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Gauges 101

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Original Curbster

Joined: 22 Jun 2005
Age: 57
Posts: 8071
Location: TX, USA

1969 Mercury Cougar XR7

PostPosted: Tue Dec 06, 2016 5:20 pm    Post subject:  Gauges 101 Reply with quote

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

- 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.
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Last edited by TheRktmn on Sat Sep 09, 2017 4:46 am; edited 3 times in total
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Original Curbster

Joined: 22 Jun 2005
Age: 57
Posts: 8071
Location: TX, USA

1969 Mercury Cougar XR7

PostPosted: Tue Dec 06, 2016 5:37 pm    Post subject:   Reply with quote

RED circuit board (C9WF-10C956-01) pin-out for a 1969 XR7.
Note that this view is from the back of the cluster with tach on left.
Also note the opening shape - the connector can only go in one way w/o forcing it.

<a href=""><img src=""></img></a>
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