All Ford Gauges, for fuel oil pressure and temperature are the same internally. They are grounded through the sender. Period. No other grounds are involved.
They are all fed by the Instrument voltage regulator. The factory unit puts out pulses of 12 volts that average out to 5 volts of continuous power. You cannot test these unless you have an oscilloscope or a meter with the capability to average over a very long period of time ($$$$$) The solid state after market IVRs typically put out 5 volts continuously. You can measure output from these. Both styles MUST be grounded through the case to the car.
All senders are variable resistors. The ones for oil pressure and fuel level use a sweep arm that moves over a wire wound resistor. The temp sender uses a negative temperature coefficient thermistor. That means resistance goes down, when temp goes up.
Shorting or grounding the sender wire is a good way to damage a gauge. DO NOT leave it grounded for long. If you see all the gauges pegged on maximum do not operate the car. The gauges are going to get fried. This is almost always an indication that the IVR has failed.
Gauges read according to resistance.
73 ohms bottom of scale.
60 ohms =0
35 ohms =1/8
28 ohms =1/4
23 ohms =1/2
15 ohms =3/4
10 ohms = 1/1
Notice that the scale is not linear. Resistance values has a plus of minus variance to so the bigger the number, the bigger the range of values that is still considered correct. That is why calibration occurs at the top of the scale.
Many aftermarket senders use a linear resistor. It can be accurate at empty and full and be wildly off everywhere else. For example when the gauge shows 1/2 you really have 3/4 tank. When you see 1/8 of tank you really have 1/2.
60 ohms =0
54 ohms =1/8
48 ohms =1/4
35 ohms = 1/2
23 ohms = 3/4
10 ohms =1
Finally you have a fuel level gauge. It doesn’t measure gallons. If you put in two gallons it will barely move the needle. It is showing you the level of the accessible gas in the tank. The fuel pick up is not able to, or designed to, access the last drop.
If you have read this far you should probably copy this or share it with other Ford groups or what ever. This is not a guess. I rebuild senders professionally and I have the Ford gauge testers and literature.
Great summary, Bill! One thing I have wondered about is how much variability is normal. For example, with your gauge tester set on mid at 23 ohms where the gauge should read 1/2, would you expect to see the gauges vary from 3/8 to 5/8? Or less or more variation than this?
Very helpful to know that. My gauges are all 6-12% high at the mid and hi readings. I had always figured that was in tolerance, but it must mean my IVR output is running a bit high.
My fuel gauge reads inconsistently. Most of the time around 3/8 of a tank. It does go higher periodically (like when I fill the tank) but not necessarily right away and typically ends back up at the 3/8 level after shutting ing the car off. It does go lower than 3/8 as fuel is burned off but again, not very consistently. Thoughts?
The wire wound resistor is exposed to gasoline. When the level drops it will no longer be covered by gas. So as the gas dries it leaves a layer of shellac on the wire. The resistor is designed to be self cleaning but over time the layer builds up. This is one cause of the issue you are having. Another cause is that the electrical connections are not soldered. When oxidation occurs is swells in the space of the connection. To fix this the connection needs to be soldered. After thorough cleaning in the parts washer and blast cabinet soldering every connection is the first real step to rebuilding a sender.
In short, your sender most probably needs to be rebuilt. The gauge tester will verify this.
Hey Bill on my 68 XR-7 I’ve had you rebuild sending unit but my gas light stays on and my gas gauge doesn’t move. Thanks for any help in advance. Where should I start?
I don’t believe the amp gauge can be tested with the gauge tester. It connects in shunt across the wire that transfers current from the alternator into the battery. A fraction of the current flowing in that wire goes through the gauge to give the amps reading. Does anybody know what voltage/current the amp gauge is calibrated to measure? I have two that I need to test also.
The current is about 14 milliamps, IIRC. What I do to test it is to use a 9V battery, tie one end to one of the gauge posts, and just briefly touch the other. The needle should deflect all the way.
Here’s the results of testing a good used 69/70 amp gauge I got from WCCC.
3 VDC with a 3.3 ohm 3 watt resistor in series with the gauge gives full scale readings. You really need that series resistance to protect the gauge. Right and left are referenced from looking at the back of the gauge.
Positive to left post (current flowing from left post to right post):
1/2 scale positive = 0.42A (0.28V across gauge)
Full scale positive (+60) = 0.74A (0.55V across gauge)
Positive to right post (current flowing from right post to left post):
1/2 scale negative = 0.45A (0.25V across gauge)
Full scale negative (-60) = 0.71A (0.46V across gauge)
If you want to just do a quick test, a 1.5V AAA battery touched briefly across the terminals in both polarities is plenty to fully peg the meter in both directions.