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Author Topic: Grounding question:  (Read 621 times)
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Rex Schimmer
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« on: January 06, 2019, 05:56:23 PM »

Duke and I are planning to go to an MSD "Midget" ignition for this year which will have a dedicated 16.5 volt battery for ignition only. All other electrical functions will be provided by a second on board battery, so the question is should each battery have its own ground, which for the ignition battery would be simply the plus and minus wires on the MSD box, or should we have a common ground for both systems? If we have a common ground this would allow us to put the master switch on the ground leg of all circuits and every thing would be turned off by the master switch. If we have the ignition circuit on a separate ground we will then need to install a power relay that is operated by power from the secondary system and turn off and on by the "Ignition" switch on the drivers control panel and then when the master switch is opened the relay will drop out and the ignition will be turned off. I do have reservations about having a common ground but the system is much simpler if we go that way.

Any good suggestions???

Rex
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Rex Schimmer
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« Reply #1 on: January 06, 2019, 06:08:56 PM »

Well I should have read the MSD info first as on the first page it says that the ground must be connected to the engine which in my case makes it common! DA

If you have other thoughts let me know.

Rex
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« Reply #2 on: January 06, 2019, 08:13:56 PM »

 Is the MSD from Milwaukee huh cheers
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« Reply #3 on: January 06, 2019, 09:09:38 PM »

I've always thought you were one of the more grounded people on the forum Rex! cool
If you have a common ground, the two systems are going to try & equalize their voltage.
Some older equipment & buses had dual voltage, 12 & 24v but they had to be separate.
Dual shutoff switches with a link between them to one control will do the job.
  Sid.
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« Reply #4 on: January 07, 2019, 12:23:35 AM »

... If we have a common ground this would allow us to put the master switch on the ground leg of all circuits and every thing would be turned off by the master switch...
Is there a starter? If there is, an extremely high current switch would be required.
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Rex Schimmer
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« Reply #5 on: January 09, 2019, 03:18:17 PM »

Jack and Sid,
Yes Jack there is a starter involved and Sid I think that I agree with you regarding having a common ground with a 12 volt and 16.5 volt systems. I am now thinking about having a completely separate system for the starter that will only work when I plug in the external booster battery. I am also checking with an electronics friend that I am sure knows how to do it.

Rex
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« Reply #6 on: January 09, 2019, 03:37:24 PM »

FWIW
     On the street roadster we run one 16 volt battery, it supplies the starter, MSD digital 6 ignition, pro comp tachometer/shift light and an AEM data recorder and sensors. At Bonneville we charge the battery every day at El Mirage one charge is good for a weekend. Note this is with a mechanical controlled leak fuel injector and no engine management system.
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« Reply #7 on: January 09, 2019, 03:52:10 PM »

Just a quick bit of input on this - I think I'm reading this right.

Having just installed the Holley EFI system in the Midget, consistently in the documentation, they warn against grounding the efi-ignition ECU to a common ground and suggest that the best practice is to take an individual ground lead and individual hot lead directly to the battery.  This is to provide clean, non-noisy power to the ECU and minimize potential miss-reads.

Now you and I know that by doing this, we have made a "common ground" at the battery terminal, but what I think this layout does is assures that if there is a ground error in the ECU, it makes it easier to trace, and if there is an error in the rest of the ground system, any spikes or drops will be, in part, somewhat absorbed by the capacitance of the battery before it effects the brain box. 

Additionally, according to Holley, one risks frying the ECU is you accidentally crank the engine with a noisy battery charger hooked up.

It's so important that Holley won't warrant the box of you take it to a common ground.

Seeing as you've got two complete circuits to deal with, I'd say isolate the one with the most sensitive electronics, and minimize the common ground by taking a direct wire from the block to the negative post of the 16.5 V.

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Rex Schimmer
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« Reply #8 on: January 09, 2019, 06:25:40 PM »

The MSC controller that I have (model 6214, Programmable Midget Ignition Controller) has a pair of "power" wires, one red and one black. They are to be connected directly to the battery + and - terminals but they also require that the battery be grounded to the engine. I have just talked to  a close friend that is a great electrical engineer and is very "hands on" and he says that having the ignition power battery (16.5 volts) and the on board 12 volt battery both grounded to a common point (the engine) will not be a problem. So with that info I will use a common ground.

Rex
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« Reply #9 on: January 09, 2019, 06:51:15 PM »

Rex,
Common ground point is the best option for all cases except where ignition EMI spray is messing with digital electronic's. Then the digital stuff needs to DC float.  Meaning a separate power source and >10K ohms between the two system commons.
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Rex Schimmer
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« Reply #10 on: January 10, 2019, 07:11:35 PM »

Good info Pops which is the reason that we avoid any of that digital stuff! Constant flow (sprinkler ) injection rules!

Rex
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