The Hummer Knowledge Base
Grounding as a Source of Elecrical Problems:
> I am having some problems.  The dome lights would come on while
> driving and turn off all by themselves.  They would stay on after turning
> the key off and I would have to turn it back on to get the lights to go off
> after parking.  Hope we both learn something from the group.
On the subject of electrical craziness....
I had a spate of weird electrical problems when I first bought my truck. Although the symptoms were different from those described, they were similar in that they seemed to effect a bunch of systems that one would not normally consider "related". And, sometimes the gremlins would be present, sometimes not, but when present, they were usually the same set of "gremlins".
I spent quite a bit of time discussing my woes with a couple of Hummer techs, and did get my problem solved. What I found out was this:
- Most vehicles use "chassis ground" or ground everything to the body/floor pan. Since the chassis and/or body are usually made of steel, and steel is a good conductor, grounds can usually be made securely and easily. The Hummer has an aluminum body. By itself, aluminum is a decent conductor, but is prone to corrosion, and the corrosion layer is non-conducting. The problem is exacerbated when dissimilar metals (such as aluminum and steel) are brought into contact. Because of it's aluminum body, and steel chassis, the Hummer does not use chassis grounding, but uses "hard-wired" grounds. This means virtually every switch and circuit actually has a grounding or ground-side wire associated with it.
- These grounds are usually brought together at a number of main "grounding points" or posts. If one of these ground wires comes loose, an insufficient ground exists, which means that, when energized, certain circuits can actually "feed back" voltage/current through their "hot" or "positive" side to other circuits, or thru the ground side which is now isolated and floating. This can cause "ghost or gremlin" problems, where certain circuits are energized even though the switches or actuators normally used to "close" them are "open".
- The most common problem area is the hood, which is totally non-conductive, and thus, truly must rely on hard-wired grounds. The hood has the headlamp circuit, turn signals, and side marker lights. Because the hood is non-conductive, it has two main grounding points, which are located next to each headlight. They are behind the "Composite Light Cover(s), which are the black, steel access covers or plates to the side of each leadlight (accessible with the hood open, one has three bolts, the other has four). If you remove these covers, you'll find grounding posts, with all of the right-hand or left-hand grounds stacked together on their respective sides. These need to be secure. If these, or main ground for the hood comes loose (I think the main harness for the hood terminates back somewhere near the CTI pump), you can get all sorts of weird problems.
- To see if this is the problem, do the following the next time your "electrical craziness" problems manifest themselves. Open the hood, remove one or both of the access plates noted above, and make sure all of the grounds are secure. If this doesn't solve the problem, run a wire from one of the grounding posts inside the cover directly to the negative battery terminal. I actually used my jumper cable to do this, as it has connectors on each end, making this pretty easy, but even a simple length of wire, with the ends stripped, can serve. MAKE VERY SURE THAT YOU ARE ONLY CONNECTING THE GROUNDS - OTHERWISE, YOU'VE GOT THE FOURTH OF JULY AND CAN DAMAGE THE HARNESS! Doing this provides a known, good ground for the hood - if the problems go away, at least you've isolated it to the hood ground, and can either trouble-shoot further, or, simply add connectors to a length of 12 ga. wire, and install a permanent "jumper wire" in case the stock grounds ever come loose again.
- To show this can work, my symptoms were - truck starts fine, when I switched on the PARKING LIGHTS , both turn indicators on the dash would illuminate, the turn signals themselves would not work, and when switched to the HEADLIGHTS position, I wouldn't get any headlights at all. With a jumper wire installed, everything worked just fine. The first loose connection found was behind the fuse panel in the cabin, (just above the driver foot well). When it happened again, a loose connection was found near the CTIS pump. With both of these grounds tightened, I've since been able to remove the grounding jumper that I installed, and everything works fine. But always carry one with me in case the problem happens again - nothing is worse that driving without turn signals and headlights!
- Apparently the same situation exists for all of the lights on the "rear" of the truck, but I haven't had problems there, and don't know where the ground points are....
A loose ground on items that have a ground in common can cause a lot of unusual behavior.
As an example, suppose that one brake / tail light has a loose or open ground. When you turn on the running lights, the light with the open ground will light up because it is grounded as follows: through the common ground, through the brake light, through the brake light circuit to another brake light, through that light, and then to the ground of that other light.
When the brake lights are turned on, this path no longer works for the tail light (there is +V at both sides of the non-grounded tail light, one side from the running light circuit, the other from the brake light circuit) so the tail light goes off, and the brake light fails to light up.
If the tail lights are not on and you light the brake lights, it will find a similar ground through the running light circuit and will appear to work normally.
So, when you have loose ground wires, the electricity will try to find a ground path through all items connected to the common ground, providing reduced power (and reversed polarity) to all of these common items. Depending upon what they are, very unusual things can happen.
General rule of trouble shooting... if the symptoms are very strange and affecting more than one item, suspect a poor or bad ground connection.