Hi all, I thought both worked at one time but haven’t worked in awhile. I have a 1967 standard and changed the wiring harness to a refurbished one from WCCC awhile back. The fuel gauge was going up when I would fill but now reads empty on what should be 3/4 of a tank. Can someone tell me the steps to debug? I have two dash clusters that should work and I can Frankenstein parts together if not.
Instrument Voltage Regulator deeds both. Check that first. You should get a pulsing light. My gauge tester will come in handy but you can verify the IVR with a test light.
https://desertclassicparts.com/product/gauge-tester-gt-ford-1/
Voltage regulator under the hood was changed fairly recently. Is there another one on the dash cluster? How does your tool differ from a volt meter? Thanks.
From the ‘Gauges 101’ post on The Curb:
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 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.
Sorry for the super late response. TheRktmn, thank you for the thorough breakdown and instructions. I probed the IVR unit and got a pulse between 0-12volts going in and 0-6volts going out with my digitial multi-meter. I probed the fuel sender which read 55ohms. When reading elsewhere online, it should read 0 for full and 150 for empty which would tell me I have 2/3 of a tank. Lastly, I had my friend momentarily ground the fuel sending wire which shot the fuel gauge to full and when he removed it, it dropped back down to zero.
I’m confused as I’m getting voltage readings from the IVR but grounding the sending unit seems to rule out the sending unit problem. Any suggestions on what else I can look at or should I just replace both?
Thanks in advance!
-Rex
Sounds like the sending unit is bad or it may have a bad ground. If the gage goes full register when you ground that wire you have proved the wiring is fine and the IVR is performing as expected. If the gas tank does not read 0 ohms to ground you have found the problem.
Don’t believe everything you read on the Internet. The range of standard fuel sending units is 13 (high) to 73 (low) ohms. It is not linear between the two, either. 55 ohms means it is somewhere near half full give or take a quarter tank.
I think that most of you know that I developed a gauge tester. And that I rebuild and calibrate fuel sending units. In order to do that I had to thoroughly research these gauges.
Here are the actual facts:
All Ford Gauges use the same movement. They are referred to as 10 / 73 gauges. That means 10 ohms for full, hot or high oil pressure. 73 ohms for empty, cold, or no oil pressure. The center reading is at about 23 ohms. They are non linear and far more sensitive at the top end of the scale (10 ohms).
They are all fed by pulsed 12 volts coming from the instrument voltage regulator. The pulses, averaged out over time, produce the same amount of power as a 5 volt continuous output. The gauges work based on heat. Heat is a function of power. If you increase the amount of power then the gauge will read higher. Many people think that the gauges are designed to run on 6 volts. Several misguided suppliers have used a common solid state chip to build IVR replacements that do out put 6 volts. If you apply higher voltage (more power), then the gauges will read higher for a given resistance. For this reason many people think that 12 or 13 ohms is correct for the Full Hot or High reading, as that is what you would use with 6 volts.
Most suppliers have now figured out that the 7805 chip will provide 5 volts and that is what they are using now.
Thanks xr7g428, I’m more then happy to order your tester and it sounds like I need to order the IVR unit. Would you recommend the electronic IVR from WCCC? What else should I order to sort out these issues? I think my fuel sender is working but my temperature sending unit looks old and some previous owner painted over it. They seem to be out of stock on WCCC. Where else can I find a correct one?
The original temp senders are pretty rugged. The thermistor is completely sealed inside and doesn’t contact the coolant directly. I would clean up the one you have and verify that when you measure resistance between the outer brass casing and the block that you get very close to zero ohms.
Once you get the tester you will be able to see if the problem is with the gauges or the senders.
About solid state IVRs: The advantage of what Ford did was that when you start the engine the gauges are being fed the first big pulse of 12 volts. This makes the needles move faster off the peg. It hardly matters for temp or fuel, but a good reading of oil pressure shortly after starting is important. This is the one thing idiot lights actually do better: they show that you have some oil pressure essentially immediately. The solid state IVRs are nice because they are pretty much right on voltage. (chips are available in different tolerances). So one variable is eliminated. The Chrysler guys have a vendor that has really done a great job of engineering a complete solution that actually captures the best of both. I don’t see the product listed on the web site any more so it may no longer be available.
My opinion the replacement IVR’s for our cars are useless. They don’t provide any voltage for 10 - 15 seconds. The factory ones from my experience never fail. I have not seen a failed one. Ever.
Hi all, thanks for all the extensive information. I was able to fix both gauges over the weekend which turned out to be a combination of problems.
Royce, you are correct. The IVR is good and I’ll send the one back that I ordered. XR7G428, your tester was a big help.
I swapped the temperature sending unit and pulled the broken gauge out which had a broken needle. Your tester confirmed the new gauge worked and was further confirmed as I let the car warm up. I then tested the fuel gauge with your tester and it worked which isolated the fuel sending unit. I drained the fuel which I only had 2 gallons, changed the float and filled her up. Everything is working great now and is subsequently much safer to drive.