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Cluster FAQ (1966-1977)

General

The instrument panel is composed of 6 gauges: the speedometer, odometer, ammeter, coolant temp, oil pressure, and fuel level. The Instrument Voltage Regulator (IVR) supplies power to the coolant temp, oil pressure, and fuel level gauges only.


Speedometer

The speedometer system includes a gear installed in the transfer case, a cable, and the gauge. If your gauge bounces a lot, try disconnected the cable from the back of the speedo and dripping some light oil or WD40 down the inside of the cable to loosen it up. Replacing the entire cable is sometimes necessary as these guys get pretty worn out and gunked up.

Calibration Info

Odometer

Not much here.

Ammeter

Broncos use a loop-type ammeter where the current from the alternator passes near a coil pickup and then moves on to the battery. This is different than a shunt-type or a direct type ammeter. If you install a 3G or another high output alternator, you'll most likely bypass the ammeter and wire the output directly to the battery.

Instrument Voltage Regulator (IVR) Supplied Gauges

The IVR does not supply a constant voltage, it's like a flasher, it goes on and off. The gauges are thermal, this regulator goes on and off at a varying rate that makes the gauge accurate at different temperatures. Some people are going to tell you that you can replace the regulator with a three terminal voltage regulator IC. You can but it will not work correctly. To test this regulator, check the terminals at the back of the gas gauge with a voltmeter. You should see the needle "pulse" from 0 to about 11 volts. The IVR is fed by an 8.5 Ohm resistor from the "A" terminal of the ignition switch, the current then passes through a radio noise filter choke (some Broncos), and on to the IVR. The IVR must be grounded.

Troubleshooting
All Gauges

To check the gauges, disconnect from the dash and read across the terminals with an Ohm meter.
10 to 14 Ohms is considered good. Anything else is a bad gauge.
The fuel, oil, and temp gauges are all in the same range so you can hook the oil pressure
gauge to the fuel sender or temp sender and vice versa.

IVR

NOTE: If you use a test lamp instead of a voltmeter be sure it has a small bulb or you'll destroy the IVR.
If you check the IVR output with a needle type volt meter, the needle should bounce up and down around the 5 volt mark. A digital meter wont work. If two of your three gauges are working fine, the IVR is probably OK.

Senders

All of the senders are variable resistances. You can disconnect the wire to the gage in question and connect the Ohm meter to the sender. If you read anything near 10-73 Ohms for the oil/temp and 33-270 Ohms for the fuel you're OK (270 Ohms at empty and 33 ohms at full for the fuel sender). For the fuel sender, it's more important to check that Ohm meter doesn't go to infinity at some point. This would indicate a defective sender. If the sender is out of the tank, you can check for 'open' spots. Move the float arm and watch for open spots. (The needle will go to infinity.)

Reference

Instrument Voltage Regulator (IVR) P/N B9MZ-10804-C

Fuel gauge
33 Ohms = full,
100 Ohms = 1/2,
270 Ohms = empty.
Oil pressure
10 Ohms = high
75 Ohms = low.
Temperature,
10 Ohms = High
73 Ohms = low.

Removal Tips

  • Disconnect the Speedometer cable.
  • Remove the 2 or 3 bendable metal straps attached to the back of the cluster
  • Remove the cluster (be careful with the metal ammeter loop, it breaks easily)
  • NOTE: For a good view of the back of the cluster you can remove the driver's side air vent chamber in the engine compartment.