Although the Ford Fox Mustang instrument clusters were basically the same from 1979 to 1986, and again from 1987 to 1993, variations surfaced as Ford weaved improvements into the Mustang. Basic instrumentation function didn’t change from 1979 to 1993. Speedometers are cable-driven mechanical units. Electronic speedometers were never factory installed in 1979–1993 Mustangs.
The rest of the Fox Mustang’s instrumentation was traditional for the 14 model years these Mustangs were produced. Fuel, temperature, and oil pressure gauges work on a principle based on a heating element, bimetallic spring, and a needle. Ignition-switched power flows to the heating element in the gauge movement and then to ground via a sending unit. The sending unit is a variable resistor that controls current flow to ground, which affects how hot the heating element in the gauge gets, which affects where the needle sits. Fuel, temperature, and oil pressure sending units are variable resistors that control current flow to ground.
This Tech Tip is from the full book, FOX BODY MUSTANG RESTORATION 1979 – 1993. For a comprehensive guide on this entire subject you can visit this link:
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When the fuel level, oil pressure, and engine coolant temperature are high, resistance to ground is low and current flows aggressively. High current flow to ground makes the heating element hot, expanding the bimetallic spring and moving the needle higher.
When the fuel level, oil pressure, and coolant temperature are low, resistance to ground is high and current flow is low, causing very little needle movement.
Current to the gauges passes from the ignition switch through a voltage limiter, which reduces current flow to approximately 5 volts to the gauges. When gauges max out or don’t work at all, you have a defective voltage limiter, which is located on the back of the instrument cluster. On the back of a 1979–1993 Mustang instrument cluster is a printed circuit that routes power to and from the instruments and lights. The instrument voltage limiter is easy to replace and snaps into place.
Gauges require virtually no maintenance. It is important to understand that gauges are not linear; they don’t indicate exactly what conditions are but are approximate. They can be calibrated, but it isn’t necessary. The tachometer is the only other electronic instrument. It functions on pulse input from your Mustang’s Duraspark ignition system.
The speedometer is a cable-driven mechanical instrument that utilizes the eddy current principle where you have a steel drum on which an indication needle is attached. Inside the drum is a magnet driven by the speedometer cable, which is driven off the transmission’s output shaft. The faster the cable and magnet spin with increasing vehicle speed, the higher the needle moves across the speedometer face. Speedometer service, if needed, is best performed by a professional. It is not something you should tackle yourself.
Instrument Cluster
1. From 1979 to 1986, dash pad installation was straightforward and certainly not as complex as 1987–1993. The instrument bezel, which is secured with an assortment of Phillips-head screws, is removed to gain access to the dash pad trim screws.
2. Dash pad screws are located all around the perimeter of the dash pad. Once these screws are removed, the dash pad lifts out. Make sure you don’t drop any screws down the defroster outlets.
3. From 1979 to 1986, the instrument cluster is secured with hex-head sheet metal screws around the perimeter. Once these screws are removed, the cluster comes free.
4. This is the basic 1979–1986 Fox instrument cluster, which is a modular unit tied to the electrical system via multiplex plugs and to the speedometer cable. The backside of a Fox cluster looks like this with printed circuits and multiplex plugs. The instrument voltage limiter is on the left with two button-style connections.
5. Dakota Digital offers a cool replacement instrument cluster for 1979–1986 Mustangs. This cluster drops into place, replacing the factory cluster. This drop-in package uses the stock gauge bezel and brings your 1979–1986 Mustang up to date with fully electronic instruments; red, blue, or white LED backlighting; and nearly unlimited expansion. It is available with silver or black alloy or carbon fiber faces; you can keep tabs underhood without hanging A-pillar additional instruments.
This Tech Tip is from the full book, FOX BODY MUSTANG RESTORATION 1979 – 1993. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
SHARE THIS ARTICLE: Please feel free to share this post on Facebook / Twitter / Google+ or any automotive Forums or blogs you read. You can use the social sharing buttons to the left, or copy and paste the website link: https://www.diyford.com/how-to-install-gauges-in-a-fox-body-mustang/
White Face Gauges
1. The Mustang’s instrument panel for 1987–1993 wasn’t much different from its 1979–1986 predecessor. The cluster is fastened to the main dashboard and is removed the same way. Cluster removal begins with the steering column covers.
2. The instrument cluster bezel, which houses thumb switches, is removed next. The instrument cluster is fastened to the dashboard. The bezel wraps around the cluster.
3. Thumb/tab switches are disconnected from the main wiring harness like this, taking extra care not to break the retainer tabs.
4. With the steering column covers and bezel removed, the cluster is removed next. Screws are located at each end of the cluster. Remove these screws to free up the instrument panel.
5. The 1987–1993 instrument cluster connects to the main wiring loom the same way as 1979–1986 with a multiplex plug like this one and a printed circuit panel.
6. The 1987–1993 Mustang has a mechanical speedometer just like 1979–1986, which disconnects from the speedometer as shown.
7. With the instrument cluster removed and disconnected, carefully remove the instrument lens as shown. Rinse the lens with soap and water. Never wipe a dirty lens with a dry cloth; otherwise, you will scratch the lens. Replace milky, scratched-up lenses as reproductions are now available.
8. Next, remove the diffuser like this and set it aside. Again, be very careful with the cluster because it is so easily damaged.
9. Using the same tool you use to pop door panels and trim, gently pop the speedometer and technology needles. The speedometer shaft should be at rest when you have removed the needle. Make sure the shaft is at rest at zero when you reinstall the needle.
10. Because we’re installing white face gauges, the old black dark face graphics must be removed like this.
11. The Fox Mustang speedometer undraped looks like this. As with its predecessors, this speedometer works on the eddy current principle where there’s a rotating magnet within a drum attached to the speedometer needle. The magnet is propelled by a cable and drive gears tied to the transmission’s output shaft. The spinning magnet levitates the drum and moves the needle off zero as vehicle speed increases. Speedometers need lubrication only at the drive head and odometer gears.
12. Brothers Performance does this cool white face instrument conversion for both 1979–1986 and 1987–1993 Mustangs. Remove the factory black face graphics and lay in the white.
13. Because we’re converting from the 0–85 mph instrument face to a 0–200 mph white face, speedometer calibration is necessary. Speedometer calibration can be performed via this calibration system from Brothers Performance or by changing speedometer drive gears at the transmission. For the 0–200 mph face, you’re going to need a driven gear with a greater number of teeth.
14. Speedometer and tachometer needles are carefully pressed back onto their shafts and positioned at zero.
15. The instrument cluster is reinstalled as shown, followed by the bezel and switch assembly. Now is the time to replace the instrument voltage regulator and any marginal switches.
Written by Jim Smart and republished with permission of CarTech Inc
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