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Junior Charter Member
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Discussion Starter · #1 ·
After reading many suggestions, I decided to put my battery disconnect on the negative side of the battery. I then ran a ground wire directly from the battery to the computer memory ground (two heavy black/green wires). The theory is that the computer will always have power to the memory, but none of the rest of the car will have power. I also don't have to worry about a fuse or breaker across the disconnect as you do if the disconnect is on the positive side of the battery. Here is the problem. When the disconnect is open (key removed), and I turn on any switch or push a button, some of the relays around the car click on for a few seconds. It sounds like the fuel pump relay and the computer power relay are kicking on for those few seconds. Any ideas on this? I am wondering since the computer is grounded all the time, it is allowing current to "backflow" through it (like if it isn't isolated internally from the case or other ground source).
Thanks,
Dave
 

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FFCobra Fanatic
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I would check the computer memory ground for resistance between it and the normal computer ground. Check both directions (use + on mem ground, then switch leads and check again). If there is a path, you're risking running the entire car through that one wire. I can tell you that one wire with that much current will very quickly create a lot of smoke and vaporize the insulation on the wire.

The other reason that I would not wire the kill switch this way is because if you turn it off in a safety situation, there's no disconnect from the alternator. Some cars will continue to run with the kill switch off if there's enough capacitance in the system to keep the computer on.
 

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I'm with Jack on this one. Having everything on the car powered up with the only the dedicated computer ground as the path to the battery creates a strange brew. A Dr. Lukas type short can and will make the fuel pump run "backwards" with the key off by turning on the light switch.

Along with the rest of you I read the negative battery cable theory of relativity. The French used to import Beneteau sailboats to the US wired this way and every one of them were trouble. They now build some of their boats in the US but no longer use a negative battery switch for the marine electronics.

Everyone should build their FFR the way they want it. However, many of us have trouble free boats and FFRs by keeping it SIMPLE STUPID.
 

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Ahh, that's KISS: Keep It Simple Stupid. Something I have to remind myself of often.

Weren't the old MGB's and MG run with the Negative Battery Cable Theory and weren't they also a major pain in the arse? Here's a question: If it has the word "negative" in it can it be positive?

Good luck
 

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Another posibile source for short-term power to disconnected electronics is a filter capacitor holding a charge, if you have one. I have one for the MSD, but no computer, and see the same effect.
 

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Feng Shui Master
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The computer provide grounds to activate certain functions outbound of the computer. ie. to trigger a relay. If you have the positive applied but have provided a ground to the brain the other circuits with low inpedance will try to ground though the circuit with least resistance. (this may be the computer itself. I do not understand why could you not use the positive cable as your disconnect?
 

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My computer is a Haltech F-10, but I'm sure it operates the same as the Ford computer. Technology doesn't change much, just the application.

Anyway, the switches and relays are controlled by switching the ground side of the relay. With your jumper wire the circuit is always live. The whole car is now grounded through the computer.

The only thing you've done is decrease the amount of power/amps the circuits can carry. With the battery swith off, and the ignition key on, check around and see how many circuits are live.
 

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There are two separate ground circuits involved in the EEC-IV system: case and power. According to Ford, "case" refers to ground paths that use the chassis as the path back to the battery and "power" refers to ground paths that use dedicated copper wires as the path back to the negative terminal on the battery. Which ground did you attach directly to the battery? If you chose the "power" ground circuit, you chose wrong. The power ground circuit is for relays, sensors and switches that are either inputs or outputs for the EEC. The case ground is for the KAM. Pin #'s 40 and 60 (black-white and black-light green, respectively) are the power grounds feeding into the EEC. Pin #20 (black) is the case ground for the EEC...and for the KAM.

The correct ground wire to use is the "case" ground and ONLY the case ground. Pin 20, that is. The EMF shunt attached to the ring connector (the bare wire that meanders around the engine via the alternator harness) can be separated and grounded on the chassis, or on the switched sided of the disconnect switch. This is how I wire disconnect switches and it works fine.

Installing the disconnect switch isn't any more or less complicated than doing it on the positive side. It's simply not in people's mind to understand BOTH the positive side and negative side of electrical systems. Many European vehicle manufacturers use positive ground systems, as did Mack Truck Company (they may still). Also, most truck and off-road equipment manufacturers that use battery cutoff switches install them on the negative side, so it's not like you're going where no man has gone before... :rolleyes:

[ October 27, 2004, 12:06 AM: Message edited by: Mercury ]
 

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One additional note: make sure your ECA module is not touching the frame/chassis in any way. That also goes for the screws/bolts you're using to attach it to the firewall, etc...they cannot be allowed to touch both the ECA module and the chassis. The module needs to be totally insulated from any possible contacts to electrical paths. The best way is to use the original holster.
 

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Discussion Starter · #13 ·
Thanks for the input everyone. Mercury, I did connect the wrong grounds from the EEC. I will go and switch the ground to the Pin #20 ground. By isolating the ECA, are you talking about the computer (EEC)? I also used the plastic holster, but one of the holes I used is the one with the metal springy ground strip. I will have to remove that. Roger, it really is simple, I just picked the wrong ground wire because I don't have a schematic showing which grounds in the EEC are for the memory. If you think about it, it really doesn't matter where you break the electrical "loop". I just prefer this way so I don't have to put a fuse or resetable breaker across the cut-off. Many switches in the car work this way. For example, the horn button. When you push the button, it completes the circuit to ground. This is not the same as a positive grounded car.
 

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Cobra,
Yeah, "ECA" refers to the computer (Electronic Control Assembly) while "EEC" (Electronic Engine Control) refers to the entire system. It's the control assembly box I was referring to.

I forgot about the ground strip on the holster...you're correct in removing it. In addition to that, you may have to install a piece of rubber or some other kind of insulating material between the ECA and the mounting surface to ensure that it doesn't make contact. I use an OHM meter or diode tester (makes a beep if there's continuity) to make sure its insulated.

How far along are you with your wiring? If you want, I can see if I can email you a wiring schematic to help you along. I have access to the Mitchell OnDemand5 repair & troubleshooting system...it has excellent schematics. Just need the year and model (GT or LX) of your donor harness.

Regards,
Mercury
 

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Junior Charter Member
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Discussion Starter · #15 ·
Thanks,
I re-wired the grounds this weekend and everything is working great. I only have the pin 20 grounded all the time. A few more pieces of aluminum and I am ready to put the body on. I go-carted about 20 miles this weekend. Too much fun!
Dave
 
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