Gauges 101

This applies to the 'accessory' gauges: OIL, FUEL, TEMP.  It does not apply to speedometers, tachometers or warning (idiot) lights.  I wrote it in plain, simple English so you don't need to be an Electrical Engineer to understand it.  It is a basic overview, not a technical bible. 

Ford kept things very simple with the gauges - less moving parts, less components to fail and for the most part no numbers. The exception to that last one is the oil pressure gauge.  

The general rule of thumb with Ford gauges (other than fuel) is that centered is normal, slightly either side of centered is okay.  

The acceptable low Oil pressure rule is 10 PSI for each 1000 RPM, so 2500 RPM = 25 PSI.  That is pretty useless considering that the Oil gauge is the same functionally as the Fuel and Temp gauges. 

Also, the gauge movements are NOT linear - 0 on the scale is 70 ohms, Full scale is 12 ohms.  You would think that 1/2 way would be 40 ohms, but it is NOT.  Half scale is 26 ohms! 

Okay, how do they work?
There are 4 components involved:

The Gauge: Internally the gauges have a thin metal strip with a wire wound around it.  This metal strip has the pointer (needle) attached to it.  Electricity traveling through the wound wire heats it up.  This heat is transferred to the metal strip, causing it to flex.  The flexing makes the pointer move.  Basically it is a mechanical mechanism triggered with electricity. Simple, right?  

The Sending unit: The sending unit is actually a receiver, not  a sender.  Internally it is a variable resistor.  The sending unit regulates how fast the voltage flowing through the gauge gets to ground.  This resistance is what causes the heat that moves the needle.

The Voltage Regulator:  Known as a CVR (Cluster Voltage Regulator) or IVR (Instrument Voltage Regulator). Do not confuse this unit with the big voltage regulator over by the battery. This voltage regulator is a small silver box that lives on the back of your instrument cluster.  It's sole purpose in life is to limit the voltage going to your gauges.  (The factory gauges are designed to use 5.5-6 volts DC (VDC) - if you hook 12VDC to one the smoke will come out and you will be looking for another gauge.)  
The IVR is extremely simple and dependable.  It uses the same coil, metal strip, heat principle as the gauge, but the metal strip has a set of points that the flexing metal opens and closes.  This happens very quickly, resulting in an average output of about 6VDC.  The two connections on the IVR are input and output.  The IVR must be grounded through the case or mounting tab to function.  There is one IVR to supply power to all of the accessory gauges.

The wiring:  Gauges are probably the simplest, least understood circuits.  Ready?  
12VDC goes into the IVR. 
The 6VDC output from IVR goes to the gauge. 
The voltage passes through the gauge to the sending unit.
The sending unit regulates how 'fast' the voltage gets to ground.
It's almost too easy.

Simple Troubleshooting Tips:

All gauges swing to full scale:
 - Faulty IVR
 
All gauges are dead:  
 - Check voltage into IVR.  It should be >= 12VDC.
 - Check voltage out of IVR.  It should be 5.5 - 6VDC.

One gauge is dead:
 - Disconnect wire from sending unit, momentarily connect wire to ground (preferably through a 10 ohm resistor).  Gauge should swing to full scale.  
 -- if gauge moves then sender is at fault.  Check that the sending unit case is grounded with little to no resistance, replace sender if necessary.  *It is common for replacement senders to be either the wrong value or wrong type.  Always buy senders from a reputable supplier and double check the part number if possible.
 -- If gauge does not move: check voltage at both sides of gauge, perform grounding test at output side of gauge (post not coming from IVR), check wiring circuit from gauge to sender.

Causes for inaccurate gauge:
 - Faulty / incorrect sender
 - Bad connections / high resistance in wiring circuit
 - Gauge is out of calibration

Notes
 - Do not use teflon tape or silicone on the sending unit threads as it can cause higher resistance.  The sending unit should have a small band of sealer on the threads and will seal when tightened normally.
 - A 'modern' sending unit will have 150 ohms of resistance so the pointer will not move at all.
 - Oil pressure sending units for gauges are large bell shaped senders.  The warning light sender is a small quarter sized unit.
 - The constant heating and cooling cycles that the metal strips in the gauges and IVRs have gone through for 40+ years can cause the gauges to become inaccurate. Both can be calibrated with the right equipment. 
 - You will need an 'old' analog volt meter to check IVR output.  The new digital meters will not read it correctly. You will not get a steady reading on the meter, it will pulse high & low and the average voltage will be about 5.5 volts.

 

[midlife]

Couldn't have said it any better. 

 

[mrgmhale]

This applies to the 'accessory' gauges: OIL, FUEL, TEMP. It does not apply to speedometers, tachometers or warning (idiot) lights. I wrote it in plain, simple English so you don't need to be an Electrical Engineer to understand it. It is a basic overview, not a technical bible.

Ford kept things very simple with the gauges - less moving parts, less components to fail and for the most part no numbers. The exception to that last one is the oil pressure gauge.

The general rule of thumb with Ford gauges (other than fuel) is that centered is normal, slightly either side of centered is okay.

The acceptable low Oil pressure rule is 10 PSI for each 1000 RPM, so 2500 RPM = 25 PSI. That is pretty useless considering that the Oil gauge is the same functionally as the Fuel and Temp gauges.

Also, the gauge movements are NOT linear - 0 on the scale is 70 ohms, Full scale is 12 ohms. You would think that 1/2 way would be 40 ohms, but it is NOT. Half scale is 26 ohms!

Okay, how do they work?
There are 4 components involved:

The Gauge: Internally the gauges have a thin metal strip with a wire wound around it. This metal strip has the pointer (needle) attached to it. Electricity traveling through the wound wire heats it up. This heat is transferred to the metal strip, causing it to flex. The flexing makes the pointer move. Basically it is a mechanical mechanism triggered with electricity. Simple, right?

The Sending unit: The sending unit is actually a receiver, not a sender. Internally it is a variable resistor. The sending unit regulates how fast the voltage flowing through the gauge gets to ground. This resistance is what causes the heat that moves the needle.

The Voltage Regulator: Known as a CVR (Cluster Voltage Regulator) or IVR (Instrument Voltage Regulator). Do not confuse this unit with the big voltage regulator over by the battery. This voltage regulator is a small silver box that lives on the back of your instrument cluster. It's sole purpose in life is to limit the voltage going to your gauges. (The factory gauges are designed to use 5.5-6 volts DC (VDC) - if you hook 12VDC to one the smoke will come out and you will be looking for another gauge.)
The IVR is extremely simple and dependable. It uses the same coil, metal strip, heat principle as the gauge, but the metal strip has a set of points that the flexing metal opens and closes. This happens very quickly, resulting in an average output of about 6VDC. The two connections on the IVR are input and output. The IVR must be grounded through the case or mounting tab to function. There is one IVR to supply power to all of the accessory gauges.

The wiring: Gauges are probably the simplest, least understood circuits. Ready?
12VDC goes into the IVR.
The 6VDC output from IVR goes to the gauge.
The voltage passes through the gauge to the sending unit.
The sending unit regulates how 'fast' the voltage gets to ground.
It's almost too easy.

Simple Troubleshooting Tips:

All gauges swing to full scale:
- Faulty IVR

All gauges are dead:
- Check voltage into IVR. It should be >= 12VDC.
- Check voltage out of IVR. It should be 5.5 - 6VDC.

One gauge is dead:
- Disconnect wire from sending unit, momentarily connect wire to ground (preferably through a 10 ohm resistor). Gauge should swing to full scale.
-- if gauge moves then sender is at fault. Check that the sending unit case is grounded with little to no resistance, replace sender if necessary. *It is common for replacement senders to be either the wrong value or wrong type. Always buy senders from a reputable supplier and double check the part number if possible.
-- If gauge does not move: check voltage at both sides of gauge, perform grounding test at output side of gauge (post not coming from IVR), check wiring circuit from gauge to sender.

Causes for inaccurate gauge:
- Faulty / incorrect sender
- Bad connections / high resistance in wiring circuit
- Gauge is out of calibration

Notes
- Do not use teflon tape or silicone on the sending unit threads as it can cause higher resistance. The sending unit should have a small band of sealer on the threads and will seal when tightened normally.
- A 'modern' sending unit will have 150 ohms of resistance so the pointer will not move at all.
- Oil pressure sending units for gauges are large bell shaped senders. The warning light sender is a small quarter sized unit.
- The constant heating and cooling cycles that the metal strips in the gauges and IVRs have gone through for 40+ years can cause the gauges to become inaccurate. Both can be calibrated with the right equipment.
- You will need an 'old' analog volt meter to check IVR output. The new digital meters will not read it correctly. You will not get a steady reading on the meter, it will pulse high & low and the average voltage will be about 5.5 volts.

I sure wish I had come across your description of gauge circuitry and behavior a long time ago. It would have spared me a lot of typing! I find it is, not surprisingly from you, clear and accurate. I hope you are not offended by my coping your words into a file for future reference and sharing with others when questions come up about how gauge circuitry wk, and how to test the circuitry. I will, of course, give credit for this work as having come from you as opposed to vilely attempting to take credit for it myself. If you prefer I not share your info with others just let me know.

[email protected]

 

[73 Mach 1 4 me]

This applies to the 'accessory' gauges: OIL, FUEL, TEMP. It does not apply to speedometers, tachometers or warning (idiot) lights. I wrote it in plain, simple English so you don't need to be an Electrical Engineer to understand it. It is a basic overview, not a technical bible.

Ford kept things very simple with the gauges - less moving parts, less components to fail and for the most part no numbers. The exception to that last one is the oil pressure gauge.

The general rule of thumb with Ford gauges (other than fuel) is that centered is normal, slightly either side of centered is okay.

The acceptable low Oil pressure rule is 10 PSI for each 1000 RPM, so 2500 RPM = 25 PSI. That is pretty useless considering that the Oil gauge is the same functionally as the Fuel and Temp gauges.

Also, the gauge movements are NOT linear - 0 on the scale is 70 ohms, Full scale is 12 ohms. You would think that 1/2 way would be 40 ohms, but it is NOT. Half scale is 26 ohms!

Okay, how do they work?
There are 4 components involved:

The Gauge: Internally the gauges have a thin metal strip with a wire wound around it. This metal strip has the pointer (needle) attached to it. Electricity traveling through the wound wire heats it up. This heat is transferred to the metal strip, causing it to flex. The flexing makes the pointer move. Basically it is a mechanical mechanism triggered with electricity. Simple, right?

The Sending unit: The sending unit is actually a receiver, not a sender. Internally it is a variable resistor. The sending unit regulates how fast the voltage flowing through the gauge gets to ground. This resistance is what causes the heat that moves the needle.

The Voltage Regulator: Known as a CVR (Cluster Voltage Regulator) or IVR (Instrument Voltage Regulator). Do not confuse this unit with the big voltage regulator over by the battery. This voltage regulator is a small silver box that lives on the back of your instrument cluster. It's sole purpose in life is to limit the voltage going to your gauges. (The factory gauges are designed to use 5.5-6 volts DC (VDC) - if you hook 12VDC to one the smoke will come out and you will be looking for another gauge.)
The IVR is extremely simple and dependable. It uses the same coil, metal strip, heat principle as the gauge, but the metal strip has a set of points that the flexing metal opens and closes. This happens very quickly, resulting in an average output of about 6VDC. The two connections on the IVR are input and output. The IVR must be grounded through the case or mounting tab to function. There is one IVR to supply power to all of the accessory gauges.

The wiring: Gauges are probably the simplest, least understood circuits. Ready?
12VDC goes into the IVR.
The 6VDC output from IVR goes to the gauge.
The voltage passes through the gauge to the sending unit.
The sending unit regulates how 'fast' the voltage gets to ground.
It's almost too easy.

Simple Troubleshooting Tips:

All gauges swing to full scale:
- Faulty IVR

All gauges are dead:
- Check voltage into IVR. It should be >= 12VDC.
- Check voltage out of IVR. It should be 5.5 - 6VDC.

One gauge is dead:
- Disconnect wire from sending unit, momentarily connect wire to ground (preferably through a 10 ohm resistor). Gauge should swing to full scale.
-- if gauge moves then sender is at fault. Check that the sending unit case is grounded with little to no resistance, replace sender if necessary. *It is common for replacement senders to be either the wrong value or wrong type. Always buy senders from a reputable supplier and double check the part number if possible.
-- If gauge does not move: check voltage at both sides of gauge, perform grounding test at output side of gauge (post not coming from IVR), check wiring circuit from gauge to sender.

Causes for inaccurate gauge:
- Faulty / incorrect sender
- Bad connections / high resistance in wiring circuit
- Gauge is out of calibration

Notes
- Do not use teflon tape or silicone on the sending unit threads as it can cause higher resistance. The sending unit should have a small band of sealer on the threads and will seal when tightened normally.
- A 'modern' sending unit will have 150 ohms of resistance so the pointer will not move at all.
- Oil pressure sending units for gauges are large bell shaped senders. The warning light sender is a small quarter sized unit.
- The constant heating and cooling cycles that the metal strips in the gauges and IVRs have gone through for 40+ years can cause the gauges to become inaccurate. Both can be calibrated with the right equipment.
- You will need an 'old' analog volt meter to check IVR output. The new digital meters will not read it correctly. You will not get a steady reading on the meter, it will pulse high & low and the average voltage will be about 5.5 volts.

Thank you your explanation is very easy to understand I guess I’ll try replacing the Ivr. This was a sudden problem everything was working fine and next thing it wasn’t.

 

[carguy47]

Rktmn:
*It is common for replacement senders to be either the wrong value or wrong type. Always buy senders from a reputable supplier and double check the part number if possible.

Do you have the part numbers and the reputable dealers from whom to purchase these from??

 

[TheRktmn]

Rktmn:
*It is common for replacement senders to be either the wrong value or wrong type. Always buy senders from a reputable supplier and double check the part number if possible.

Do you have the part numbers and the reputable dealers from whom to purchase these from??

The part numbers may vary depending on year, engine, etc.
I always get mine from NPD or RockAuto. Avoid the big box part stores like Autozone, O'Reilly, etc.

 

[73MustangCoupe]

Rocketman
, that writeup was awesome. Do you have something like that for troubleshooting idiot lights?

 

[boxgranch]

We had some battery issues and I noticed that the IVR seems touchy about input voltage and the gauges don't read until the voltage comes up. This is a new IVR. Are the older ones less sensitive? I noticed the same issue with the tach conversion. It doesn't read until the voltage comes back up. I think we also got a bad battery which is not helping.

 

[Fredensborg]

Thank you for your post, stuff like this really helps us far less experienced tinkerers! I have a couple non functioning gauges...I will be using this info!!

 

[midlife]

We had some battery issues and I noticed that the IVR seems touchy about input voltage and the gauges don't read until the voltage comes up. This is a new IVR. Are the older ones less sensitive? I noticed the same issue with the tach conversion. It doesn't read until the voltage comes back up. I think we also got a bad battery which is not helping.

The circuitry for the gauges is intentionally designed (via the resistor input wire) to be slow in reacting to the initial input of voltage. It takes something like 10 seconds before the gauges settle down to their normal routine.

 

[boxgranch]

We are actually seeing the gauges stay dead until the voltage gets up to about 13.

 

[kevken1959]

I just read this post. I'm trying to diagnose an issue with my oil and temp gauges. The oil gauge seems to read to the low side but when I was driving at night with the headlights on the oil pressure went up to the high range. The engine temp reads to the high side, but I can verify that the prior owner installed a 196 degree thermostat. I've replaced both sending units. I plan to replace my engine gauge feed wire. I want to rule out a bad instrument voltage regulator and just go ahead and replace it and see if it has any effect. I have several new ones in my stash. My main question is "Is there a difference between the voltage regulator for a gauge car and a car with idiot lights?" Here is a picture of the box of the NOS one I plan to install. GR-513, D1AZ-10804-A. I want to leave swapping out the gauges for last, because the gauges in there now at least work. I have tons of gauges I could use, but I'm sure most would do nothing if I installed them.

 

[midlife]

No. Idiot lights are not powered by the CVR.

 

[mrgmhale]

I just read this post. I'm trying to diagnose an issue with my oil and temp gauges. The oil gauge seems to read to the low side but when I was driving at night with the headlights on the oil pressure went up to the high range. The engine temp reads to the high side, but I can verify that the prior owner installed a 196 degree thermostat. I've replaced both sending units. I plan to replace my engine gauge feed wire. I want to rule out a bad instrument voltage regulator and just go ahead and replace it and see if it has any effect. I have several new ones in my stash. My main question is "Is there a difference between the voltage regulator for a gauge car and a car with idiot lights?" Here is a picture of the box of the NOS one I plan to install. GR-513, D1AZ-10804-A. I want to leave swapping out the gauges for last, because the gauges in there now at least work. I have tons of gauges I could use, but I'm sure most would do nothing if I installed them.

I have had a few older MustangShelby instruments read incorrectly over the years. In one case (69 GT500 428CJ) the factory temp gauge was reading in the upper part of the "Normal" range. I picked up an infrared thermometer and verified the upper radiator hose was showing a temp of 180 - 182 degrees, which is usually on the lower half of the Normal Range with a factory temp gauge. I decided to replace the sending unit, after doing some quick tests (I began with sending unit indicated it was not providing the correct level of resistance), and considering the grief I would be running into were I to have to replace the temp gauge or Instrument Voltage regulator. I got lucky. The new sending unit did the trick. The more important part of this is getting an infrared thermometer and verify the temp you are running at.

As for the oil pressure gauge, if the reading changed when you turned on the headlights, you have a ground "somewhere" with resistance. What year Mustang are you working with? Depending on the year Mustang you have I may be able to provide the location of the various wiring grounding locations. I would expect the ground issue will be found to be with the instrument panel lighting circuit, with a ground location where there is a common grounding location for one of the circuits involved with both illumination and the instrument gauges. If it is a grounding issue it does not take much resistance to cause baffling looking kinds of problems like the one you mentioned (oil pressure reading. You will need to check voltage drop levels at every relevant ground for the instrument gauges and lighting circuits until you find the ground that has higher than normal resistance. The amount of voltage drop I like to see is 0.1volt at the connection being tested (or per connection if I am checking a circuit with multiple serial connections between the endpoints I am checking the voltage drop for).

As for the IVR question, I have never seen a different part number for an IVR for First Generation Mustang equipped with idiot lights or gauges. For an idiot light vehicle the IVR is still used to manage the Fuel Gauge, whereas for gauge equipped instrument panels they are used for the oil pressure and engine coolant temperature as well as the fuel gauge circuit. For the voltage levels for the IVR (CVR) the very first message posted in this thread gives some really good information. How good is the information? Well, I saved it as a consolidated document for my own refernce in the future, as opposed to me needing to check in several different areas in various shop manuals.

 

[kevken1959]

No. Idiot lights are not powered by the CVR.

Thanks Randy!

 

[kevken1959]

I have had a few older MustangShelby instruments read incorrectly over the years. In one case (69 GT500 428CJ) the factory temp gauge was reading in the upper part of the "Normal" range. I picked up an infrared thermometer and verified the upper radiator hose was showing a temp of 180 - 182 degrees, which is usually on the lower half of the Normal Range with a factory temp gauge. I decided to replace the sending unit, after doing some quick tests (I began with sending unit indicated it was not providing the correct level of resistance), and considering the grief I would be running into were I to have to replace the temp gauge or Instrument Voltage regulator. I got lucky. The new sending unit did the trick. The more important part of this is getting an infrared thermometer and verify the temp you are running at.

As for the oil pressure gauge, if the reading changed when you turned on the headlights, you have a ground "somewhere" with resistance. What year Mustang are you working with? Depending on the year Mustang you have I may be able to provide the location of the various wiring grounding locations. I would expect the ground issue will be found to be with the instrument panel lighting circuit, with a ground location where there is a common grounding location for one of the circuits involved with both illumination and the instrument gauges. If it is a grounding issue it does not take much resistance to cause baffling looking kinds of problems like the one you mentioned (oil pressure reading. You will need to check voltage drop levels at every relevant ground for the instrument gauges and lighting circuits until you find the ground that has higher than normal resistance. The amount of voltage drop I like to see is 0.1volt at the connection being tested (or per connection if I am checking a circuit with multiple serial connections between the endpoints I am checking the voltage drop for).

As for the IVR question, I have never seen a different part number for an IVR for First Generation Mustang equipped with idiot lights or gauges. For an idiot light vehicle the IVR is still used to manage the Fuel Gauge, whereas for gauge equipped instrument panels they are used for the oil pressure and engine coolant temperature as well as the fuel gauge circuit. For the voltage levels for the IVR (CVR) the very first message posted in this thread gives some really good information. How good is the information? Well, I saved it as a consolidated document for my own refernce in the future, as opposed to me needing to check in several different areas in various shop manuals.

Thanks MrGMHale! That is great information. I found the ground going from the negative battery cable to the shock tower was loose. When I tightened it, the engine temp reading on the gauge went down about 1/16". I suspect there are more ground issues. I plan on installing the engine block braided ground wire also which is missing. I need to get one of those infrared thermometers as well. If you find a link for the locations of the grounds please send them to me or post here. This car is a 1973, but I only collect 71, 72, 73s. Thanks again. Kevin.

 

[Don C]

These cars didn't come with the braided wire from the firewall to the engine block, just the one cable from the negative battery terminal to the voltage regulator to the engine block. I believe that it is a good idea to add the braided wire.

 

[Vicus]

I just read this post. I'm trying to diagnose an issue with my oil and temp gauges. The oil gauge seems to read to the low side but when I was driving at night with the headlights on the oil pressure went up to the high range. The engine temp reads to the high side, but I can verify that the prior owner installed a 196 degree thermostat. I've replaced both sending units. I plan to replace my engine gauge feed wire. I want to rule out a bad instrument voltage regulator and just go ahead and replace it and see if it has any effect. I have several new ones in my stash. My main question is "Is there a difference between the voltage regulator for a gauge car and a car with idiot lights?" Here is a picture of the box of the NOS one I plan to install. GR-513, D1AZ-10804-A. I want to leave swapping out the gauges for last, because the gauges in there now at least work. I have tons of gauges I could use, but I'm sure most would do nothing if I installed them.

Kevin
I have had similar readings on water temperature and oil pressure gauge when head lights were on and correct readings when head lights were off. An additional ground cable from engine to body fixed this problem instantly.
Frank

 

[kevken1959]

Kevin
I have had similar readings on water temperature and oil pressure gauge when head lights were on and correct readings when head lights were off. An additional ground cable from engine to body fixed this problem instantly.
Frank

Awesome! I have one ready to install. Can't wait till I try it. Thanks Vicus.

 

[kevken1959]

These cars didn't come with the braided wire from the firewall to the engine block, just the one cable from the negative battery terminal to the voltage regulator to the engine block. I believe that it is a good idea to add the braided wire.

Thanks Don!