78 f250 electrical problems
Lead Driver
Joined: Nov 2006
Posts: 5,217
Likes: 506
From: Pa.
I know you need to get your rig going but on something like this it's best to work with one person and FMC 400 is the best on diagnosing/helping with electrical problems .. He's on here usually every day .. Just listen what he says and you'll have it fixed soon
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Please excuse my absence; my work schedule has been demanding lately.
This says that the instrument cluster as a whole is not getting grounded properly. That explains the full-scale reading on the fuel gauge. There is a solid BLACK wire within the cluster connector responsible for ground. This bolts to dash metal with an eyelet terminal. Without this connection secure, the fuel gauge will peg to full scale, the cluster lights will not work properly, and there will be interactions among everything within the cluster itself.
The negative number is a sign that something else in the truck is turned ON.
What switch was ON? The headlight switch? My instructions early on where key OFF, lights OFF. The multimeter measures resistance by applying a voltage and measuring the current. It's an active device. You don't want any interaction from voltage applied by the TRUCK's electrical system. This probably explains the negative number you saw.
In short: no, you did not do this correctly. All your readings are suspect. Based on what you appear to be working with and the difficulty we seem to be having in relaying instructions from me to you, my recommendation is that you trace out the three connections by hand. This includes the instrument cluster ground described earlier, and the two bare wires. You will probably have to peel back harness wrap for this.
Your goal is to ensure a proper ground connection from the cluster to bare metal behind the dash, and the origin of the two bare wires presumably hacked up by a previous owner. There is most likely a relationship among all three.
This says that the instrument cluster as a whole is not getting grounded properly. That explains the full-scale reading on the fuel gauge. There is a solid BLACK wire within the cluster connector responsible for ground. This bolts to dash metal with an eyelet terminal. Without this connection secure, the fuel gauge will peg to full scale, the cluster lights will not work properly, and there will be interactions among everything within the cluster itself.
What switch was ON? The headlight switch? My instructions early on where key OFF, lights OFF. The multimeter measures resistance by applying a voltage and measuring the current. It's an active device. You don't want any interaction from voltage applied by the TRUCK's electrical system. This probably explains the negative number you saw.
In short: no, you did not do this correctly. All your readings are suspect. Based on what you appear to be working with and the difficulty we seem to be having in relaying instructions from me to you, my recommendation is that you trace out the three connections by hand. This includes the instrument cluster ground described earlier, and the two bare wires. You will probably have to peel back harness wrap for this.
Your goal is to ensure a proper ground connection from the cluster to bare metal behind the dash, and the origin of the two bare wires presumably hacked up by a previous owner. There is most likely a relationship among all three.
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Senior User
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Yes the light switch was turned on. Wire 2 is a black wire coming off the instrument cluster connector. I will redo these tests with key off and light switch off. So from what you said, wire 2 would be the cluster ground? I will trace wire 1 to where ever it goes. But wire 2 comes out of the connector. Sorry for the misunderstanding not much experience with wiring.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
No worries. I don't know what the two bare wires are; that's my whole point. My original suggestion was to "smoke" them out using the meter, but at this point I think it's just best to peel things back and see where they're coming from. That will eliminate all doubt. Not too uncommon to have to "research" a previous owner's "artwork."
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New tests:
Wire 1: key off switch off 2.0 ohms
Wire 2: key off switch off 34 ohms
Regulator: key off switch off 2.0 ohms
All 3 tests done with black wire on multimeter on the bare metal behind dash.
Wire to wire is 34 ohms key off switch off. Wire 2 is in the top row, 2nd slot from the right in the connector. Top row meaning towards the top of the cluster.
Wire 1: key off switch off 2.0 ohms
Wire 2: key off switch off 34 ohms
Regulator: key off switch off 2.0 ohms
All 3 tests done with black wire on multimeter on the bare metal behind dash.
Wire to wire is 34 ohms key off switch off. Wire 2 is in the top row, 2nd slot from the right in the connector. Top row meaning towards the top of the cluster.
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Wire 1 I followed to a taped connection of 2 more black wires. Other side of this connection 4 black wires come out which continues down the main harness.
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After a little more digging I found that wire 2 on the connector has 2 black wires coming out of the socket. The other wire is solid and follows the rest of the wires down the harness. The instrument cluster connector is each socket suppose to have 1 wire only or does 1 socket have 2 wires. If only 1 then I can remove the cut wire as I dont believe it should be there. The other wire in this socket I will look at again today to confirm the color.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Don't cut any wires coming from the cluster connector. Cutting wires is what gets folks into messes like this one.
Going back to the original problem, you've indicated that the gas gauge pegs FULL regardless of the level of fuel in the tank. You've also indicated that there are some solid BLACK wires hanging below the dash, and your investigation indicates that these wires are probably coming from the factory harness.
Since a bad cluster ground is a common cause of the gauge pegging FULL, one would guess that the free-hanging wires are actually what's left of the cluster ground. However, your results indicate minimal resistance between the ICVR body (where it seeks ground) and bare metal behind the dash (actual ground). Coupled with the fact that your cluster illumination works properly, this would indicate that the cluster is in fact grounded properly. That means the bare wires you're seeing are 1) ground wires, but not part of the instrumentation, 2) not ground wires at all, or 3) something added by the previous owner after all.
So:
1) Gas gauge reads almost FULL: a common cause is bad cluster ground, but your measurements seem to eliminate that possibility. Try unplugging the sending unit at the tank. If the level drops, the problem is the sending unit. If the level does not drop, there's a short somewhere downstream of the gauge, between the gauge and the sending unit.
2) Bare wires: I honestly don't know what to tell you at this point. It's anybody's guess as to what the last owner had in mind. I'm sorry; I don't think we're going to be able to ID these over the internet. The best I can tell you is to try and trace them back by hand. Your latest descriptions say they continue down the main harness, but that doesn't tell me anything. You need to see where they go. If they're obviously part of the original harness and not something added by the last owner, then you'll have to go by jacket color. Solid BLACK is ground, but note that many things can look solid BLACK when they are in fact not (for example, solid BLACK with a BLUE stripe or faint dots).
Going back to the original problem, you've indicated that the gas gauge pegs FULL regardless of the level of fuel in the tank. You've also indicated that there are some solid BLACK wires hanging below the dash, and your investigation indicates that these wires are probably coming from the factory harness.
Since a bad cluster ground is a common cause of the gauge pegging FULL, one would guess that the free-hanging wires are actually what's left of the cluster ground. However, your results indicate minimal resistance between the ICVR body (where it seeks ground) and bare metal behind the dash (actual ground). Coupled with the fact that your cluster illumination works properly, this would indicate that the cluster is in fact grounded properly. That means the bare wires you're seeing are 1) ground wires, but not part of the instrumentation, 2) not ground wires at all, or 3) something added by the previous owner after all.
So:
1) Gas gauge reads almost FULL: a common cause is bad cluster ground, but your measurements seem to eliminate that possibility. Try unplugging the sending unit at the tank. If the level drops, the problem is the sending unit. If the level does not drop, there's a short somewhere downstream of the gauge, between the gauge and the sending unit.
2) Bare wires: I honestly don't know what to tell you at this point. It's anybody's guess as to what the last owner had in mind. I'm sorry; I don't think we're going to be able to ID these over the internet. The best I can tell you is to try and trace them back by hand. Your latest descriptions say they continue down the main harness, but that doesn't tell me anything. You need to see where they go. If they're obviously part of the original harness and not something added by the last owner, then you'll have to go by jacket color. Solid BLACK is ground, but note that many things can look solid BLACK when they are in fact not (for example, solid BLACK with a BLUE stripe or faint dots).
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Don't cut any wires coming from the cluster connector. Cutting wires is what gets folks into messes like this one.
Going back to the original problem, you've indicated that the gas gauge pegs FULL regardless of the level of fuel in the tank. You've also indicated that there are some solid BLACK wires hanging below the dash, and your investigation indicates that these wires are probably coming from the factory harness.
Since a bad cluster ground is a common cause of the gauge pegging FULL, one would guess that the free-hanging wires are actually what's left of the cluster ground. However, your results indicate minimal resistance between the ICVR body (where it seeks ground) and bare metal behind the dash (actual ground). Coupled with the fact that your cluster illumination works properly, this would indicate that the cluster is in fact grounded properly. That means the bare wires you're seeing are 1) ground wires, but not part of the instrumentation, 2) not ground wires at all, or 3) something added by the previous owner after all.
So:
1) Gas gauge reads almost FULL: a common cause is bad cluster ground, but your measurements seem to eliminate that possibility. Try unplugging the sending unit at the tank. If the level drops, the problem is the sending unit. If the level does not drop, there's a short somewhere downstream of the gauge, between the gauge and the sending unit.
2) Bare wires: I honestly don't know what to tell you at this point. It's anybody's guess as to what the last owner had in mind. I'm sorry; I don't think we're going to be able to ID these over the internet. The best I can tell you is to try and trace them back by hand. Your latest descriptions say they continue down the main harness, but that doesn't tell me anything. You need to see where they go. If they're obviously part of the original harness and not something added by the last owner, then you'll have to go by jacket color. Solid BLACK is ground, but note that many things can look solid BLACK when they are in fact not (for example, solid BLACK with a BLUE stripe or faint dots).
Going back to the original problem, you've indicated that the gas gauge pegs FULL regardless of the level of fuel in the tank. You've also indicated that there are some solid BLACK wires hanging below the dash, and your investigation indicates that these wires are probably coming from the factory harness.
Since a bad cluster ground is a common cause of the gauge pegging FULL, one would guess that the free-hanging wires are actually what's left of the cluster ground. However, your results indicate minimal resistance between the ICVR body (where it seeks ground) and bare metal behind the dash (actual ground). Coupled with the fact that your cluster illumination works properly, this would indicate that the cluster is in fact grounded properly. That means the bare wires you're seeing are 1) ground wires, but not part of the instrumentation, 2) not ground wires at all, or 3) something added by the previous owner after all.
So:
1) Gas gauge reads almost FULL: a common cause is bad cluster ground, but your measurements seem to eliminate that possibility. Try unplugging the sending unit at the tank. If the level drops, the problem is the sending unit. If the level does not drop, there's a short somewhere downstream of the gauge, between the gauge and the sending unit.
2) Bare wires: I honestly don't know what to tell you at this point. It's anybody's guess as to what the last owner had in mind. I'm sorry; I don't think we're going to be able to ID these over the internet. The best I can tell you is to try and trace them back by hand. Your latest descriptions say they continue down the main harness, but that doesn't tell me anything. You need to see where they go. If they're obviously part of the original harness and not something added by the last owner, then you'll have to go by jacket color. Solid BLACK is ground, but note that many things can look solid BLACK when they are in fact not (for example, solid BLACK with a BLUE stripe or faint dots).
Wire 2 is a cut off wire coming out of the instrument cluster connector. In the same pocket as wire 2 on the connector a solid black wire comes out as well. This wire goes from the connector, up and around the dash to my heater box. Ok do you have a wiring diagram I can look at to see if wire 2 was original or added by the p.o? My connector sais these 2 wires are in pocket 6.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Nowhere in my instructions do I ask you to observe the fuel gauge's behavior with the mystery wires grounded. In fact, I highly recommend against applying power or ground to mystery wires; that's a good way to start an electrical fire.
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
Thread Starter
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Senior User
Joined: Apr 2014
Posts: 271
Likes: 0
Nowhere in my instructions do I ask you to observe the fuel gauge's behavior with the mystery wires grounded. In fact, I highly recommend against applying power or ground to mystery wires; that's a good way to start an electrical fire.
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
The P.O was a 78 yr old guy that gave the truck to his 15 yr old grandson 6 months before I bought it. Everything that ive seen is original besides an aftermarket radio the grandson added which ive removed. A tach which I installed. And a few connections under the hood that are a little fishy when compared to my 74 half ton. But I know these 2 trucks are wired totally different.
Thread Starter
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Senior User
Joined: Apr 2014
Posts: 271
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Nowhere in my instructions do I ask you to observe the fuel gauge's behavior with the mystery wires grounded. In fact, I highly recommend against applying power or ground to mystery wires; that's a good way to start an electrical fire.
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
My point is this: I don't think I'm going to be able to help you identify these mystery wires. That's the one thing I can't do over the internet, is sort out a hackjob by a previous owner. The best I can tell you is that if they are truly solid BLACK, and come out of the harness, they are more than likely one end of a ground wire. Either it's the ground connection of something upstream in the dash that needs to be grounded, or it's ground itself.
I'm having a tough time following what you're saying because you introduce too many variables at once. When it comes to electrical, it's imperative you only change one thing at a time. Specifically, back to the fuel gauge issue:
The numbers are changing. One time you've got the switch on, then off, and sometimes you've got mystery wires grounded, sometimes not, or I'm not sure what. At no point in this thread have I asked you to ground mystery wires, so why are you peppering that variable into your experiments?
You're hooking up wires, asking to cut wires, replacing clusters - none of which are part of any of my recommendation. That's why this thread is three pages long and 6 weeks old. Let's try this one last time:
1) You've stated your fuel gauge pegs at full scale. Coincidentally, you've indicated that there are multiple BLACK wires that appear to be factory, hanging below the dash. My first thought is that the cluster is not grounded properly, and maybe these dangling wires have something to do with that.
2) Leave the BLACK wires alone. We don't know what they're for, and we don't want to see what they'll do yet. We're going to find out IF they're related and can only do that if they're left alone.
3) Turn the truck completely OFF, as dead as it can get. Doors closed, headlights off, all OFF as if the battery was out of the truck.
4) Connect the cluster.
5) Press one meter lead into the ICVR body, and the other meter lead into bare metal behind the dash. Write down the number.
6) Leave the truck and take your meter with you. Press the two meter leads together and see what the meter says. It should be less than an ohm.
If the numbers in steps 5 and 6 are about the same, then assume the cluster is grounded properly. Unplug the sending unit and see if the gauge is still pegged.
Note that at no point in any of these instructions are you being asked to do anything with either mystery wire. Do not connect them to anything, do not tie them together, do not let them touch bare metal.
Lastly:
I thought you said the dash lights work after replacing the cluster? What changed?
Step 6 lead to lead was under an ohm
Both tests done as you said in step 3. No wires were touched as you said in step 2.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
OK then, next: