Military Vehicles: M-37 Heater

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Installing an M151 Heater into an M37

These instructions describe how to install a heater unit from the M-151 series jeep into an M-37 3/4 ton truck.

If you have any questions, please email me.

Good luck,
Harris Neil

Parts List:

  1. 1, Heater, 24V, (Hunter Mfg. Co., Model #HW-20-2, Stock # NSN 2540-00-869-0424; originally designed for M151
  2. 1, " NPT brass nipple, 3 inches long, or, if you can find them, 2, 1/2" NPT to 5/8" I.D. hose brass fittings
  3. 2, rubber grommets, 1 1/16" I.D.
  4. 9 feet, 5/8" inside diameter heater hose
  5. 1,Heater shutoff valve, with outlets for 5/8" hose
  6. 1, push/pull type valve control cable
  7. 6, hose clamps (7/8" to 1 1/8" size)
  8. 2, " x 6" metal straps
  9. 7, " x " pan head machine screws, with nuts and lockwashers
  10. 10, " fender washers
  11. 4, " flat washers
  12. 1, toggle switch
  13. 1, "T" tap (to tap into your electrical system)
  14. 4 feet, 10 gauge stranded electrical wire
  15. 1, butt splice (a 10 gauge butt splice will plug onto the Packard connection that comes on the heater)
  16. 4, cable ties


Prior to installation, check your glove box. If it shows any sign of rust or weakness, you may want to reinforce it by running a metal strap from the exterior front side (facing firewall) of the box, to the bottom of one of the bolts that secure the passenger side hood hinge. I did not do this, and have had no problems.

Installation Instructions:

Take the 3" brass nipple, 1/2" NPT (National Pipe thread), and cut it in half.

Remove two pipe plugs; one from the top, passenger side rear of the engine head (the word "heater" is cast into the head adjacent to it) The other from your lower radiator pipe. If the lower radiator pipe has no fitting (as mine did not) use the fitting on the top of the elbow atop your water pump. If you have changed the water pump, and the elbow is no longer there, you will have to remove the lower radiator pipe, drill into it, thread the half nipple into it, then solder it. This works, as it is what I had to do.

Get a 1 and 5/16" hole saw. Drill two holes in the fire wall above, and slightly to the passenger side of the rear hot water outlet, where you put the nipple. You will have to drill from the engine side to the passenger compartment. Triple check your location, to avoid hitting wiring, speedometer cable, or vacuum line.

Get two grommets (PCV valve grommets are what work) that have the diameter to let 5/8" I.D. heater hose pass through. Install these in the holes you just drilled.

Work 5/8" heater hose onto the two nipples. It will be very difficult, but it will stretch enough. Use WD40 or petroleum jelly to ease it along. Start with a 6' piece from the lower radiator hose, and a 3' piece on the rear. You will trim to fit when the time comes.

Get the heater shutoff control, and the push/pull control cable. Install the valve into the line exiting the rear of the engine. It should have only enough hose between the top of the engine nipple, and the valve to allow correct fitment of the clamps. The control cable can be trimmed to length by removing the inner wire, clipping off the sheath, and then re-inserting the inner wire, and clipping it, so approx. 6" extends. install the knob end in the location of your choice, on the dash. The cable can be fed to the engine compartment through the same bracket the choke and hand throttle cables come through, since there should be an extra space from when the manual PCV control was there. Then attach the end of the control cable to the valve. Attach the remainder of the output heater hose to the other side of the valve, then feed both hoses through the grommets in the firewall. I routed the return hose (the one hooked to the lower radiator pipe) to the passenger side of the engine compartment, then around the rear of the air cleaner. I secured it to the hood support bracket rod with cable ties. (Just to the rod, not the support bracket itself of course).

Fabricate two brackets out of the metal straps. These should be 3/4" to 1 1/4" wide, by 6" long. Bend them 90 degrees, at the three inch point. Drill one hole in one leg, and attach (using panhead screws, with a flat washer on either side of the heater brackets, then a lockwasher, and nut) one strap to each of the holes in the radiator grill end of the heater, so that they go up (towards the top of the heater, as determined by the heater data plate). The other end should point toward the motor end of the heater. This leg of the bend should be parallel with the top of the heater. Drill two holes in the other end of the straps, at least 1 and 1/2" from each other.

With the straps installed on the heater, turn the heater upside down, on a sheet of standard office paper (8.5 x 11) place the hole in the bracket, near the motor end 1 inch from an 8.5" end of the sheet, centered between the 11" sides. Align the heater with the sheet (make it square). Draw marks at each of the holes in the bracket, and straps. Take the paper, and tape it to the bottom of your glove box, so the edge with the mark of the motor end bracket is flush with the driver's side edge of the bottom of the glove box. The mark should be center between the front and rear of the box. Use a punch to locate your holes. Remove the paper, and drill the 5 holes, each 1/4".

Use 5 pan head machine screws 3/4 x 1/4" place a fender washer on each, and drop them through from the inside of the glove box. Have someone hold your heater in place, then install a fender washer, lock washer, and nut on each of the five screws. Hook your output (from engine through valve) hose to the input line (the longer brass pipe) of the heater, after trimming to length. Trim and hook the return hose to the other line.

Use good quality hose clamps at all connections, even those very tight ones at the nipples.

Use the toggle switch, tapped into the line from your main "On/Off" switch, to control the blower motor. Since my truck had a plug where the PCV manual shutoff used to be that is where I put the toggle switch. With a little hunting, you can find one that looks very mil spec.

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