M998 HMMWV: Modifications
Updated January 25, 2004
Updated January 25, 2004
This page describes the modifications I have made to my truck. In general, I like to keep my truck in a correct military configuration. However, some modifications were necessary to make it street-legal, for safety, and for convenience. Wherever possible, I made these modifications completely reversible, by using existing bolt-holes for mounting, and by making wiring adapters which tap in to the wiring harness without cutting any wires. Where I added wires, I used the correct connectors, obtained from Jeff Smith, and I used silicone-rubber motor lead (SRML) wire, which is very similar to the wire used in the HMMWV's wiring harness. I sealed all crimped connections with adhesive-lined heat-shrink tubing, and covered wires with braided polyethylene tubing where possible for abrasion resistance. The Thin-walled adhesive-lined heat-shrink tubing and braided polyethylene tubing are available from Mouser Electronics, and many types of heat-shrink tubing, with and without adhesive lining and in various wall thicknesses, are available from McMaster-Carr.
I started with a heavy-duty license plate light from NAPA Auto Parts (part number 9366W). It came with a 12V GE168 wedge-base bulb, which I replaced with a 24V GE464 bulb. I replaced the wire with a length of SRML wire and a standard male 1-pin rubber connector, and made a waterproof splice between my new wire and the old one inside the light housing using a crimped butt splice and adhesive-lined heat-shrink tubing. I drilled a couple of tiny seep holes in the lens, because I didn't feel that the housing gasket would keep water out forever. To connect the lamp to the electrical system, I used a "Y" adapter from Front Line Military Vehicles to tap in to the left rear marker light circuit. A separate ground wire connects the light housing to the grounded marker light bolt, because the bonded and painted truck body may not provide a reliable ground.
I fabricated mounting brackets out of some thick aluminum plate that I bought at a hobby shop. One bracket attaches to the top of the light housing at one end, and at the other end, it cantilevers out from the bolts which attach the U-shaped plate which protects the left rear tail-light connectors. The other bracket attaches to the rear of the light housing, and holds the license plate. I backed up the license plate with a thinner piece of aluminum. I painted all of the aluminum parts flat black. All of the hardware is either 1/4" grade 8 hardware, or #10 stainless steel hardware, and the nuts are all locknuts.
This modification uses existing bolt holes and doesn't require cutting the wiring harness, so it is completely reversible. People who display their truck in parades and shows may wish to use a different approach that is easier to remove for display, but I plan to use my truck as a semi-daily driver, and opted for a more secure mounting.
I did most of the metal work with a drill press and a metal-cutting bandsaw. It should be possible to fabricate a bracket like this one with hand tools, too, with a bit more effort. If you don't have access to the correct connectors and crimper, you can cut off a wire lead and connector from a dead military headlamp. You'll still need some sort of "Y" adapter if you don't want to cut into your wiring harness.
I may make a bracket for the front license plate someday, but for now it's attached to the front bumper with cable ties.
Most military HMMWVs do not have back-up lights, but the ambulance versions do. The two-litter soft-top ambulance, which has a soft rear cover similar to the one on my truck, has a pair of back-up lights mounted on brackets under the tailgate. Hard-top ambulances mount the lights in cutouts in the ambulance body. The ambulances use different wiring harnesses that have the extra wires for the backup lights and switch. Luckily, the shift lever housing in the basic trucks includes a threaded hole for mounting the backup light switch used in the ambulances.
I ordered the lights and brackets used in the two-litter soft-top HMMWV ambulance, as well as the backup light switch (not cheap!) from Surplus Enterprizes, and guessed where to mount them based on a scanned picture of one of the ambulances which another collector sent to me. The mounting holes for the brackets were not already present, so I needed to drill some holes in the horizontal lip at the lower rear edge of my truck's body.
The rubber boot around the shift lever housing should have a nipple around the neutral start switch, and another sealed nipple where the backup light switch goes. It will be necessary to remove the shift lever housing in order to get the boot off and install the switch. Be very careful, because the boot will probably be firmly stuck to the body, and it's thin and tears easily. I tore mine and needed to replace it, and it is not cheap! It's a big pain in the butt to remove and install the shift lever housing because of the contortions necessary to reach the lock nuts under the body through the engine access cover. I decided to ream the mounting holes in the body out and install 1/4" rivnuts, so it will be a lot easier to remove the housing in the future. You may need to cut off the end of the nipple for the reverse switch, but my boot was the ambulance version, with the nipple end already cut off.
I had to make a separate wiring harness for the backup light switch. I did not want to cut into my wiring harness, and I wanted the blackout lighting system to continue to function properly. In ambulances, the backup lights derive power from the same electrical net that powers the horn (since the horn is shut off in blackout mode in a HMMWV), so I simply tapped my backup light power from the horn at the front of the truck. I used SRML wire, and covered it with braided polyethylene tubing. This time, instead of using a pre-made "Y" adapter, I built my own Y adapter into the end of the front wire. I spliced the two short leads with connectors to the longer lead with a special butt splice which is made for connecting a large wire to a small wire, or two wires to one wire of the same gauge. I got it from McMaster-Carr. I covered the splice with adhesive-lined heat-shrink tubing. I used the same technique to make the "Y" where the two backup light leads and the long wire from the switch join together. Each backup light has its own ground wire connected to the grounded lug on the nearest marker light. I routed the long wires along the existing wiring harness (that wasn't easy!), and secured it to the existing harness with black UV-resistant cable ties.
The military HMMWV does not have a glove box, so there's no convenient place to put small items which you might want to reach from the driver's seat, but which you don't want rolling or blowing around in the truck, like registration and insurance cards, a flashlight, a cell phone, sunglasses, etc. Furthermore, there's no way to lock up the interior, which makes the truck pretty inconvenient as a daily-driver.
For a glove box, I simply drilled two holes in the bottom of a .50 caliber ammo can, and bolted it to the floor of the truck using the two existing rivnuts where a fire extinguisher would go. I plan to mount a fire extinguisher in the alternate location under the front of the driver's seat. I may modify the box later to make it lockable.
For a lockable trunk, I bought a Knaack Model 32 jobsite storage chest. I chose this chest because I liked the way it was made, and Knaack has good dimensional drawings on their website, including the height of the box with its lid open. That was a critical dimension when determining what box might fit in my truck. The Model 32's lid handle lightly scrapes along the inside of the cargo cover when I open it, just because the cover sags down a bit, but I don't think it'll scuff the cover. I drilled small holes in the feet of the chest, and then drilled larger holes inside the floor of the chest so that I would be able to get at the smaller holes from inside the chest with a wrench. I bolted the chest to a piece of 3/4" plywood (painted 383 green, of course!) with bolts and tee nuts, and then bolted the plywood to the floor of the truck using the existing tiedown ring mounts. I covered the large holes inside the chest with plates of thin aluminum, glued in place with silicone, to keep small stuff from getting trapped in the chest's feet. I suppose somebody could unbolt the plywood and walk off with the chest, but it would take a while to do that, and I plan to keep that chest nice and heavy! Eventually, I plan to paint the chest 383 green, but I was in a hurry to get it installed in time for my trip to the 2000 MRCG Meeting.
I mounted the chest far enough forward to allow a spare tire to fit between it and the tailgate. Someday, I might make spare tire mounting bracket that I can attach to the chest.
I considered many methods for securing my truck from theft. The military HMMWV doesn't have an ignition key... it just has a stop/run/start switch, and the only built-in security device is a cable under the dash which can be wrapped around the steering wheel and padlocked. I spent a long time thinking of different electrical ways to disable my truck, but decided that any of them could be easily defeated by anybody with basic knowledge of how a car works. I finally decided to use a Pit Bull Tire Lock, which works on the same principle as the famous Denver Boot, but is a bit more convenient to carry around and use. It was necessary to modify the lock a bit so it would fit around those big HMMWV tires. When I carry a spare tire, I'll make sure it's locked up so that somebody can't simply replace my rear wheel with the unlocked spare. There's an attachment for the tire lock that covers the lug nuts, and I might add one of those on sometime.
Of course, I have other methods for securing my truck, but I won't
share those... 
Now, I'm converting my truck from an M998 cargo truck to an M1037 communications shelter carrier. It will carry this S-250/G shelter, containing an AN/GRC-122B radio teletype (RTTY) set.
The hardest part of the conversion consists of replacing the rear suspension with a heavy-duty version. The lower control arms, lower ball joints, shock absorbers, springs and spring seats all need to be replaced. The new springs are one turn longer than the original ones. The new spring seats are castings, rather than being built up from sheet metal, and their seating surface is higher to make room for the longer springs. The pictures below show the old suspension on the left side, the new suspension on the right side, and the old and new spring seats side-by-side.
I also need to install a frame in the bed of the truck which will hold the shelter. The tailgate is removed and reinstalled on the frame. The cargo bulkhead and the rear seatbacks (which also cover the footwells in the rear in most 2-door configurations) are removed. The frame mounts using a bunch of existing holes, including some of the tiedown ring bolts and those extra holes in the back, as well as several new holes which must be drilled. The tailgate chains and tailgate latches are also replaced. The pictures below were taken before reinstalling the tailgate, replacing the chains and latches, and drilling the new holes.
The shelter carriers should have the heavy rear "airlift" bumper. I recently found one on Ebay, but I still need to get mounting brackets for it. I guess I'll need to relocate the license plate frame and backup lights when I install that.
There's supposed to be a pair of panels which close off the rear footwells from the front seats in this configuration. I don't have them yet, but I have located a set.
Here are some pictures with the shelter mounted on the truck. I still need to install the footwell plates and rear bumper, and realign the suspension. I've removed the radios to fix them. I'll take pictures of the shelter interior after the radios are reinstalled.
