Simple Wiring Question

[jacob80]

Hey guys,

 

Well I'm doing some re-wiring on my Megasquirt and have a question about grounding. I want to run all ground/power leads to 10 position terminals strips (one for ground, one for power). Does this sound like a good idea? I was thinking I would run a heavy gauges wire directly from the battery negative to a 10 position terminal strip and run grounds for everything on my Megasquirt there, such as the fan, distributor, and relay board ground. My theory is that this will make it much easier to wire and keep all grounds together. The same will apply for power leads. Thanks!

[RTz]

To the best of my knowledge, all modern cars have their EFI grounded to the engine. Battery is grounded to the engine. Chassis is grounded to the engine. This helps reduce ground loops and related gremlins.

[jacob80]

Thats the thing. The reason I think this is a good idea is beacuse: strip>battery>engine gound path. Wouldn't this be fine?? Here's my potential diagram:

 

[Zmanco]

It's most important that you have a common ground for the small signal devices such as MS and O2 sensors. It's less important for things like eFans. In fact there's an argument to be made that they should be wired separately so that they won't have as much impact on the voltage sensitive devices like MS and O2 sensors when the high-load and noisy devices like eFans turn on and off. But the bigger and better your wires and connections are, the less impact high load devices will have. So it's as much a function of the quality of your wiring as it is how you wire it.

 

That said, I have my MS negative buss (an 8 position terminal IIRC) wired to the negative side of the battery and the plus side wired via a big fuse to the output of the alternator and have not had any electrical noise issues since I switched to Magnecor spark plug wires.

[jacob80]

Well what do you think of my wiring diagram? From what I understand, you wouldn't recommend have the electric fan and megasquirt, dizzy, and fuse block all grounded together?

 

I just want to keep everything together, you know? Simplicity is key here.

 

Thanks!

[Zmanco]

Sorry Jacob80, I'm in a hotel room looking at a small laptop screen and just can't follow your wiring diagram. But you should have a relay controlling the efan which you can mount on the radiator with +12V coming from the alt (with a fuse of course) and ground going to one of the bolts that attach the radiator, so why route that all the way back to your MS? It makes more sense to just route the ground wire that controls the eFan relay back to the MS by itself. Keep in mind that the eFan pulls on the order of 10-20 amps whereas the relay will pull a few tenths of an amp at most. Why would you want to route that much current over a longer distance than is absolutely necessary?

[jacob80]

The whole idea is to bring all grounds and power leads (whether they be switched or constant) back to one central location for simplicity and organization. I am also trying to keep my wiring and engine bay as clean and minimal as possible. I'm also going to configure my circuit board so that I can use the fast idle relay for my fan on my relay board. I believe this also makes troubleshooting easier.

 

What do you think??

[Zmanco]

The whole idea is to bring all grounds and power leads (whether they be switched or constant) back to one central location for simplicity and organization. I am also trying to keep my wiring and engine bay as clean and minimal as possible.

I just told you that this is not the best approach from an electrical point of view. If you want to go ahead and do it because it looks better on a wiring diagram, then go ahead and try it. It may work well enough. I was suggesting an approach that's more likely to give you less electrical noise. You need to choose Form vs. Function.

Edited June 11, 2010 by Zmanco

[jacob80]

Easy there, I'm new to conceptual wiring, so give me some slack.

 

I do agree that things such as electric fans and fuel pumps should not be commonly grounded with megasquirt and things Ike a wideband.

 

Question: Are you not supporting my fan control via the fast idle relay that is already installed on my relay board?

 

My thought is that introducing multiple power sources makes troubleshooting even more complicated than it needs to be. I understand that circuits that draw a lot of amperage will create more noise than those that do not. I agree that instead of running a power lead all the way around the engine bay back to my terminal strip is probably not the best idea noise wise, but what if i use a shielded wire and take as many precautions to prevent noise?

 

Like I said, I am looking for simplicity and organization all while trying to maintain a solid, noise free EFI wiring harness. Thanks!

Edited June 11, 2010 by jacob80

[Zmanco]

Please don't get defensive, but you asked for an opinion on your approach, I told you that I would recommend a different approach and explained why. If you want to try it the way you've proposed, then please by all means do so. As I said in post 8, it may indeed work well enough. Part of the reason I have my Z is that it's a platform for me to try experiments and learn from them. So I encourage you to experiment away!

 

But please don't expect me to tell you what you want to hear - routing noisy high current loads across long lengths of wire is not the best approach - period. That doesn't mean you can't make it work, and if it's important to you for ease of troubleshooting and appearance, then go for it!

 

PS. Even with a larger monitor in my office, I still cannot read the labels on your diagram - I think the resolution of the picture is too low.

 

PPS. I use FIDLE to control the fan as well, but as I said in post 6, I mounted the relay on the radiator tab. The high current wires are on the order of a foot or so in length and only 1 wire goes back to MS. To me, that's a cleaner approach than routing power and ground to it as well.

[jacob80]

Please don't get defensive, but you asked for an opinion on your approach, I told you that I would recommend a different approach and explained why. If you want to try it the way you've proposed, then please by all means do so. As I said in post 8, it may indeed work well enough. Part of the reason I have my Z is that it's a platform for me to try experiments and learn from them. So I encourage you to experiment away!

 

But please don't expect me to tell you what you want to hear - routing noisy high current loads across long lengths of wire is not the best approach - period. That doesn't mean you can't make it work, and if it's important to you for ease of troubleshooting and appearance, then go for it!

 

PS. Even with a larger monitor in my office, I still cannot read the labels on your diagram - I think the resolution of the picture is too low.

 

PPS. I use FIDLE to control the fan as well, but as I said in post 6, I mounted the relay on the radiator tab. The high current wires are on the order of a foot or so in length and only 1 wire goes back to MS. To me, that's a cleaner approach than routing power and ground to it as well.

 

I think this is a good method to run the fan. Are you using a relay board? I'm under the impression that I could actually use the relay that has been built onto the relay board (which I don't use). Perhaps you're not running a relay board?

 

I'm with you on the noise thing, now I just need to figure out how to run wire to my big stuff all while keeping the runs fairly short.

[Zmanco]

If you are mounting the relay board in the engine compartment then by all means, try using the FIDLE relay for the fan. In my case I didn't like the idea of either cutting a big hole in the firewall to fit the DB37 connector through, or just building it up in the car. So I mounted the MS and Relay board under the passenger seat and ran the wires individually through the firewall. Hence it made no sense to run the heavy gauge wire all that way, and to have it side by side with low voltage sensors and possibly induce noise in them.

[jacob80]

Other than the fuel pump and fan, is there anything else i should be worried about giving off a lot of interference to my other sensors?

 

That's another thing. I would rather loom the big amperage sensors together to have them separate from my other sensors.

 

We have been running our relay board in the engine compartment ever since we started.

 

Thanks!

[Zmanco]

Fuel Injectors are another high current device to pay attention to, but it can be hard to route those wires away from the sensors. If you use a PWM Idle Air Control Valve, that's another one to be careful with. In my case I routed them with everything else and haven't had problems, but if you can run them away from the sensors that's better.

[jacob80]

The fuel injectors will be routed separately. The majority of my sensors that are up front (tsp, clot, iat, dizzy, etc) will be in one loom and run around the driver side rough the cabin and back over by the battery where the relay board is. My injecot harness is simply going to curve around the back of e motor and directly to the relay board.

 

Hopefully I won't end up with any noise with this configuration.

[jacob80]

It's most important that you have a common ground for the small signal devices such as MS and O2 sensors...

 

I was thinking about this today. Since my fuel pump and coil are powered through my relay board, would my MS be considered a big signal device?

[Zmanco]

I was thinking about this today. Since my fuel pump and coil are powered through my relay board, would my MS be considered a big signal device?

I'm not sure what you mean by "big signal device".

 

I used the term "small signal" earlier to refer to things like the O2 sensor where the output voltage needs to be measured precisely in the mV range. Things like motors (eFans) that pull tens of amps at 14V can generate electrical noise that can be on the order of many mV, so it's best to wire them separately when possible so the O2 signal doesn't get "drowned out" by the other noise.

[jacob80]

I'm not sure what you mean by "big signal device".

 

I used the term "small signal" earlier to refer to things like the O2 sensor where the output voltage needs to be measured precisely in the mV range. Things like motors (eFans) that pull tens of amps at 14V can generate electrical noise that can be on the order of many mV, so it's best to wire them separately when possible so the O2 signal doesn't get "drowned out" by the other noise.

 

My mistake. What I meant is what you said, items that will pull large amounts of amperage (fan, fuel pump, etc.)

 

So basically, keep things like the MS and 02 sensor off the same aftermarker fuse block as my fans, fuel pump, correct?

[Zmanco]

So basically, keep things like the MS and 02 sensor off the same aftermarker fuse block as my fans, fuel pump, correct?

If you can, that's better. As I said above, my fuel pumps are powered off the relay board which is mounted next to the MS under the passenger seat. I suppose I could have mounted a remote relay closer to the pumps, but this was good enough.

 

There are no black and white rules - just best practices. Follow them as well as you can, but don't go crazy unless there's a really good reason.