Although the winters in the section of the country where I live are mild enough that I usually have been comfortable driving the M37 without heat, there are those days when I've thought that it would be a great addition to the truck.
The heater that the military installed on the truck for heating the passenger compartment came in two basic versions, a gas fired unit (that burned fuel supplied from the vehicle's main tank) or a hot water unit (used coolant flow from the engine). Both designs placed the heater core and fan on the left front fender and ducted the heat into the passenger compartment through a hole in the cowl (knockout is located in front of the driver's door roughly in line with the top edge of the door). These units are getting scarce (and thus pricey) and tend to clutter up the clean lines of the fender (and obstruct access to the engine and with the Cummins in my truck, most of the service points are on the left side...).
One solution to putting heat in a M37 that has been used by a lot of fellow enthusiasts is to install the hot water heater originally designed for the M151 series of jeeps inside the passenger compartment of the truck. These heaters are relatively compact, and since the military surplussed a lot of the units when the trucks were taken out of service (but not surplussed as complete vehicles), they are relatively inexpensive. A bare heater unit (fan unit with heater core) or complete heater kit (fan unit with heater core, ducting, hoses, switches, and hardware) can be obtained from a variety of surplus dealers who carry jeep parts.
After looking at the installations performed by several other M37 owners, and obtaining installation instructions from two of them, I decided to order a M151 Heater Kit from Sam Winer in Ohio.
Once I received the kit, I took it out of its boxes and looked at everything, but other than that I just let it sit for a while.
After driving to the beach for New Years' 2002, I decided that having heat in the truck would be a really good idea (temps ranging from 25-40F for the three hour trip down, slightly higher for the trip back the following week. The holes in the firewall and floor pan cut during the engine conversion but not yet covered yet did allow some heat to come into the cab, but not enough to be real comfortable.
I strayed from both sets of instructions that I had received in that one set was for tucking the heater under the glovebox without any provisions for ducting of hot air, the other set was for fitting the jeep defroster setup into the truck along with the heater under the glovebox. I decided to mount the heater under the glovebox with the diffuser box mounted to the fan output. My thoughts are that I could install defrost and air distribution at a later date when I had decided exactly how I wanted to tackle that part of the installation.
After attaching the air distribution box to the fan, I played around with placement location for the unit and marked locations for holes that needed to be drilled. I attached the heater to the kick panel on the side of the right passenger footwell using four angle brackets that had been bolted together to form a pair of U-brackets and a piece of steel strap. One screw holding the brackets to the truck is shared with the fire extinguisher bracket. These brackets are attached to the lugs located on either side of the heat exchanger inlet and use a piece of steel strap to bridge across the opening since the alignment of the brackets did not match the lugs. A washered bolt runs down from the glovebox to the lug on top of the heater box between the heat exchanger and the fan.
I then drilled a pair of holes for the hot water hoses in the firewall panel located immediately above the transmission cover. I wanted to avoid running the hoses through this panel an in effort to avoid having to undo the hoses when I wanted to remove the panel, but the grommets I had are designed to go through a single layer of steel, and the locations in the firewall where I could route the hoses and only have to cut one layer where limited. Once I drilled the holes and touched up the edges with a small grinder, I installed the hose grommets that came with the heater kit.
I then placed the heater hose retainer clip on the right front inner fender panel. I had to flatten the clip as it was designed to fit on a rounded panel and the one in the M37 is flat. I also had to drill a hole in the clip and enlarge a hole already in the panel (located above the wiring harness connector retainer clips). This clip serves to keep the heater hoses from flopping about in the engine bay.
The wiring for the heater is pretty basic. A power feed goes from a convenient source such as a circuit breaker to the center of the control switch. One line from the switch goes through a resistor to a y-tap, the other line from the switch goes directly to the y-tap. The third leg of the y-tap is connected to the heater motor. In one position, the switch provides line current to the heater for high speed, in the other position the line current runs through the resistor and is reduced by half for low speed. I fabricated a wiring extension with a metal male end on one end of a piece of wire and a female rubber connector on the other end in order to plug the switch for the heater into a spare circuit breaker in the truck while placing the switch in a convenient location on the dash. I drilled three holes in the defroster knock out panel in the dash, enlarging the center hole with my smallest hole saw and mounted the fan switch. I then mounted the resister used to reduce fan input voltage for the low speed setting on the driver's side of the glovebox. I finished connecting the wiring harness supplied with the heater kit using some wire ties to clean up the installation somewhat.
I then used a pair of 5/8" ID heater hoses (one five feet long, one five and a half feet long) to connect the heater to the engine. I ran the hoses through the grommets installed on the firewall panel and pushed the ends onto the pipe ends sticking out of the heater box and clamped them using clamps from the kit. I then tucked the hoses underneath my air cleaner and routed them through the hose retainer clip I had previously installed. I attached the lower hose to a nipple on the water inlet neck of the Cummins, and the upper hose to the nipple on the water pump neck. I then topped up the coolant.
Pictures of the installation can be found on photo page six.
Future plans include the design and installation of a defroster setup. At this point, I'm leaning to running a small diameter pipe with holes drilled in it across the top of the dash with the air supplied through a hose running up the face of the dash similar to a setup I've seen in another truck. I'm a bit leery of cutting the holes in the cowl needed for installation of the jeep setup, especially since I have stuffed a few extra gauges into the truck that look like they might interfere with air hose routing. The military setup, that seems pretty hard to find, used a sheet metal duct that went across the top front of the dash that was fed through a hole in the dash (where I located my heater control switch) that looks like it obscured the gauges a bit.
M151 Heater Kit (basically used the heater, air distribution box, hose clamps, hose grommets, switch and related wiring parts. I also plan on using some of the air hoses for defrost and distribution of hot air).
Four inch angle brackets (four, purchased at hardware store)
Assorted sheet metal screws, nuts and bolts from the heater kit.
Heater hose (5/8" ID, two pieces six feet long. Pieces in kit too short for my routing).
Flat steel bar (1/8" x 1")
Copies of the two sets of instructions (including photos provided by the authors) may be found on the heater instruction page.