Crazy Wiring Issue!! I Need Help!!

Anthony Finuf · Jan 29, 2017

So i have a 88 GT and i have a cluster wiring issue. When i turn on the brights my temp gauge jumps up, my battery gauge goes down, my RMP gauge goes up and also the speedo. Also all of the lights shut off on the cluster. When i turn on the left blinker the arrow shows up but when i turn on the right the needle for RPM and the right side of the RPM gauge blink and there is no arrow. I've tried different MultiFunction switches but the current one the wipers dont work, another one the headlights didnt work, and the other one the brights stayed on. So i bought a brand new one and im waiting for it to come in. I've taken apart the cluster housing around the cluster to check for bad wires but nothing. I've taken apart the cluster as well and i didnt notice any bad connections inside. The plastic little backing that has all the copper connections had a few bent parts on it and i slightly fixed it but nothing happened. My low oil light is always on and i messed with the wiring a bit and i had no coolant and the light for that came on and the oil light went off and now the oil light is back on and no low coolant light. IDK if it matters but i have a 140MPH cluster and i think the 88 came with the 85MPH ones but correct me if im wrong. The low oil comes on on the cluster not on the little light housing right of the steering column. Im completely lost. Im looking into buying a 88 original 85MPh cluster to hope it fixes the problem but id like some input on it. I KNOW THERE IS ANOTHER THREAD SIMILAR TO THIS ONE BUT THERE WAS NO FIX!! I do think thats all the info. Oh also when i hooked up a LED bulb to the passenger headlight the cluster thing would go crazy without turning the brights on.

jrichker · Jan 30, 2017

The temp gauge is simple to fix and it may fix some of the other problems as well.

See step #2 below...

Grounds

This checklist applies to all Mustangs , not just the EFI equipped cars. Some of the wiring will be different on carb cars and carb conversions

Revised 28-Oct-2012 to add signal ground description & possible problems if it is bad

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.

Make sure that all the ground places are clean and shiny bare metal: no paint, no corrosion.

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. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

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

All the grounds listed in items 1,2 & 3 need to bolt to clean, shiny bare metal. A wire brush or some fine sandpaper is the best thing to use to clean the ground connections.

4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors. This ground is inside the computer and connects pin 46 to pins 40 & 60, which are the main computer grounds. If this internal computer ground gets damaged, you won't be able to dump codes and the car will have idle/stall/ performance problems

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.

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. .

Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections

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.

General karthief · Jan 30, 2017

I have found subtle differences in clusters between years, I think 89 is the first for the low fuel/oil lights in the lower right corner, not sure, I have several clusters in a box if you want to I will pick one close to your build date if you can't get yours figuared out.
PM me you need one

Mustang5L5 · Jan 30, 2017

Step #1. Identify the year of your cluster. There are two year spans. 87-89 and 90-93. The wiring is NOT interchangeable so if you install a 90-93 cluster in an 87-89, you need to rewire it or things get messed up.

So, does your cluster look like this
87-89 square flat bottom. The bars for the "dividers" between various gauges are flat against the face.

Or this 90-93 with a hump for the airbag wheel and an AIR BAG warning light on the tach. The bars between the gauges are a raised plastic rounded edge

You'll notice a big difference in the bottom "hump" between the two

Anthony Finuf · Jan 30, 2017

So i have looked at the ground wire behind the intake and it was fine i also ran a multi meter to it and it was fine as well. My cluster is the square bottom but the gauges aren't 85MPH?

Anthony Finuf · Jan 30, 2017

Correction it is the humped bottom cluster style.

Anthony Finuf · Jan 30, 2017

Would it maybe fix the problem if I switch clusters to a proper year?

jrichker · Jan 30, 2017

Anthony Finuf said:
So i have looked at the ground wire behind the intake and it was fine i also ran a multi meter to it and it was fine as well. My cluster is the square bottom but the gauges aren't 85MPH?

Did you do the voltage drop test on the ground wire? The connections cold be dirty or corroded and cause the same problem.

Anthony Finuf · Jan 30, 2017

jrichker said:
Did you do the voltage drop test on the ground wire? The connections cold be dirty or corroded and cause the same problem.

No not a drop test. How would I perform that?

Anthony Finuf · Jan 30, 2017

jrichker said:
Did you do the voltage drop test on the ground wire? The connections cold be dirty or corroded and cause the same problem.

whats weird is I've torn apart my whole dash dam near and I didn't find anything loose or frayed.

jrichker · Jan 30, 2017

Anthony Finuf said:
whats weird is I've torn apart my whole dash dam near and I didn't find anything loose or frayed.

I didn't ask you if you found anything loose or frayed, I asked you if you did the voltage drop test on the firewall to engine ground.

General karthief · Jan 30, 2017

@jrichker, he asked how to do the drop voltage test in the post before
#9
Also looks like he has the wrong year cluster from his description.

Anthony Finuf · Jan 30, 2017

I responded and said I hadn't performed a test such as that and asked how to perform that test. The second post was just something else.

Anthony Finuf · Jan 30, 2017

jrichker said:
The temp gauge is simple to fix and it may fix some of the other problems as well.

See step #2 below...

Grounds

This checklist applies to all Mustangs , not just the EFI equipped cars. Some of the wiring will be different on carb cars and carb conversions

Revised 28-Oct-2012 to add signal ground description & possible problems if it is bad

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.

Make sure that all the ground places are clean and shiny bare metal: no paint, no corrosion.

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. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

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

All the grounds listed in items 1,2 & 3 need to bolt to clean, shiny bare metal. A wire brush or some fine sandpaper is the best thing to use to clean the ground connections.

4.) All the sensors have a common separate signal ground. This includes the TPS, ACT, EGR, BAP, & VSS sensors. This ground is inside the computer and connects pin 46 to pins 40 & 60, which are the main computer grounds. If this internal computer ground gets damaged, you won't be able to dump codes and the car will have idle/stall/ performance problems

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.

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. .

Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V bolt together connections

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.

I know you put it on here but not sure how to test it with that specific ground wire?

General karthief · Jan 30, 2017

Anthony Finuf said:
No not a drop test. How would I perform that?

Lets see if I get this correct, connect positive side of volt meter to the positive battery post, connect the negative to the ground you want to check, crank engine noting voltage before and during cranking. I would think you want less than 1volt difference.

General karthief · Jan 30, 2017

I also think you have the wrong instrument cluster.

Anthony Finuf · Jan 30, 2017

karthief said:
Lets see if I get this correct, connect positive side of volt meter to the positive battery post, connect the negative to the ground you want to check, crank engine noting voltage before and during cranking. I would think you want less than 1volt difference.

I think that it's the wrong cluster may be causing the whole problem. I'm waiting for the new multi function switch to come in and I'm going to buy a cluster tomorrow for the proper year plus it'll give it the original look. I'll do the drop down test once I get home and post results. Should I also do it on the ground next to the washer fluid? I noticed in the picture that's the ground but in your writing you wrote that it's the ground behind engine?

General karthief · Jan 30, 2017

There are several grounds on an efi setup. The two mentioned and I think there is one by the ecm in the right kick panel, my brain is saying there is a fourth one but it's not telling me where it is.

jrichker · Jan 31, 2017

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. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

Voltage drop testing of connections and grounds.

Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 16-18 gauge wire for the test leads if you have to lengthen them.

Voltage drop increases with the increase of current in a circuit and it also increases with heat. Put a maximum current load on a bad wire or connection and it gets hot and drops more voltage across the wire or connection. As it heats up, resistance increases which makes more heat. Round and round you go in a vicious circle until something catches fire or fails.

Voltage drop testing must be done while the usual load is on the circuit. If it is a starter, it has to be tested while cranking the starter. If it is lights, A/C or fan, they must be turned on high while testing. Fail to do this and you will not get accurate results

1.) Most grounds use the negative battery post as their starting point. Keep this in mind when checking grounds.
2.) The voltage will be small if the ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) Follow the steps shown in the diagram...

5.) Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections (sensors are low voltage devices and small drops can have a large effect on the devices dependent on sensor accuracy)
0.0V Connections
A voltage drop lower that spec is always acceptable.
6.)
See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .

Anthony Finuf · Feb 1, 2017

Performed a drop test today. Everything seemed fine. I'm now a waiting on the new cluster to come in and see if that changes. I'm also going to solder new wire to the grounds and re ground them. I will update when I complete those tasks.

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