HMMWV Upgrades: How to make a DIY HMMWV Grounding Harness
Do you need a Humvee grounding harness?
Good question. In theory, the body wiring harness of the HMMWV should provide sufficient electrical grounding. However, in practice, many experienced HMMWV enthusiasts swear by the benefits of adding an additional grounding harness.
Common issues that can be caused by poor grounding:
- Blinking “wait” light
- Failure of start control box (S3 Box, EESS)
- Gauges that work intermittently, show erroneous or variable readings
- Other unexplained “gremlins”
Anecdotal account posted by Steve on a Humvee site:
“I have had trucks that would do all of the issues you guys post about and were fixed with just this harness. I have eaten my fair share of expensive control boxes as well. Bad boxes will eat glow plugs.”
What does a HMMWV grounding harness do?
AM General used an aluminum body on the HMMWV to save weight. However, aluminum doesn’t provide a good electrical ground. So, AM General build a grounding harness into the factory wiring harness. This was adequate when new, but most Humvees that find their way into civilian hands are at least a decade or two old. Over time corrosion, physical abuse, ham fisted mechanics and electrolysis can cause failures or degradation in the factory grounding wires. Poor electrical ground is really tough on electrical components, especially the expensive start control boxes, as well as parts of the charging system. This easy DIY HMMWV grounding harness provides a strong ground to all of the critical components on the Humvee.
How to make a Humvee grounding harness
Supply list with Amazon links:
- 8 gauge AWG flexible wire – need about 14 feet, but it comes in 25 foot rolls. I had a mix of black and red on hand, but you should use black. And I’ve heard of people using 10 gauge wire. I recommend using at least 8 gauge, if not larger… like this 6 gauge wire.
- Ring terminals – this link is for four, which is the number needed
- Large 2/0 solderless connector that has a 3/8 lug to mount wires to the engine ground point – need one, but this has two.
- Terminal crimper
Use a large gauge flexible wire for this harness. I have read that 8 gauge wire from a car stereo shop is good because is made of very fine stranded wire that holds up well where flexing is necessary. You might also consider replacing the original firewall grounding point with a 1″ stainless threaded rod , as the original firewall grounding point had a tendency to corrode easily. Attach the wires using star washers to guarantee good contact.
Start by cleaning off every ground connection on the truck:
- all the lights
- all the gauges
Locate the engine ground
Remove the dog house cover from between the front seats, exposing the rear of the engine. The factory engine ground termination point is on the aft vertical face of the left side of the engine. This, or the empty bolt hole above it, are where each of your wires will terminate.
Make as you go
I recommend that you start with your wire on the spool and pull each section of wire from the item to be grounded to the engine grounding point, then cut the wire. That way your wires are the exact correct length. It is also easier to fish the wires through the various zip ties, clamps and conduits along the way without the connectors crimped to the ends.
Here are the points to be grounded:
- Generator ground
- Start control box – One of the bolts that mounts the start control box (PCB, EESS, S3)
- Dash ground – Stud on the firewall that feeds all the instrument gauges. You have to take the instrument cluster out to get to it. The stud passes through to the engine area near the fuel filter, one of the body harness grounds terminates there.
- Starter ground
Or make in advance
However, I’ve seen various folks list the following measurements for the grounding harness wire lengths. If you prefer to assemble them all ahead of time, then be my guest.
It is a 5 point harness:
- point 1- left rear cylinder head ground terminal
- point 2 label A – 25″ TO DASH/FIREWALL
- point 3 label B – 33″ TO GENERATOR GROUND
- Point 4 label C – 35″ TO CONTROL BOX MOUNTING BOLT
- Point 5 label D – 73″ TO STARTER GROUND
More HMMWV articles can be found under the Humvee category menu above or by clicking HERE. You might also like the HMMWV section of our Amazon store where we have listed lots of replacement and upgrade parts for Humvees. We have also found a lot of HMMWV parts on eBay (link).
Please leave a comment if you have suggestions on how to better install LED tail lights in a HMMWV.
The Project Humvee Battlewagon
Gear Report acquired a real HMMWV (High Mobility Multi Wheeled Vehicle) from the US Army for use as our official field and shooting range vehicle. We have already posted about some of our upgrade projects (Project Humvee Battlewagon article links) and have a lot of really cool projects on the way. Consider subscribing so you don’t miss any of the cool reviews as we post them.
Here is our Project Intro (link) and our list of DIY HMMWV upgrade projects.