good grounds - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 12 Old 05-27-2016, 09:52 AM Thread Starter
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good grounds

Doing a battery in truck relocation. Does anyone have a good rundown on proper grounding, for the entire system. ECM ground, engine block, ect. What should go where.

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post #2 of 12 Old 05-27-2016, 10:13 AM
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My battery ground cable goes to the bolt on the rear sub-frame that held the quad shock. I use a 0 gauge ground cable. The engine is solid mounted to the K-member so the block is in extension grounded to the frame. If not using solid mounts, then a ground cable from the block to the K-member will be needed. My Fast XFI ECU has the main ground cable tied/bolted to the roll cage that is welded/bolted to the chassis. Do not use undersized cables. My main negative and positive cables are all 0 gauge. The longer the cable, the bigger the gauge should be.

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post #3 of 12 Old 05-27-2016, 11:41 AM
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^ this. 0 welding cable for both + and -. Ground through grommeted hole in trunk to quad shock frame bolt secured with P clamps. Make sure all grounds are clean, shiny and tight.

+ cable under the car secured with P clamps up to front of car. I have an inline 175 amp fuse on the + cable about a foot out from the battery box. Some guys use a breaker switch.

I'd also if not done so, I'd also install a mini starter
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post #4 of 12 Old 05-27-2016, 12:25 PM Thread Starter
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What configuration for the ECM ground? The reason I ask is I'm getting wildy erratic behavior/readings when datalogging. RPM readings have shown 8-10,000 at times, load% are jumping from 10%-90% at idle. Everything I have read is 99% of computer issues are actually grounding problems.
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post #5 of 12 Old 05-27-2016, 12:42 PM
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Quote:
Originally Posted by boostup89 View Post
What configuration for the ECM ground? The reason I ask is I'm getting wildy erratic behavior/readings when datalogging. RPM readings have shown 8-10,000 at times, load% are jumping from 10%-90% at idle. Everything I have read is 99% of computer issues are actually grounding problems.
Grounding could be an issue. Another common thing on these old PCMs are bad capacitors. Pull your PCM and take off the cover to inspect those capacitors. If all is original there's a very good chance some are not in the best of shape.

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post #6 of 12 Old 05-27-2016, 12:51 PM Thread Starter
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ECM was swapped for a known good one. The car straightened out 100% better for about 5 min. Then started acting up again.
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post #7 of 12 Old 05-27-2016, 04:38 PM
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As an electrical engineer in the automotive industry, I can tell you that the best thing to do would be to run the ground up to the engine block. Running it through the frame has more resistance and therefore more voltage drop. You also lose power on power circuits. The frame still needs to have a connection to B- though because other circuits in the car are grounded to the frame.

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post #8 of 12 Old 05-28-2016, 07:09 PM Thread Starter
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I'm 99% sure I have found my issue. The ECM ground wire behind the passenger side kick panel looked fine, but I had never removed it before. Today when I took it loose it was a nice shade of green. Cleaned it up and now the car runs consistent enough to try and dial everything in. Good grief I know better to overlook ground wires and this one cost me a week's worth of head scratching, but oh what a relief.
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post #9 of 12 Old 05-28-2016, 07:19 PM
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I'm 99% sure I have found my issue. The ECM ground wire behind the passenger side kick panel looked fine, but I had never removed it before. Today when I took it loose it was a nice shade of green. Cleaned it up and now the car runs consistent enough to try and dial everything in. Good grief I know better to overlook ground wires and this one cost me a week's worth of head scratching, but oh what a relief.

you can check all ground potentials without removal

using a dvom on voltage just measure on the bolt head with one lead, and batt negative with the other

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post #10 of 12 Old 05-29-2016, 11:21 AM
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Good find, that one is very overlooked. One thing I have noticed all foxbody's I have worked on have the ground attach hardware with a green tinted coating. You will see that on other grounds on the car also.

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post #11 of 12 Old 06-01-2016, 10:04 PM
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Quote:
Originally Posted by nathan67stang View Post
As an electrical engineer in the automotive industry, I can tell you that the best thing to do would be to run the ground up to the engine block. Running it through the frame has more resistance and therefore more voltage drop. You also lose power on power circuits. The frame still needs to have a connection to B- though because other circuits in the car are grounded to the frame.
As a managing electrical engineer formerly in the automotive industry, as an aftermarket ancillary electronics equipment designer, and as someone who routinely solves problems other people cannot seem to fix, I disagree.

The body shell in a typical unibody car has FAR less resistance from from to rear of the vehicle than any practical cable could ever have. I have seen this over and over again with direct measurement. It also calculates that way if you compare resistivity of steels to copper, and then calculate the cross sectional area of the body and compare it to a practical cable.

Not only does the body have less resistance, it has radically lower impedance for high frequency signals like noise.

It is foolish to run a cable from a trunk battery to the block. I can *prove* this with either measurements or by calculation, as can anyone who know how to measure things.

The best possible noise ground and vehicle distribution in a unibody car is the chassis itself. If the battery is near the engine, then a block connection makes sense for the start and charge currents ONLY. Any distribution should be chassis centric, and any sensitive devices should be chassis centric.

To my point is this:

Quote:
I'm 99% sure I have found my issue. The ECM ground wire behind the passenger side kick panel looked fine, but I had never removed it before. Today when I took it loose it was a nice shade of green. Cleaned it up and now the car runs consistent enough to try and dial everything in. Good grief I know better to overlook ground wires and this one cost me a week's worth of head scratching, but oh what a relief.
Ford uses the CHASSIS as the noise reference ground. They do this for good reason. If you lose that chassis connection, even though the EEC is grounded to the fender at the battery ground point, the system will generally noise up.


If the battery is in the rear, the block negative return becomes too long. The best solution is a good solid short block to subframe ground in front, and a good solid ground from the battery negative to frame or shell at the battery.

Nothing should ever tie to a battery negative except the block ground and/or the chassis ground. Never. The only thing going to the battery negative should be the block and/or chassis ground.

My 1989 LX coupe has a 00 cable from battery to front. That 00 cable has about three times the resistance of my chassis path. This is using a four wire measurement technique. My car is chassis centric grounded.

If my battery was in front, the starter and charging would be block centric and the chassis would be the distribution for all other electronics.
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post #12 of 12 Old 06-10-2016, 12:19 PM Thread Starter
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Good write-up. Also discovered my ECM was bad. Had no tps, ect, act, readings when datalogging.
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