What gauge should an engine ground strap be?

"If only EssExTee could be so grossly incandescent" (essextee) 02/16/2019 at 13:30 • Filed to: None

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A workplace acquaintance’s block ground wire corroded through and the car won’t run. 2001 Civic. I’ve been pup py-dog-eyed into fixing it.

farscythe - makin da cawfee! > If only EssExTee could be so grossly incandescent 02/16/2019 at 13:37

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10 or up

Urambo Tauro > If only EssExTee could be so grossly incandescent 02/16/2019 at 13:41

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I would assume the correct size would  be equal to the alternator and starter cables.

jimz > If only EssExTee could be so grossly incandescent 02/16/2019 at 13:41

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you might want to replace it with a flat braided strap of the appropriate length. the flat ones are more flexible and less prone to fatiguing.

edit: most of those will be of an appropriate equivalent gauge.

slipperysallylikespenguins > If only EssExTee could be so grossly incandescent 02/16/2019 at 13:43

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I personally like #2, but that i s mostly because I can snag some from work.

Spamfeller Loves Nazi Clicks > If only EssExTee could be so grossly incandescent 02/16/2019 at 13:44

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Varies. If it’s actual wire then match factory gauge. If it’s a STRAP then it needs to be a strap for RF reasons. For straps, factory AWG equivalent minimum (usually 8 or 10) and only use tin-plated copper. (Remember, lower gauge = bigger!) Jacketed straps for corrosion resistance are fine, just not wound or solid conductor.

If only EssExTee could be so grossly incandescent > Spamfeller Loves Nazi Clicks 02/16/2019 at 14:02

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It’s a strap, but I can’t imagine a 2001 Civic would have any subsystems that would suffer from RF interference. Parts stores here are closed for the weekend so i f I can just replace it with hardware store wire I can get the car on the road quicker.

Spamfeller Loves Nazi Clicks > If only EssExTee could be so grossly incandescent 02/16/2019 at 14:14

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And you would be very, very, very wrong. Or do you think that cars don’t have radios and alternators? Yeah. Strap versus wire matters .

That said: as a strictly temporary fix, you’re probably not going to break anything. I won’t guarantee it because I haven’t looked at Honda’s setup deep enough yet. But you do need to be at LEAST equivalent to the strap’s effective gauge, which is often quite high.

For example, a 1/4" wide braid is going to be equivalent to at least 13AWG and more commonly 10AWG. At a half inch, you’re talking 8AWG. (AWG capacity increases with thickness as well, so this is just guideline.) If it’s off the alternator, it often needs to be greater than alternator amperage - so typically 4AWG equivalent (~125A). Things get weird, yes.

But, here’s the thing. If you’ve got a strap that’s busted in the middle? If you can find equivalent or greater braid to splice in with a bunch of of solder? It’s a suitable permanent fix. Can’t splice braid to solid, but can do braid to braid and solid to solid. If it broke at the eyelet, you can reterminate as a permanent fix as long as you have enough braid. It’s a simple crimp.

If only EssExTee could be so grossly incandescent > Spamfeller Loves Nazi Clicks 02/16/2019 at 14:19

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It’s engine to unibody, and it snapped at the fastener. The whole thing's covered in green corrosion so I'm not just gonna re-crimp a new connector on the end.

Spamfeller Loves Nazi Clicks > If only EssExTee could be so grossly incandescent 02/16/2019 at 14:32

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Nah. Green corrosion’s irrelevant on these. They’re carrying ground and made up of thousands of tiny strands. If the continuity passes, the braid is good. Honestly if there’s enough cable left, I’d hit it with vinegar on the end and re-terminate with an equivalent size crimp and eyelet. As long as you’ve got reasonably clean contact at the crimp it’s fine. Throw some nail polish OVER the crimp (LIGHTLY because it’ll wick,) sand clean the bolt points, and call it a day.

gogmorgo - rowing gears in a Grand Cherokee > Spamfeller Loves Nazi Clicks 02/16/2019 at 18:18

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So riddle me this. My Jeeps all have alternators and radios. They d o come with a ground strap on the head going to the firewall , but also a 4awg ground from the block back to the battery, serving as the starter ground. Many of the sensors ground to the engine block, either via the sensor body itself or by wire to the dipstick hold-down stud in the block, although some of them including the injectors do ground to bolts on the intake manifold which would ground through the head. But I’ve had the ground strap corrode and break off completely and I only noticed while hunting for a vacuum leak , because everything still worked . So clearly there’s also a path to ground via the battery ground cable, or the engine would’ve died with no injectors. So that begs the question, what is that ground strap actually doing that the ground cable isn't ?

Spamfeller Loves Nazi Clicks > gogmorgo - rowing gears in a Grand Cherokee 02/16/2019 at 19:46

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Again: varies. Everybody’s is different. Some Ford EEC-IV has a ground strap to the body; if it has so much as a slightly bad connection? Car is dead. Entirely. Chrysler LH has a ground strap from the cylinder head to the firewall that you can rip off and be none the wiser. Ram has a body-to-frame connection that can kill the PCM if it breaks sometimes.

Some designs have multiple ground paths for one ground either to use smaller gauge wires or reduce loads . Others have multiple ground paths and multiple grounds for circuit isolation. Others just say “fuck it” and use the housing and screws for a ground path.

None is more or less valid than any other. Just depends on the exact circuitry, the desired result, and how they elected to implement it. But, because of that, you do not want to deviate over much from that design. Especially in anything after about, oh, ‘85-86. But as long as you’re using the same type of conductor (solid, stranded, braided) and equal or higher gauge, you’re generally safe.

gogmorgo - rowing gears in a Grand Cherokee > Spamfeller Loves Nazi Clicks 02/16/2019 at 21:58

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The more I look into it, the more it seems the braided ground straps are just used because of greater flexibility for connecting moving or vibrating components. Sure there were some small RF issues  associated with early high-energy ignition systems, but those all got mitigated pretty quickly, and are virtually non-existent with coil-on-plug ignition.

The OBD1 Jeeps are probably fairly similar to the Chrysler LH, considering it's a Chrysler designed system from a similar period. But at any rate, a ground strap is a ground. The strap will just last longer than a copper wire that work hardens and breaks under vibration. 

Spamfeller Loves Nazi Clicks > gogmorgo - rowing gears in a Grand Cherokee 02/16/2019 at 22:51

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So, here’s the thing: sometimes yes, sometimes no . It is absolutely dead wrong and invalid to attempt oversimplifying like that. Demonstrably so.

If that were the case, why wouldn’t manufacturers use braided cables for the battery connections and nearly everything else? They hate electrical warranty claims. They hate having to solve vibration. It’s expensive.

So no, you absolutely cannot say that for even the majority of them. It’s not correct, it’s not accurate, and it’s simply not true. Not only that, it’s completely ignorant of PWM, high-frequency switching, and everything else that’s involved, much less the drastically increased external interference from cellular phones and everything else.

Not only that, but braided conductors have vastly superior heat dissipation and temperature handling capabilities compared to solid and stranded, which can be a significant design factor. It may be for 400MHz and 900MHz rejection. It may be to prevent 88-108 MHz interference. There’s a literal mountain of FCC, EU, and per-country rules around this stuff that the manufacturer must pass on demand or in testing (which varies by country as well .) So u nless you actually ask the electrical engineers responsible, you cannot possibly know.

And no, you cannot compare apples to oranges like that. It’s not how it works. Just because they’re both based on the SBEC does not mean they have absolutely anything else in common. That 60-pin brick was introduced in 1983 and remained largely unchanged aside from consolidating from a LM/PM architecture into a single board (hence, Single Board Electronic Control.)

The very late SBECs were made of nothing but bailing wire and duct tape, because they were trying to make 60 pins (including power ,) connected to an 8-bit microprocessor running with a whopping 64 bytes of RAM, 256 to 512 kilobytes of non-volatile , and clocked at a whole 2-5 MHz depending on application. That’s it .

And on the LH they were trying to tack on the PCI bus (not to be confused with PCI Local Bus; Chrysler PCI is SAE J1850 single-wire PWM,) ABS computer, body computer, and a transmission computer on top of it. Which operates at a whole 10.4Kbps with a minimum message time of 200uS. And these circa-1984 parts designed for nothing bigger than 4 cylinders were successfully doing OBD1 and OBD1.5.

That it worked at all is a testament to the unmatched skill of Chrysler’s software and electrical teams, period. Nobody has ever achieved anything even remotely close. Megas quirt is far less capable despite having more than four times the CPU, and many times the storage. Honda had to use an M66207 (16-bit, 10MHz, 32KB ROM, 32KB RAM) just to build something functional before   OBD1 kicked in, forget OBD-II!

gogmorgo - rowing gears in a Grand Cherokee > Spamfeller Loves Nazi Clicks 02/17/2019 at 12:45

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¯\_()_/¯

Do what you want. I’m not wasting money on braided steel that will just corrode and fall off again. Braided cables are far more susceptible to corrosion than insulated stranded copper. Battery terminals are highly corrosive places. Braided cables are more difficult to reliably insulate, because the conductor is so much more flexible than most insulations. Battery cable runs are also much longer than your typical engine to firewall ground strap, meaning the lower flexibility isn’t as much of an issue.

Any instance where interference between the signals is an issue, it wouldn’t go through a common ground anyhow. Those few inches of braid are meaningless compared to the feet of ground path through the chassis  to the battery. 

brianbrannon > If only EssExTee could be so grossly incandescent 02/17/2019 at 20:12

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I’ve never seen a ground strap keep a civic from starting. Usual cause is fried ECU from loose alternator bolts