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Thread: Battery negative side - dedicated ground cable vs chassis for ground

  1. #1

    Yes, I love Technology
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    Battery negative side - dedicated ground cable vs chassis for ground

    I got interested in whether the chassis as a ground is as good as a negative side power cable. Since copper is such a better conductor than steel, it seems a reasonable question if one should run a ground cable or not. If your battery is next to the engine this is not worth contemplation, but my battery is in the front.

    I ran some rather low current tests with cables I had laying around and concluded maybe a ground cable would be a good idea. My experiment did not include cranking the engine, so it was not a definitive test. But the scraps weren't pretty/sexy so I bought a length of #2 at the local parts store. My negative battery lead is run to close by to the right side steering rack mount frame/bolt. Cleaned, shiny and tight. The engine ground strap is at the chassis left engine mount support.

    Engine is an H6-3.0 liter, connected to a 98 Forester manual 5 speed. Starting was done with cold engine, temp was about 50F, with the clutch engaged as I had not installed the clutch slave cylinder yet. Transmission was in neutral. Test equipment is Fluke 87 meter at starter 12V terminal to starter frame. A high end Wavetek meter at the battery and a Beckman DC current clamp set to 400AMP range.

    Battery is a used one, and not exactly the most healthy, but a big Group model used in our Dodge Charger V8's, and I think delivered ok for this test:

    No ground cable, chassis ground only -
    crank = 160 Amperes, 11.3V at battery terminals, 10.53V at starter

    Added ground cable, #2 guage copper -
    crank = 160 Amperes, 11.3V at battery terminals 10.60V at starter

    While you may get different results, or in current lingo YMMV, I am thinking I don't need the ground cable. On the other hand, it will get some red shrink at each end and be my new, bigger battery positive cable front to back.

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    You've got to think what is going to happen a year from now or ten years. That short connection, bolted to the frame, may just become corroded over time.

    I've constantly read here and on other forums about electrical failures: Bad ground is many, many times the problem.

    My battery is in the rear of the car so I did both. A short run to the frame and a black cable to the front of the car to a negative post and worked many grounds off of it and to the frame. My opinion-you can't have too many grounds as they are many times the weak link to good electrical flow, especially over time.

    Just as an example: much of my dash wiring is grounded to the chassis and to this negative post. If that rear frame ground connection goes bad I've got a backup in front of the car where the negative post has a cable to the battery and is grounded to the frame and to components needing ground.

    I am building an FFR Roadster but the concept is the same. Many will say I've overthought it but that's just me and I'm enjoying it.

    George
    Last edited by GWL; 05-06-2019 at 10:47 PM.

  4. #3

    Steve >> aka: GoDadGo
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    My take is if you want to run an extra ground to the engine, then do it.
    I've got redundant grounds to the frame for all of my lights and the cooling fan.
    My block has two ground directly to the frame for obvious reasons and they are easy to get to.
    To keep them in good shape I used a little dielectric grease on all connectors and attachment points to the frame.

    On another note, here is a little wire size calculation tool that I like to use:

    https://www.wirebarn.com/Wire-Calculator-_ep_41.html

    Good Luck From The Dark Dart Side!
    Last edited by GoDadGo; 05-08-2019 at 12:54 PM.

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    Senior Member AC Bill's Avatar
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    A frame ground is perfectly fine. Resistance is minimal with that much steel. Ensure the ground cable attachment point is clean, and free of any powder-coat or paint. The frame being ground, can be a benefit for grounding other electrical items to it.

    After it's attached, you still could always smear some dielectric grease over the cable end, but if the car is stored inside out of the weather, corrosion is unlikely to be a real issue.

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    I was originally going to make a comment about ground loops. Though, upon looking through your build I think you know what your doing! I didn't expect to come across some lines of code in a build :]

  7. #6

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    I've got a long history in electrical/electronics ranging from Kilovolt stuff to microvolts at the wee end of it and a wide range of products and devices. Even so there is always something more to learn at every turn.

    For the car build I was focusing on the the starter power draw. I view the connections themselves as the real bug-a-boo due to the environment cars suffer. Chassis ground paths work until a connection gets flakey or gone. My son-in-law, who designs and sells wire harnesses for industrial vehicles, fire trucks, ambulances, etc. read my thread and emailed this comment below. I think I'll keep the cable ground in there, but will be moving the chassis tie point up out of the road dirt zone, at least up front.

    I'm willing to have some grounds tied to the chassis but I do believe I will run some dedicated - direct to battery - grounds to the dash area, etc.

    What is interesting is every thread where grounding via chassis comes up, it really gets inputs rolling on both sides of the fence. I've dug through the other builds a while back specifically reading the grounding discussions.

    My son-in-law says -

    "My two cents on the ground cable is, there shouldn’t be any such thing as a chassis ground. Always do a two wire system with dedicated ground wiring. Cable back to battery ground is the only option.

    Just my opinion, but learned this by experience as a fleet manager with 125 ambulances running 24/7. Dedicated ground clean happy electrical system, chassis ground dirty troublesome electrical system."

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    This is very interesting, as for the Subaru wiring diagrams the grounds for a lot the individual components is the chassis.
    dieting the wire harness, I followed this practice throughout the build. For a factory vehicle it is probably cheaper to not run individual ground wires. There were however some circuits with copper crimp tubes and black tape to share grounds, some of those were loose! These were all corrected during the diet procedure.

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    Senior Member edwardb's Avatar
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    Haven't done an 818. Only Roadsters and now a Coupe. But general comments about the FF chassis and grounding. The one piece welded steel chassis that makes up all FF designs provides an opportunity for grounding that's different than your average DD, and also including most industrial vehicles, fire trucks, ambulances, etc. Not sure this thread and responses fully takes that into consideration. As my friend Jeff K told me once, "Ground is ground is ground..."

    Having said that, I too am a proponent of multiple grounds. My standard build practice is (1) Battery ground with #2 cable to a solid and clean to bare metal connection on the chassis. (2) Sandwiched onto this connection is one of two #4 cables to the engine block. (3) Second engine block ground from one of the starter bolts to a second solid and clean to bare metal connection on the chassis. (4) Main RF harness ground to solid and clean bare metal connection under the dash, (5) Added grounds at front a back corners along with lighting and cooling fan circuits.

    The common backbone to all of these connections is the uninterrupted welded steel chassis, and I don't see the need to add additional interconnections between the grounds. I've never had a ground problem (knock on wood). As already stated, these builds are typically stored indoors and don't see a lot of extreme weather. At least not on purpose. So the chances for the connections, assuming they were solid in the first place, to remain good is very high. A little dielectric grease or electronics grade RTV (e.g. non-corrosive) over the finished connections doesn't hurt though.
    Last edited by edwardb; 05-08-2019 at 09:30 AM.
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    Senior Member AC Bill's Avatar
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    If I build an ambulance some day, I'll keep your son-in-laws advice in mind..

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