ECU Ground

[stang22]

Daggar is going to shoot me, but...

There are 2 wires connected to the ECU ground. One is the ECU ground (imagine that), and the other is ground wire going to the battery. When I relocated my battery to the trunk, I cut the small wire coming from the battery to the ECU ground, but the ECU is still grounded. Becuase my battery is now in the trunk and grounded to the frame, do I need to re-connect that ground I cut? I grouned the engine to the sway bar mount, but that one ground is bothering me now. I am going to double check all of my grounds today.

All of this is stemming from my coil not getting any fire and I am at just about a complete loss of what else to do.

Some pics from another post of someone elses car of the ground I am reffering to in its stock location. The one with the quick disconnect (ECU ground) is still connected.

Tim

 

[jrichker]

All the grounds need to be connected, especially the computer's power ground at the battery.

Rear mounted battery ground wiring. Follow this plan and you will have zero ground problems.

One 1 gauge or 1/0 gauge wire from battery negative post to a clean shiny spot on the chassis near the battery. Use a 5/16” bolt and bolt it down to make the rear ground. Use a 1 gauge or 1/0 gauge wire from the rear ground bolt to a clean shiny spot on the block.

One 4 gauge wire from the block where you connected the battery ground wire to the chassis ground where the battery was mounted up front. Use a 5/16” bolt and bolt down the 4 gauge engine to chassis ground, make sure that it the metal around the bolt is clean & shiny. This is the alternator power ground.

Crimp or even better, solder the lugs on the all the wire. The local auto stereo shop will have them if the auto parts store doesn't. Use some heat shrink tubing to cover the lugs and make things look nice.

The computer has a dedicated power ground wire with a cylindrical quick connect (about 2 ½”long by 1” diameter. It comes out of the wiring harness near the ignition coil & starter solenoid (or relay). Be sure to bolt it to the chassis ground in the same place as you bolted the alternator power ground. This is an absolute don’t overlook it item for EFI cars

here's a list of the important grounds...

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.

4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or head bolt.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.

See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103#volt for help troubleshooting voltage drops across grounds

 

[stang22]

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

Joe,

How is the foil shield grounded?

All of my grounds checked out perfect, but here are the voltage reading from my TFI connector harness. My wires are different colors, but you get the idea. From top to bottom:

Blue: 4.26v
Yellow: 12.00
Red/wht: 0, but 7.5v when trying to start (battery needs to be charged).
Red/grn: 12.00 Power to the coil.
Grn/wht: 1.56 negative side of the coil?
Blk/red: 4.56 Ground?

The wire in question is the grn/wht wire that runs to the negative side of the coil. That's the one that is losing me. Any help would be great, thanks!

Tim

 

[DMAN302]

Hey Tim,
Aside from relocating the battery cable have you made any other changes in wire routing?..Is the ground possibly not making good contact due to paint on the frame?. No other changes if you have the ECU ground, and the neg battery tail ground in place then you should be getting fire...unless the TFI decided to punk out on you..as mine did without warning. Also my car would not even try to turn over with less than 11.5V reading on battery...thought this was strange but true.

 

[stang22]

I have tripled checked all of my grounds and they are all perfect. The car turns over just fine, but no spark. Other than deleting the A/C, I haven't made any changes to any wiring whatsoever. I had the TFI tested at AutoZone and they said it checked out ok, but I still don't trust it. My friend is bring his TFI to me today so I can test it. If it does the same thing, I am going to plug his computer into mine and give it a whirl.

If the ignition switch was bad, would that in anyway prevent me from getting spark or fire? I am a little curious about the voltage that the wires connecting to the TFI are pulling. In particular, the one labled grn/wht. I think that is a ground (pulse?), but not sure how it works.

Tim

 

[DMAN302]

Tim I sent you a PM before actually reading you last post here..really sounds like a TFI. Can you check to see if the computer is pulling codes..may prevent you from having to pull the computer.

 

[stang22]

Nope no codes.

Here is what I am trying to figure out:

My yellow/tan wire (5 from the top of the TFI) goes from the TFI to the coil and splits off and has an ignition suppressor in between it before it goes to the computer. The wire after the suppressor goes to pin 4 on the computer. The supressor keeps high voltage out of the computer? With the key on, the yellow/tan wire on the plug that goes into the TFI is getting about 1.5volts. The negative side of the coil is getting 1.5volts as well. The positive going to the coil is getting 12volts. On the other side of the ignition supressor (which I can't locate), nearest the computer going into pin 4, it is getting around 2.5volts.

The problem is where or how in the world does the yellow/tan wire that goes to the TFI and coil get its 12 volts from? That is my delima right now.


Tim

 

[jrichker]

The foil shield connects to the case ground near the computer under the passenger side kick panel.

At this point if you have 12 volts at the red/lt green wire on the coil, either the TFI module or PIP sensor inside the distributor are bad.

See http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp for more wiring diagrams

 

[90mustangGT]

Good thread, something often overlooked.

This week I am in OBDII courses and voltage drops across grounds was about all we talked about today. A bad ground can cause all kinds of issues and really mess with sensors. I always knew it was important but didn't know just how much. Even a few milivolts of a drop can greatly effect sensors. I think I will go all out re-doing all the grounds in my car since I plan to stay EFI.

 

[stang22]

At this point if you have 12 volts at the red/lt green wire on the coil, either the TFI module or PIP sensor inside the distributor are bad.

Something just doesn't add up with the voltage going through the negative side of the coil and to the TFI. Even though I have tested the TFI and it passed, I am not ruling it out. But, even with the connection to the TFI disconnected I have not getting 12volts to the #5 wire on the TFI and negative side of the coil.

If I am getting this right, the coil sends power from the positive side to the negative side and 12volts to the TFI #5 wire. I am only getting 1.5 to the negative side of the coil as well as the #5 wire to the TFI. That would lead me to think that the coil is bad, but have 2 good coils. I replaced the PIP yesterday.

Just when I think I am getting closer...

Tim

 

[jrichker]

A voltmeter is handy if you are familiar with how to use it to find bad connections. Do a voltage drop test across the connections to the coil. Disconnect the TFI module & ground the tan/yellow wire. All the voltage should drop across the coil. Measure the voltage drop across a connection: more than .5 volts across a connection indicates a problem.
See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103 for help

 

[stang22]

It is FIXED!

Like Dagger said, "You watch, it's going to be something really stupid or simple".

Well, I took the coil out and tested the ohms from pos to neg and it was 0. I checked the negative side again just to make sure it was still only getting 1.5volts versus 12volts. The negative side connector was getting 12volts, but the wire itself was still only getting 1.5volts. The wire going to the metal connector in the clip looked ok, it looked connected, but I decided to take it apart. The wire cover was attached really well and I gave it a good tug to make sure it wasn't going to come out. Well, it turn out that either the wire inside either retracted back into wire cover or the cover expanded itself and the wire was not making contact with the connector. Simple fix!

I feel like an idiot but at least it is fixed. I learned a heck of a lot about voltage, ohms, electricity the way the computer operates and how to troubleshoot a problem the right way. I used to hate electrical problems, but I definately feel a lot more comfortable trying to fix them and understand them.

I owe a huge "Thanks" to Daggar, Joe Richkter, Tom (TMOSS), JT (Hissin) and anyone else who gave me any pointers along the way. Once again, "Thank You".

Tim