I am not sure anyone can help here, but it might generate some useful comments for us all.

In the Mega Squirt system, the MAP, IAT, ECT, and O2 sensors were all grounded at a common ground on the engine. In the ACcel system, only the )2 sensor in this group is listed as having a ground. The others list what would be the ground wire as a "signal return" wire, and they are all ganged together at a common terminal at the ECM, but not grounded externaly that you can see. If these are ground wires, they are grounded inside the ECM box circuit.

Do you think these sensors are likely simply grounded inside the box, or are they maybe used some way other than MEga Squirt used them. My understanding is the sensors are just a variable resistor that changes the voltage the entire circuit will pass. I have an e-mail into Eli on this, but I would appreciate your opinions here as well. If it is simply a ground that is completed inside the box, I would like to double up the ground on the box just like Mega Squirt does, if ti will not hurt anything. On military vehicles, grounds are very redundent, in case one ground gets blown up, or shot away. It can;t hurt.

I am wondering if I have a weak ground system now with the ACcel system. With my battery in the trunk, I am wondering if I should ground the battery directly to the engine via a long ground cable just like the positive ground cable that I have run from the trunk to the starter.

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Yes it is likely the Accel grounds them in the box.

The sensors themself use a very small amount of current - without the fuel pump or injectors or ignition the megasquirt is rated at 120ma. The only reason it wants more ground wires is because it switches the injectors on and off on the negative side. Hence why heavy ground, but small positive feed. The high current devices get their power direct from battery through fuses or relays.

I don't trust body grounds that go through spotwelds. For anything this sensitive such as efi I go direct to battery.

Theres really only one way to use sensors of this nature. You have a known voltage, passed in series through a known resistance, passed in series through the unknown resistance (the sensor) then to ground. With simple voltage divider equations the computer determins the unknown value.

Thanks Pierre, I think you are telling me tht Accel's way of doing this is safer, and more sure, since it does not use the body as a ground path. . But do you think it is a bad idea to ground the sensors on the engine as well? Should I wait for Eli's response to know for sure? Do your think it is a good idea to ground the engine to the battery as well, even though the battery is in the trunk. I have a hunch that this ground concern might be compounding some of the troubles I have had here for awhile.
Sam

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I would wait. Many computer circuits, do not ground the "signal return" wire right after it enters the computer. The signal wire goes into the computer, and is connected to a resistor, the other end of the resistor is connected to ground, internally in the computer. The computer, takes its signal information from the connection inside the computer, at the end of the internal resistor, that is NOT connected to ground.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

If the sensors have one specific terminal to ground it on in the ecu then leave it as is don't ground the sensors anywhere else.

In terms of battery grounds, I would run one fat wire (2awg? or whatever your using for the positive) from battery to block, then ground everything (including main ecu ground) at that point. Maybe use the starter hold down bolt since your positive is going to starter stud, then add some small bulk capacitance there while your at it.

You may even notice your starter spinning a little faster or increased battery voltage. Passing high currents for the alternator, headlights, starter, etc through all that sheetmetal ground, while it may work, isn't ideal.

The Megasquirt grounding depends on whose wiring harness you're using. The setup I have looks, from the outside, the same as the Accel setup. Some other builders use a single sensor ground wire that returns to the Megasquirt, or engine grounds.

There is one advantage to grounding all sensors to the ECU: It makes for just a little more accuracy as it ensures there is no voltage difference between the sensor ground and the ECU ground. This is not likely to be much of an issue if you ground the ECU and sensors to the engine block, but it may be more of a factor if you've grounded one to the block and the other to the dash.

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"Mad Scientist" Matt Cramer
'66 Dart - turbocharged 225
My blog - Mad Scientist Matt's Lair

I'll be the first to admit that I'm no electrical guru, but I always thought that having multiple grounds introduced the possibility of "ground loops." Am I missing something?

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Used to own:
'82 Dodge D150
Erson 270 Cam, O/S valves, mild port work, ~9.5:1 compression

Currently fighting with an '85 VW Cabriolet

My other passion

Thanks guys for all your input here. This is what Eli wrote back:

Hi Sam

E2 is ground, if you wish to ground these sensors to ground, the ground has
to be away from any coil, ignition wires, or any wire that carries high
current to prevent EMI. That is why they are grounded in the ECU.

Make sure your engine is grounded to your frame and there is not paint or
grime between the terminal and the frame surface.

Have a great day.
Eli

Can anyone speak to CStryker's question about ground loops?

I still have some concerns about the internal integrity of this harness. I was wondering if I could double up the ground for the sensors and bypass any internal weak link. Maybe I should bypass the harness with a new sensor ground that goes to both the block and E2 on the ECM connector.
I do plan on running a long jumper cable wire from the battery cable ground to the engine. Things are so crowded around the starter, that I thought I might use one of the bell housing bolts on the passenger's side. My positive cable is run inside the car, but I might just run the ground cable under the car for siimplicity's sake.
I was away for the weekend preparing a concert grand piano we had sold to a rich kid prep school. The event was the dedication of a new building, and included some big name talent. I was fun to rub elbows with fame for awhile, but it is really nice to get back to the Forum after 4 days away. It is neat to have this group, and great to see some responses to this question. Thanks again.
Sam

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I do not think the battery needs to be grounded directly to the engine. The car body is a HUGE conductor, so as long as you are well grounded (thick cable - 4GA or bigger, and big fasteners - 5/16" or bigger) from the block to the body and the body to the battery, you will be fine.

I believe it will be better to put the sensor grounds directly back to the ECU grounds, as in the MS relay box, but that is probably not critical either as long as grounds are solid.

I had a little trouble running when I forgot to reinstall my ground strap from engine to body, but it was all good once I hooked that up again (braided strap).

Ground loops might pose a problem if you have multiple ground wires going willy nilly near the ignition system (could amplify RFI), but if you keep it simple you will be fine.

Lou

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Home of Slant6-powered fun machines since 1988

The car is indeed a huge conductor, but its one held together by spotwelds that weaken after 30-40 years.

Grounding direct to block is not a necessity but may be a good idea overall. Originally you had 2-3 feet of actual wire from battery to block, but with battery in rear now you have who knows how many spotwelds. Which may be fine for tail lights and other nonsensitive loads, but if you want to deliver full current to starter / from alternator and help eliminate any issues with sensitive efi stuffs a direct block ground is closer to ideal.

Sam I think if you ground the sensors direct to the computer you will be fine, as long as the computer has a solid ground itself.

Keep in mind that one good spot weld has roughly the same cross sectional area as a 4GA cable.

Lou

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Home of Slant6-powered fun machines since 1988

This is true, but the sheet metal itself is what, 20awg? There may be multiple layers in places, but still, not effective a ground as a direct run. Its the difference between "good enough" and "better".

I believe Dan also suggests running direct to battery grounds for headlights to deliver full voltage to headlights and avoid differences in brightness between one lamp and the other. If it makes a difference for headlights that have grounds going through a small amount of sheet metal, imagine running starter/alternator current the whole length of the car.

So does anyone want to take on explaining what a ground loop is? This explanation needs to be for dummy me. As I said before, I might not be too bright, but I make up for it by being slow. I have heard this term used many times but always kept a cool silence about my lack of understanding. Well, the cool is gone. 'Splain me dis.
Sam

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Heck, don't you watch war movies? That is what a aircraft does, when trying to land after being all shot up.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

This is a fairly good explanation of ground loops.

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Used to own:
'82 Dodge D150
Erson 270 Cam, O/S valves, mild port work, ~9.5:1 compression

Currently fighting with an '85 VW Cabriolet

My other passion