whats up w/ dash gauges

[XMAN JR]

On my dad 68 GTX the fuel &temp gauges don't work. Now some time ago I changed the fuel flout & then the fuel gauge worked. Now this year it doesn't. When you turn the key on it moves up to the first line & stops. There is over 5 gals. of gas in it. I tested the gauge by grounding it & works then. Now for the temp gauge it never worked. My dad tested the wire under the hood & it was fine. He tried several different sending units. Could it be the voltage limiter ? I also grounded out the temp gauge wire & the gauge works

 

[FrnkNsteen]

Low voltage from the voltage limiter should cause all of your gauges to run low. I believe the output from the voltage limiter feeds 5volts through your control board to the Fuel Gauge, the Temperature gauge, and the oil pressure gauge through the same strip on the control board. If only two of them are not working, I wouldn't think it would be the voltage limiter.

It could be bad wiring to the gauges from the sensors. The lower the resistance, the higher the gauge reads, so if you had an issue with the wires causing high resistance, that would cause your gauges to read low or not at all. You said your Dad tested the wires under the hood,... What did he test? Resistance through the wire to the control board?

When I test my gauges on the bench, I run 5 volts to the hot side of the gauge, then put various resisters (10ohm, 23ohm, and 75ohm) inline between the ground side and the ground source. The 10ohm should bring the gage to full range (not pegged, but at the top line). The 23ohm should bring the gauge to about middle position, and the 75 ohm should just barely move the needle. There shouldn't be any, or very little resistance between the sensor wire and the connector to the control board.

Is it possible the sensors are not grounding properly? I could be wrong, but I believe the sensor needs to ground properly and the resistance changes with changes in pressure, temperature, or fuel level. Maybe you are having grounding issues at the sensors and it's not actually the sensors themselves that are bad. Did someone use thread sealant on the sensors that is insulating it? Could your fuel level sensor not be grounded properly. My Barracuda has a ground strap from the pickup unit to the chassis to ensure grounding.

 

[XMAN JR]

Low voltage from the voltage limiter should cause all of your gauges to run low. I believe the output from the voltage limiter feeds 5volts through your control board to the Fuel Gauge, the Temperature gauge, and the oil pressure gauge through the same strip on the control board. If only two of them are not working, I wouldn't think it would be the voltage limiter.

It could be bad wiring to the gauges from the sensors. The lower the resistance, the higher the gauge reads, so if you had an issue with the wires causing high resistance, that would cause your gauges to read low or not at all. You said your Dad tested the wires under the hood,... What did he test? Resistance through the wire to the control board?

When I test my gauges on the bench, I run 5 volts to the hot side of the gauge, then put various resisters (10ohm, 23ohm, and 75ohm) inline between the ground side and the ground source. The 10ohm should bring the gage to full range (not pegged, but at the top line). The 23ohm should bring the gauge to about middle position, and the 75 ohm should just barely move the needle. There shouldn't be any, or very little resistance between the sensor wire and the connector to the control board.

Is it possible the sensors are not grounding properly? I could be wrong, but I believe the sensor needs to ground properly and the resistance changes with changes in pressure, temperature, or fuel level. Maybe you are having grounding issues at the sensors and it's not actually the sensors themselves that are bad. Did someone use thread sealant on the sensors that is insulating it? Could your fuel level sensor not be grounded properly. My Barracuda has a ground strap from the pickup unit to the chassis to ensure grounding.

the car has 55,000 original miles on it. everything dash whys is original. my dad tested the resistance on both fuel & temp gauges & I took the wires off & grounded them. Both gauges worked. There is no oil pressure gauge. No thread sealant was used. My dad isn't around any more & I don't remember what he found when he tested the resistance.

 

[FrnkNsteen]

Well,.... Not an expert on these things, but I have learned a lot from reading while I have been rebuilding mine. If you are grounding the wires from the sensor, the gauge "Should" peg if there is no resistance in the wire. If that is the case, then your wires and gauge should be ok, which brings me back to the sensors not grounding properly.

Curious,... when you ground the sensor wires, what are you grounding them too? The block?, the chassis? Something isn't right and it sounds like your gauge is acting like it should. Could it be possible that you don't have a good ground between your block and the chassis? I could be wrong, but it sure sounds like you aren't getting proper ground to the sensor. I assume your negative battery cable goes from your battery to the block somewhere. Check that connection and make sure it is clean. I would also add a clean ground strap from the motor to the chassis somewhere.

You might try attaching a jumper wire from the body of the sender to chassis ground, to the block, and to battery ground to see if any of those make the sensor react and make the gauge work.

Here's the deal,.... the gauge needs power on one side and a connection through varying resistance to ground on the other. The varying resistance is the sender (or inline resistors when testing). That's all there is to the system. If you ground the sender wire and the gauge reacts, then the gauge is working and something is going on with the sender that it is not getting ground, or there is an open circuit in the sender. If you've tried other senders, then I would look at why those senders aren't connecting to ground. The only thing I can think of is either the block isn't properly grounded, or something is interfering with the connection between the sender body and the block, or in this case, the water pump housing,.... Correct?

I guess I don't know what motor you have..... Is the temp sender in the water pump housing like a normal big block? Could it be possible that the water pump housing isn't grounding to the block. The only thing that would connect them is the bolts and the housing face that mates to the block. There is a gasket in between the housing and the block, so that may very well be insulated, leaving only the mounting bolts. If someone used thread sealant on the housing bolts, I guess it's possible it may not be getting ground to the block.

All I can say is check your grounds. It has to be an issue with it somewhere.

 

[XMAN JR]

Well,.... Not an expert on these things, but I have learned a lot from reading while I have been rebuilding mine. If you are grounding the wires from the sensor, the gauge "Should" peg if there is no resistance in the wire. If that is the case, then your wires and gauge should be ok, which brings me back to the sensors not grounding properly.

Curious,... when you ground the sensor wires, what are you grounding them too? The block?, the chassis? Something isn't right and it sounds like your gauge is acting like it should. Could it be possible that you don't have a good ground between your block and the chassis? I could be wrong, but it sure sounds like you aren't getting proper ground to the sensor. I assume your negative battery cable goes from your battery to the block somewhere. Check that connection and make sure it is clean. I would also add a clean ground strap from the motor to the chassis somewhere.

You might try attaching a jumper wire from the body of the sender to chassis ground, to the block, and to battery ground to see if any of those make the sensor react and make the gauge work.

Here's the deal,.... the gauge needs power on one side and a connection through varying resistance to ground on the other. The varying resistance is the sender (or inline resistors when testing). That's all there is to the system. If you ground the sender wire and the gauge reacts, then the gauge is working and something is going on with the sender that it is not getting ground, or there is an open circuit in the sender. If you've tried other senders, then I would look at why those senders aren't connecting to ground. The only thing I can think of is either the block isn't properly grounded, or something is interfering with the connection between the sender body and the block, or in this case, the water pump housing,.... Correct?

I guess I don't know what motor you have..... Is the temp sender in the water pump housing like a normal big block? Could it be possible that the water pump housing isn't grounding to the block. The only thing that would connect them is the bolts and the housing face that mates to the block. There is a gasket in between the housing and the block, so that may very well be insulated, leaving only the mounting bolts. If someone used thread sealant on the housing bolts, I guess it's possible it may not be getting ground to the block.

All I can say is check your grounds. It has to be an issue with it somewhere.

Yea im not sure if there is a ground between block & fire wall. When I grounded the fuel wire I cleaned a bare spot & used the frame. When I grounded the water temp wire I used the battery negative cable where it bolts on to the head. I will have to check out & make sure every thing is grounded good & then take it from there. Wiring/electrical is not my strong point. lol

 

[FrnkNsteen]

Mine either, but I am gradually learning more and more!

 

[XMAN JR]

OK I am lost ! Bought a ground strap & cleaned the paint off of the firewall & head. Bolted the new ground strap down. I also bought a new voltage limiter. I put that on. Still the temp gauge doesn't move & the fuel gauge just moves to the first line then stops.

 

[miller]

All I can say is check your grounds. It has to be an issue with it somewhere.

I haven't been into a 68. Earlier models, yes. That being said, you say you replaced the voltage limiter. Okay.
On the backside of the instrument panel, assuming it has a circuit board, that all the gauges connect to. Have you looked the circuit board over real good, for any burnt or broke spots? For the circuits on the board, there's a power side, and a ground side.

Back to basics...using an ohm meter...check for a good ground connections. Should have good ground from engine to firewall. Firewall is same as body/frame. Check from firewall to dash. Check from dash to metal instrument panel housing. All that is connected via fasteners (screws/bolts). You should have complete ground connection, from the instrument housing, all the way to the engine ground, for it all to work.

 

[FrnkNsteen]

On the backside of the instrument panel, assuming it has a circuit board, that all the gauges connect to. Have you looked the circuit board over real good, for any burnt or broke spots? For the circuits on the board, there's a power side, and a ground side.

I thought about that too, but he says that if he touches the sender wire to ground, the gauge works and reacts to getting ground, but doesn't work when connected to the sender. That tells me the gauge has power, and it must be something with the sending unit not getting connected to ground.

However,... as I am reading this, it also occurs to me that a low voltage condition would also cause the gauge to read low, or not at all. Might be a good idea to make sure you are getting a full 5 volts out of the limiter. I still think it has something to do with the grounds though.

 

[XMAN JR]

Decided to work on fuel gauge. Jacked the car up Took the sending unit ground strap off. Hooked up a jumper wire to the sending unit to a bare spot on the frame. NO GO Hooked up a jumper wire to the negative side of the gauge itself to the door striker. THE GAUGE MOVED. The back of the gauge cluster looks good. Hooked a wire to the negative side of the voltage limiter to door sticker. NO GO gauge did not move. So ordered a new fuel sending unit. What else could it be

 

[dadsbee]

Did you ground the actual wire that connects to the sender output??.. at the tank end?? If you did and the gauge didn't move then your wire to the tank is disconnected or broken somewhere. How about the tail lights, do they work.. if not it's the connector at the bottom of the A pillar by the e brake pedal.

Did you add more fuel to the tank?? 5 gallons on mine brought it just above E... needed more to determine it truly worked.

 

[XMAN JR]

Did you ground the actual wire that connects to the sender output??.. at the tank end?? If you did and the gauge didn't move then your wire to the tank is disconnected or broken somewhere. How about the tail lights, do they work.. if not it's the connector at the bottom of the A pillar by the e brake pedal.

Did you add more fuel to the tank?? 5 gallons on mine brought it just above E... needed more to determine it truly worked.

Yep I ground the wire that connects to the sender at the tank & the gauge moved. All lights work. The tank has at least 10 gallons in it.

 

[dadsbee]

Get a good focus beam flashlight to look in the filler neck, see if the float sunk! If not, try lifting it higher with a yard stick and see if you get full indication. If sunk... you already ordered the answer...

 

[XMAN JR]

Get a good focus beam flashlight to look in the filler neck, see if the float sunk! If not, try lifting it higher with a yard stick and see if you get full indication. If sunk... you already ordered the answer...
View attachment 596986

Would you still be able to do that with 10 gallons in it ?

 

[dadsbee]

With a good mag light and good fuel I would think you'd see the float. I'll peak in mine tomorrow.... mind you I did set my float and intake up before I installed the filler neck, so an easier view in that picture.

Do you have an inspection camera??? If so you could use it to snake in to place above the fuel and see if the float is floating!

 

[XMAN JR]

With a good mag light and good fuel I would think you'd see the float. I'll peak in mine tomorrow.... mind you I did set my float and intake up before I installed the filler neck, so an easier view in that picture.

Do you have an inspection camera??? If so you could use it to snake in to place above the fuel and see if the float is floating!

yea already ordered a new one but I would still like to see if I can move the float & see if the gauge moves