86 Gt Gremlins

[JimCzeb]

Gremlins with my Mustang.

Guys, I have been having some issues with my 1986 Mustang GT for a while now but recently I installed a Tweecer and discovered that it does not stay in closed loop mode once I start driving.
Current configuration - Engine 306, converted to Mass air (A9P), 5spd, E303 cam, 1.7 rollers, cobra upper and lower intake, slightly ported stock marine heads. 1 5/8 headers. 2.5” exhaust.

When I put the new engine in, (had B303 cam) the driver side exhaust was blacker than the PS as it would be if running rich. I swapped the O2 sensors and it had no effect. I bought a new O2 sensor and tried it in both places and still no effect. Cam was changed back to stock for that year and this year was changed to an E303. Still the same behavior. The Tweecer shows that the voltage for the driverside HEGO is always slightly higher than the PS. Note that this has remained constant even after pulling the engine again and changing upper and lower intake. I had built a HEGO output meter and it had been showing the values to be as expected. Recently the passenger side HEGO will run fine at times and will peg to max for a couple of seconds.

In 2007 installed a digital instrument cluster from Nordskog. If I have the headlights on and come to a stoplight and press the brake the speedometer will read around 250mph and randomly show lots of speeds. As soon as the car moves or I take my foot off the brake the speedometer reads correctly. It only does this if two things drawing high current are on such as headlights and brakes.
Everyone knows that Ford in its wisdom put the GT fog lights in the HEGO circuit. So, I wired a separate circuit for my fog lights. The lights are run from a solenoid connected directly to the battery and a switch for the lights in the dash only carries enough current to activate the solenoid. The fog lights still cause the speedometer to read several hundred mph.
Sometimes the speedo won’t go crazy if only one hi current item is on but if 2 or more are on. Headlights and brakes, Fog lights and brakes, Fan and brakes the speedo goes crazy.
Last year my Nordskog Speedo and tac got fried so, I installed a different manufacturer digital Speedo and tac. They behave the same way. Since then I have installed an electric fan, high current is almost always on. Again once I am moving the speedometer works correctly. Please note that this behavior happens even when the car is not running. Turn on the ignition, turn on headlights and the speedo goes crazy with the car OFF.
I have also changed the alternator in case that was injecting noise into the system and still no effect.

Anyone have any ideas? This has been driving me crazy for over 6 years. The next thing I am going to do is replace that whole fusible link mess with a fuse block.

Note that all my under hood wires are hidden in the fenders so that none are visible and the battery is in the passenger compartment.

Jim Czebiniak

 

[jrichker]

You have a bad missing or engine or to body ground. Check the O2 sensor ground while you are at it.

Grounds

Revised 26-Feb-2012 to add testing of voltage drops with maximum load on the circuit or connection under test.

Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.

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 or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972

Correct negative battery ground cable.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.


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

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 an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

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

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://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.

The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.

 

[JimCzeb]

You have a bad missing or engine or to body ground. Check the O2 sensor ground while you are at it.

Grounds

Revised 26-Feb-2012 to add testing of voltage drops with maximum load on the circuit or connection under test.

Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.

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 or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it.
Picture courtesy timewarped1972

Correct negative battery ground cable.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white wire.
You'll find it up next to the starter solenoid where the wire goes into the wiring harness.


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

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 an orange wire with a ring terminal on it. It is located in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or bolt on back of the cylinder head.

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

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://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. Be sure to have the maximum load on a circuit when testing voltage drops across connections. As current across a defective or weak connection, increases so does the voltage drop. A circuit or connection may check out good with no load or minimal load, but show up bad under maximum load conditions. .

Extra grounds are like the reserve parachute for a sky diver. If the main one fails, there is always your reserve.

The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop.

Thanks for the input. since my car has extensive wire rerouting the issue could be in one of many places. No connections have been intentionally eliminated but, ignorance can cause problems. This could take a while. Since I have another 5.0 in the garage I can check against stock.

Jim

 

[95BlueStallion]

Good luck man. I HATE electrical problems.

 

[jrichker]

Thanks for the input. since my car has extensive wire rerouting the issue could be in one of many places. No connections have been intentionally eliminated but, ignorance can cause problems. This could take a while. Since I have another 5.0 in the garage I can check against stock.

Jim

The clue is the rise in the temp indicator with the increase in electrical load. The main source of that problem is a bad or missing secondary engine power ground the runs from the rear of the intake manifold or driver side cylinder head to a spot just below the Baro sensor. Using the Fluke voltage drop testing method will prove my point.

 

[JimCzeb]

Ok, I will double check that ground. I have been judicious in making sure that is reconnected. The engine bay is very clean and this problem has persisted through 3 engine pulls and re-connections. Doesn't hurt to retest it though. I have a suspicion that It may be the computer ground connector mentioned above. I never knew what that was and when wiring was moved under the fender to a large standoff block I doubt I reconnected it. Will need to pull fender liner and check that out. Hope the computer end is still there. It would be nice if that solves the problem.
Thanks.
Jim

 

[Noobz347]

After you've completed JR's checklist above, take a look at grounds under the dash. I some similar problems on mine after pulling and reinstalling the dash. It turned out that I had stray voltage making it's way through the gauge cluster because there was a missing ground beneath the dash.