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Login to post Forums » Grassroots Motorsports » How to ensure that engine grounds are sufficient?

May 2, 2011 8:48 a.m. DILYSI Dave SuperDork

So, this time yesterday my joy of having a Megasquirt install nearly complete turned to horror via a puff of electrical smoke. Turned on main power, fuel pump primed, hit the starter button for a few seconds and got a smoke show near the DB37 connector. Of note, the starter stopped turning, and even the solenoid wouldn't switch at this point. I initially thought I somehow smoked the squirt, but a visual inspection says it's fine. I pulled apart the DB37 plug at the end of my wiring harness though and was greeted with burnt electric smell. Found a couple of burnt to the point of failing ground wires. 20 Ga fuses...

Given that it was behaving normally until I engaged the starter for a few seconds, I'm pretty sure the starter current was taking the ECU wiring grounds as the return path. Only reason to do that was if the big ground wire wasn't, right?

I've got the ECU harness repaired, and paid extra close attention to the engine ground that I already had (scrape paint, retap hole, star washers, dielectric grease), and have another one added just for good measure. Both are 8 gauge. I need to pick up a bolt today to bolt the second ground to a spare hole on the block.

What I want to know is, how do I keep this from happening again? How do I know that my engine is grounded well and that the starter current is taking that path? With the ECU unplugged, the grounded sensors unplugged, etc., after reworking the current ground strap I can engage the starter. That would seem to say that it's good. But I'm a little gun shy to plug the MS back in until I know for sure that I'm not re-creating the same situation.
May 2, 2011 9:14 a.m. ditchdigger HalfDork

I read the first paragraph and said "engine ground strap" out loud. Sounds like you have taken care of it.

What was the status of the ground when this happened? You described what you did to fix it but not what might have been wrong.
May 2, 2011 9:16 a.m. ncjay Reader

I always run a cable same size as the main battery cable from the engine to the frame (if applicable). I then usually run a ground wire about half that size from the engine to the firewall. Haven't had any problems with that setup. I attach the wires to the mounting points using bolts that are welded in place. That way I know it's a good grounding point.
May 2, 2011 9:19 a.m. DILYSI Dave SuperDork

ditchdigger wrote:

I read the first paragraph and said "engine ground strap" out loud. Sounds like you have taken care of it.

What was the status of the ground when this happened? You described what you did to fix it but not what might have been wrong.

The multimeter said there was continuity, but that doesn't say whether it was continuity through the ground strap - I could have been measuring through the ECU ground and not knowing it.

I ASSumed that the ground was good, but the holes it was mounted to did have paint, rust, etc. I didn't think to test continuity between engine and chassis after the ECU wire burnt, so I don't know for sure that it was lost, but I suspect it was since the starter relay wouldn't even click.
May 2, 2011 10:01 a.m. AngryCorvair SuperDork

your repair method is what i would've recommended, ie clean with abrasive then wipe clean and coat with dielectric, then use star washer or bolt with underhead serrations.. I always use the same gauge ground as power, and typically go B- to block and B- to body.

side note: i once burned through a corvair clutch cable because of missing engine ground strap.
May 2, 2011 10:05 a.m. pres589 Dork

The only thing I can think to add is to make sure your terminals are all properly sized, none of that stuff where a really spindly ring terminal is used in a situation that calls for high amperages just because it all fits together that way. And I would image you did avoid that in your rework mentioned above.

And definitely size the ground and + wires the same size as you did here. It is unfortunate that more people don't realize that is a requirement but it happens a lot.
May 2, 2011 10:37 a.m. curtis73 Dork

AngryCorvair wrote:

side note: i once burned through a corvair clutch cable because of missing engine ground strap.

66 Bonneville, same thing - welded my tranny dipstick tube to the firewall.
May 2, 2011 11:22 a.m. DILYSI Dave SuperDork

I am using properly sized terminals.

I'm considering running the battery ground straight to the block. Keep the chassis to block ground as well, but give the starter current a path back to battery that doesn't involve the chassis.
May 2, 2011 11:36 a.m. Schmidlap HalfDork

DILYSI Dave wrote:

I've got the ECU harness repaired, and paid extra close attention to the engine ground that I already had (scrape paint, retap hole, star washers, dielectric grease) ...

Where is the dielectric grease? Dielectric grease is non-conductive, so if it is between your ground wire and the body you probably don't have a good connection. If you just covered the connection with it to protect it from moisture after you had bolted the cable to the body you should be fine.

Bob
May 2, 2011 11:53 a.m. DILYSI Dave SuperDork

Schmidlap wrote:

DILYSI Dave wrote:

I've got the ECU harness repaired, and paid extra close attention to the engine ground that I already had (scrape paint, retap hole, star washers, dielectric grease) ...

Where is the dielectric grease? Dielectric grease is non-conductive, so if it is between your ground wire and the body you probably don't have a good connection. If you just covered the connection with it to protect it from moisture after you had bolted the cable to the body you should be fine.

Bob

Looks like I'll be redoing those...
May 2, 2011 12:15 p.m. ditchdigger HalfDork

Move the ECU ground to the chassis instead of the engine. That way the starter which is grounded by its mountings cannot use the ECU ground as a return path, It would have to use the ground strap and if that wasn't enough it would simply not turn over at all/fast enough or burn up the strap.
May 2, 2011 12:23 p.m. DILYSI Dave SuperDork

I can see that. The only reason I see not to is simply that the megamanual specifically calls for an engine ground.

Presumably, since the sensors are resistor based, there needs to be a reliable commonality between the ECU ground and the sensor ground.
May 2, 2011 12:40 p.m. John Brown SuperDork

And remember Dave, nothing is funnier than watching all your hopes and dreams get dashed because of not having enough interlaced grounds. Our $2003 XR4Ti would not run well at the Challenge only to find out someone (finger pointed backwards) forgot to hook up one of the grounds.

I now always go battery to body ground, body ground to dash ground, dash ground to battery. It is overkill and I can live with that.
May 2, 2011 12:46 p.m. DILYSI Dave SuperDork

I don't have a dash ground. But I am thinking that in addition to redundant ground straps on the engine, that I will take battery ground to body and to engine both.
May 2, 2011 6:58 p.m. Vigo Dork

Im jealous of everyone here with nice, easy carnage like welded dipsticks and melted clutch cables. The only time i had this problem it did the thing that would annoy me most: ruin the entire engine bay harness. AND the computer. AND the next computer i tried before i knew it was the harness. And i tried to fix that harness but it was RUINED.
May 2, 2011 7:21 p.m. pres589 Dork

In reply to Vigo:

I've seen bad grounding blow holes through a fuselage spar on a business jet where I work because someone on the line decided they didn't really need to clean the fresh paint off of a ground stud used by an engine.

Which means engines come off, interior comes out for a short section, and a new piece of aircraft gets grafted in. This is after the aircraft has gone into service, of course. And the spar is made from a single billet so it isn't cheap to make either.

So it could get worse, but at least it's company money paying for it, and it is a good lesson for the home hobbyist.
May 2, 2011 7:54 p.m. donalson SuperDork

...thanks for the reminder (as i'm working on figuring out/building my harness for the Swede-10 :)
May 2, 2011 7:59 p.m. Rad_Capz Reader

curtis73 wrote:

AngryCorvair wrote:

side note: i once burned through a corvair clutch cable because of missing engine ground strap.

66 Bonneville, same thing - welded my tranny dipstick tube to the firewall.

B&M shifter cable once for me. Path of least resistance for starter current to get back to battery if engine ground fails. Also fried a core support ground once.
May 2, 2011 10:29 p.m. DILYSI Dave SuperDork

Alright. Productive evening. Main cable to starter bolt. Cleaned up the engine to chassis ground that I had, and added another. Star washers all over the place. Got everything reinstalled and the fuel pump primes when I flip the main power switch. That's a positive sign that I might not have hurt the ECU. I'm about to go plug the laptop in and see what I see. I'm still gunshy about hitting that damn start button....

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