Grounding Various Components to the Chassis

[Lonnie-S]

I'm working on how and where to ground electrical components to the chassis. I'm trying to avoid problems, but this is my first, from-scratch, vehicle and I doubt I have it all correct. So, I'm going to expose my current thinking and see "what the smart kids think" as JD likes to say.

My 1994 Mustang donor is your standard issue 12V, negative ground system. Off the battery are two positive connections, one to the starter motor and one to the "Power Supply Module" mounted nearby. The ground from the battery goes to the chassis directly. The Power Supply Module appears to provide power directly, or indirectly, to all other components in the vehicle. Here's a mini-view of the situation in the donor:

Power-Dist-Module.jpg

My battery will be mounted at the right, rear of the vehicle. A positive cable runs from there through the transmission tunnel, and will terminate on the passenger side firewall. A stand-in for the positive cable appears in subsequent photos as the orange extension cord. To ground my electrical system in the Locost chassis, I've chosen the location shown in the photo below as a black rectangle. I've selected this particular member because it is welded directly to all the other members shown with a green check mark. The horizontal members run all the way to the opposite side of the chassis and really connect up well to a majority of the chassis tubes.

I'm not sure what kind of ready-made part is available to anchor the ground cable, but I was thinking a piece of 1/8" plate with a threaded stud welded to it, and it in turn welded to the chassis would do.

Main Chassis Ground.JPG

The starter motor (huge consumer of power when engaged) is located on the passenger side, just below where the cable from the battery will emerge. It's indicated by the blue rectangle shown in the photo below. I'm told it's best if the starter is grounded directly to the chassis rather than through the engine block ground. So, I've picked an area of the chassis adjacent to the started to put another grounding connection. It may be the stud/plate component described above for the battery cable ground at rear.

My understanding is that grounding the starter motor by itself, away from other grounding points, helps avoid problems due to the high current load of the starter when engaged, which reacts through the chassis locally.

I'm thinking that to have a situation similar to the donor setup, I'll need to feed the battery cable into some kind of mounting lug, and then have two cables running from that. One will go to the starter, and the other to the Power Supply Module, mounted up above on the firewall shelf.

Starter Grounding Point.JPG

I'm understanding all the major loads like the alternator, engine wiring harness, sensors, etc., are grounded to the engine block. The engine block is then grounded to the chassis. In my original donor setup, some components on the engine, and the starter motor itself, were grounded using a common cable. The engine block itself had a second ground which connected on the drivers side through a braided cable. That braided cable ran from one bolt on the engine block side of the motor mount around the rubber vibration damper to another motor mount bolt on the chassis side. I'm thinking I'll go directly from the bolt on the block to a grounding point on the chassis as indicated below.

Engine Grounding Point.JPG

Lastly, all the other things like the instruments, accessories, ECU computer, radio, etc., also need a ground. I'm understanding that the chassis can act as a sort of "filter" on the big activity of the starter, and having a ground for the above components somewhat removed from the starter motor ground area is a good thing. So, I'm thinking a ground on the member indicated below, but on, or close to, the driver's side of the car is a good location.

Instrument-Acc-Dash Ground.JPG

So, "smart kids" how did I do with the basic plan? Does it look fairly decent? Where should I look for some ready-mades for the chassis grounding parts and the distribution lug for the positive battery cable?

Thanks in advance,

Lonnie

[cs3tcr]

In my honest opinion, just keep it simple. The battery ground can go to any of your chassis members, no need to select one that has many members joining it. On my two old cars I just drilled a hole in the battery mount that was welded to the chassis and ran a ground cable to a bolt I put through the hole. You could always run the ground cable to a suspension bolt or any other bolt in the general area of the battery.

As for the engine ground, I've only ran one from either side of the motor mount picking up on the bolts that pass through either side. I've never had a need to run a separate ground for the starter. In regards to the instruments and other ancillary items, just find a bolt or screw that's nearby and that contacts or pierces the chassis.

Onto the starter cable, I guess you could do as you've shown with a junction set up. Though if it were me I'd have the large battery cable run to the main terminal on the starter and a slightly lighter gauge wire run from the same starter terminal up to your power distribution module.

Rod

[seattletom]

I'll need to feed the battery cable into some kind of mounting lug,

You might consider using a dash mounted battery disconnect switch as your mounting lug. Lots of good reasons for having one. And it would serve as a power distribution point as well.

[rx7locost]

For the high current from the battery to the starter and the return engine to chassis and then to the battery, Should have as few lugs as possible. these only add resistance and voltage drops for the 100-200 amps during the starting. Usually, the battery runs go direct to the starter and power electronics are fed on a separate wire from the battery, or from the starter power lug.

A cutoff switch can be in either the B+ or the B- runs depending on your requirements and any racing class requirements if that is your intention. My intention is simply to disconnect any load from the battery during times of storage. I put the cutoff switch in the B- lead between the battery and the chassis. The one lug of the cutoff switch is bolted to a weldment on the chassis itself eliminating a wire and 2 more crimps. This connection also serves as the engine ground to chassis. Take a look here at an early battery incarnation. BTW, I do not recommend this type of battery post clamp. They are very susceptible to loosening.

[Lonnie-S]

@cs3tcr
Thanks for the response, Rod. As I mentioned, the original setup on my donor grounded the starter plus a couple of other components separately. I don't know why they selected that design, but I assumed it was for some reason, and I had read that the high current load of the starter caused disturbances locally. My thought was to include a similar connection to be on the safe side.

@seattletom
I've seen those switches mounted in race cars, but never seen the back (or business) side of one, Tom. I'll have to check those out. If they have ones with two output lugs it could indeed solve my distribution problem of needing one cable for the starter and one for the power supply module. Thanks for the info. The other benefit you hint at is maybe safety in the senses that you have a quick disconnect if there's problems. I see Chuck's major reason for a disconnect was to stop battery drain during storage

@rx7locost
I could easily ground the starter and the engine block at the same location, Chuck. Is that an OK thing to do? Would there be any risk of making the engine sensors get wonky signals at start-up time if they used the same ground as the starter?

I know you're pretty knowledgeable about ECUs and their sensors too. Any rules of thumb or standard procedures for grounding the ECUs?

The B- terminology isn't familiar to me. Is that the negative earth side? I'm assuming so from your photo. I like that setup, by the way. I felt a welded positive (in my case) lug would be more reliable that a bolt-in solution.

Cheers all,

[rx7locost]

Every car I've worked on had but one (major) ground. It is the big fat wire that runs from the engine block somewhere to the chassis. It might use the bolt that the starter is mounted with. I've not seen 2 heavy wires used. Sometimes a lighter wire is used in addition this is for noise control for the signal level stuff. Generally, a single point ground at the chassis side is best. Except when it isn't for signal integrity. Then a 2nd or 3rd or 4th lighter gauge wire is sometimes used to control signal level noise. Signal noise is a black art with some basic rules. Noise seldom follows rules. My GMC Jimmy used 2 wires for noise(presumably), one on the back surface of each cylinder head to the firewall.

So, if you are a mind to, run a heavy wire from the starter mounting bolt, to the chassis and leave it at that. Just don't forget to reconnect it EVERY LAST TIME you disconnect it. Otherwise the starting current will try to find another path which will burn up the xxxx (you fill in the blank. I have seen heater cables try to carry the starter current when the heavy ground was forgotten with very poor results)

B+ and B- are battery positive and battery negative respectively as you suspect. As for a bolt to the chassis or a welded tab, I think it really makes no difference. Somewhere there will be a bolt and a terminal. there is no way around that. My point was to keep the number to a minimum. I eliminated that by bolting the cutoff switch directly to the welded tab. It could have easily been to the chassis if that is the way I chose to mount the switch.

I hope this helps.

[Lonnie-S]

@rx7locost
It does, Chuck, thank you.

Cheers,

[C10CoryM]

I wrote a long reply a while ago but it didn't make it through. Seems like you're sorted now but basically what you had in mind with a battery-disconnect switch makes a lot of sense to me. Also, do the math to ensure that your loooong cable can handle the starter current. 175A+ over 12ft is a whole different thing than over 3ft.

If the ECU has a separate ground eyelet for it, I would suggest to give it it's own ground in as clean/dry area as viable. If that ground gets corroded, you will run into some very odd issues. As mentioned, do not use the bolt-on battery cable ends like pictured. They corrode the cables.

Cheers.

[turbo_bird]

I used what looks like the same battery disconnect switch as Chuck, but mounted on my dash like Tom was talking about. It's got a removable handle and it's also my ignition key, as well as a battery disconnect and power distribution point. I have the battery + cable on one terminal and everything else on the other terminal, including the starter and alternator. There are nicer ones that have 3 lugs instead of 2, and the third lug is usually used for the alternator, so when you turn off the disconnect it will actually kill the engine. I haven't had any issue with the starter current going through the disconnect, but the engine will stay running, so I've got a seperate toggle switch for the ignition and fuel pump.
Kristian

[TRX]

> disconnect

If your preferred disconnect switch location is far from the battery, you can use a cable or a rod to actuate it.


> ground

After having more than my fair share of electrical gremlins, I went to the single ground system. The battery grounds to a starter mounting bolt. A second short cable runs to a brass stud in some convenient location. And everything else grounds there. You could also ground to the battery negative, but I usually wind up with the battery far from where most of the bits need to be grounded.

With a welded tubeframe like a Locost grounding is probably not a big deal, but unibody cars with paint, spot welds and seam sealer can be a *really strange* ground path.

[TRX]

[double tap]

[Lonnie-S]

I've refreshed (i.e., reloaded the missing photos from) my original, initial post. Then I'm going to re-read the responses other added and see if I can come up with a new, and actual, scheme to implement the grounds. I'm actually there now in terms of the work I need to do in the next couple of weeks.

Cheers,

[KB58]

I just welded a bolt head to a tube anywhere I needed a ground, then treated it as a stud. The short story is to keep high current grounds separate from low current grounds. Don't have them share a common path back to the battery unless that path is the chassis.

[Lonnie-S]

@KB58
Noted, Kurt. Thank you.

I'm going to stick with my original plan, which is to have the battery at the rear. The direct battery ground will be at rear also as shown in the second photo of the original thread post.

I already have the tabs welded in to hold the P-clips for the positive battery cable inside the transmission tunnel along the top of it. The last one already installed is shown in the photo below. The positive cable will be 8' or less from the battery to my proposed location for the disconnect switch (photo after the below one). It will be an extra 18 inches or so from the disconnect switch to the starter. Maybe 6 inches from the disconnect switch to the power distribution module.

Plus Cable Route.jpg

As to the other grounds, I think I'd like them as shown in the below photo. It does keep the big load ground and the small load ground separate, "filtered" by the chassis. All grounds will be in good chassis locations and welded to tabs welded to chassis tubes.

I think this will be good, but If I've done something stupid, please let me know.

Current thinking.jpg

Cheers,