 22:17,9,10
EngineM673: 10.1 hp at 1800 rpm / 8.4 hp at 1500 rpm; M773: 15 / 12 hp; M843: 20 / 17 hp (the M843NW3 operator’s manual gives 16.6 hp rated flywheel power)21,23,24,10
Engine oil capacityWith filter: M673L3/LD3 3.0 l; M773LW3 4.9 l on the oil-change page of its operator’s manual, 4.0 l in the data table of the same manual; M843NW3 5.5 l. Fill to the dipstick mark7,9,10
Engine oil gradeDiesel engine oil of the current API service category, multigrade preferred: SAE 15W-40 above 0 °C, 10W-30 from −23 to 0 °C, 5W-20 below −23 °C; diesel break-in oil for the first 100 h; no additives, no flushing oil7
Coolant capacityM673L3/LD3: about 2 l; M773LW3: 3.9 l; M843NW3: 4.1 l7,9,10
Coolant50 % distilled water and 50 % low-silicate ethylene glycol; no methanol-based antifreeze; pressure cap opens at about 0.5 bar (7 psi); 75–90 °C in operation7
Oil pressure20–60 psi (about 1.4–4.1 bar) in operation; must stay above 15 psi (about 1 bar)7
Valve clearance (cold)0.2 mm (0.008 in) intake and exhaust on the M673L3, M773LW3 and M843NW3; rocker cover nuts 5–8 ft-lb7,9,10
Belt tensionAbout 5 mm (3/16 in) deflection under thumb pressure midway between crankshaft and alternator pulleys7
FuelDiesel to ASTM D975 No. 2 (No. 1 below 0 °C), low or ultra-low sulphur, cetane number at least 45; no heating oil7
Fuel consumptionFull / half load, 60 Hz then 50 Hz: M673L3/LD3 2.2 / 1.2 l/h and 1.9 / 1.1 l/h; M773LW3 3.52 / 1.93 l/h and 2.72 / 1.40 l/h; M843NW3 4.50 / 2.30 l/h and 3.78 / 1.93 l/h21,22,23,24
CoolingHeat exchanger with copper-nickel tubes and rubber end caps, no zincs; keel cooling optional; gear-driven raw water pump; raw water inlet 3/4 in (19 mm); wet exhaust elbow 1.5 in (38 mm) OD on the M673, 2 in (51 mm) on the M773 and M843; fuel inlet and return 1/4 NPT21,23,24,20
Battery12 V negative ground as standard (24 V optional); M773LW3 at least 100 Ah / 640 CCA, M843NW3 at least 120 Ah / 560 CCA9,10
EmissionsM673LD3G, M673L3G, M773LW3GN, M843NW3G: US EPA Tier III at 60 Hz and IMO Tier II at 50 Hz; M773LW3G: IMO Tier II, sold for export and non-US-flagged vessels only21,22,25,23,24,29
Dry weightHeat-exchanger sets: M673L3G/LD3G 168 kg (spec sheet; 171 kg in the manual); M773LW3G/GN 244 kg; M843NW3G 339 kg per the operator’s manual (the spec sheet says 736 lb); the Gem sound enclosure adds 15.4 / 19.5 / 24 kg21,7,23,24,10
DimensionsL × W × H, open set / in the Gem enclosure: M673L3G/LD3G 698 × 426 × 521 mm / 724 × 498 × 532 mm; M773LW3GN 831 × 442 × 603 mm / 888 × 559 × 617 mm; M843NW3G 978 × 483 × 643 mm / 986 × 574 × 655 mm21,23,24,30

Good to know

  • Don’t keep cranking a set that has a waterlift muffler: after three 10-second attempts without a start, close the seacock and take the impeller out, so the muffler cannot fill and push water back into the engine. Once it starts, stop it straight away, refit the impeller, open the seacock and start again.7
  • If the point where cooling water enters the exhaust sits less than about 30 cm above the loaded waterline, the set needs a vented loop with a siphon break, and that valve needs cleaning every six months or at each oil change.31,7
  • Intervals changed between generations: the Tier III manuals ask for oil every 200 h or 6 months and a new impeller every 500 h, while the Tier II manuals (M673L2, M773LW2, M843NW2) said 250 h and 1000 h. Follow the manual for your exact set.7,32,11,33
  • There are no zinc anodes to change: the copper-nickel heat exchanger is built without them. Instead, take the core out every 2500 h or two years and clean the tubes; the rubber end caps make that easy.20,10
  • Check part numbers against the parts manual for your set: the M773LW3 operator’s manual prints the M673 air filter 24-28003 instead of 24-28602, the M843 manuals still show oil filter 24-03100, now 24-02002, and the M773LW3GN has its own thermostat, glow plugs and injectors.6,4,5,9
  • If the set shuts down on high coolant temperature, look at the sea strainer first and then the impeller. When a failed impeller comes out, find every missing piece before you close the pump.10

Videos & how-tos

Links to YouTube, nothing embedded. The maker’s own channel comes first; the rest are channels we found clear and correct for this model.

Fitted in

Boats whose builder lists it as standard or optional equipment:

  • Leopard 42 — Northern Lights 6 kW (2nd generation: charter upgrade (Moorings list July 2020, Sunsail 424 list 2025; the 2025 Moorings lists say only “Genset”))

On the marketplace

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Sources

  1. northern-lights.com/wp-content/uploads/2020/03/P673-2.pdf
  2. northern-lights.com/wp-content/uploads/2020/09/L510-Spares-Kiit-FINAL.pdf
  3. northern-lights.com/media/PDFs/manual_pdfs/P643.pdf
  4. northern-lights.com/wp-content/uploads/2020/04/P773-3.pdf
  5. northern-lights.com/wp-content/uploads/2026/07/P773N-3.pdf
  6. northern-lights.com/wp-content/uploads/2026/07/P843-3.pdf
  7. northern-lights.com/wp-content/uploads/2024/09/OM673L3.pdf
  8. northern-lights.com/wp-content/uploads/2021/06/L653-Filter-Chart.pdf
  9. northern-lights.com/wp-content/uploads/2026/01/OM773LW3.pdf
  10. northern-lights.com/wp-content/uploads/2025/02/OM843NW3.pdf
  11. northern-lights.com/media/PDFs/manual_pdfs/OM773LW2.pdf
  12. northern-lights.com/media/PDFs/manual_pdfs/P673L.pdf
  13. northern-lights.com/media/PDFs/manual_pdfs/P843-2.pdf
  14. northern-lights.com/media/PDFs/misc_pdfs/L972_Instructions_replacement_DS-71-2_voltage_re…
  15. northern-lights.com/media/PDFs/manual_pdfs/OPX300K2.pdf
  16. northern-lights.com/media/PDFs/misc_pdfs/L971_Instructions_for_replacing_22-68007_circuit…
  17. northern-lights.com/technicalinformation/
  18. northern-lights.com/m673ld3g/
  19. northern-lights.com/m773lw3gn-9-7-kw/
  20. northern-lights.com/m843nw3g/
  21. northern-lights.com/wp-content/uploads/2024/09/B182-M673LD3G.pdf
  22. northern-lights.com/wp-content/uploads/2024/09/B183-M673L3G.pdf
  23. northern-lights.com/wp-content/uploads/2025/11/S202-M773LW3GN.pdf
  24. northern-lights.com/wp-content/uploads/2024/09/B185-M843NW3G.pdf
  25. northern-lights.com/wp-content/uploads/2024/09/B184-M773LW3G.pdf
  26. northern-lights.com/media/PDFs/manual_pdfs/O101.pdf
  27. northern-lights.com/media/PDFs/manual_pdfs/OM673.pdf
  28. northern-lights.com/media/PDFs/manual_pdfs/O843K.pdf
  29. northern-lights.com/m773lw3g/
  30. northern-lights.com/wp-content/uploads/2021/04/L768-5-40-kW-Gem-Series-Sound-Enclosure-Ap…
  31. northern-lights.com/wp-content/uploads/2025/07/L192-Dont-Drown-Me_2025.pdf
  32. northern-lights.com/media/PDFs/manual_pdfs/OM843NW2.pdf
  33. northern-lights.com/media/PDFs/manual_pdfs/OM673L2.pdf

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