Tests:

Gauges:
The fuel, temperature and oil gauges operate on the constant supplied voltage principle through a voltage limiter integral with the fuel gauge. Test and service diagnosis are the same as those outlined in the 1966 Monaco-Polara Service Manuals. All gauge wires are connected directly to the respective gauges instead of being connected to a mutiple connector and routed through a printed circuit board.

Voltage Limiter:
The Voltage limiter can be tested in the vehicle or with the instrument cluster removed. To quickly test the voltage limiter in the vehicle, connect one lead of a voltmeter or test light to the temperature sending unit and the other lead to a good ground. Leave the sending unit lead wire attached to the sending unit.
Turn the ignition switch to the "ON" position. A fluctuating voltmeter or a flashing light indicates the voltage limiter is operating.
To test the voltage limiter out of the vehicle, connect a jumper wire from the positive post of a 12 volt test battery, to the "I" terminal of the fuel gauge. Connect another jumper wire from the battery negative terminal to the instrument cluster housing.
Connect the negative lead of a voltmeter to the battery negative terminal. Touch the positive lead of the voltmeter to the "I" terminal of the fuel gauge. A reading of 12 volts should be shown on the voltmeter.
Move the positive lead of the voltmeter to the "A" terminal of the fuel gauge. A fluctuating reading between 0 and 7 volts should be shown on the voltmeter.
The same reading should be shown at the "S" terminal of the fuel gauge. Any other readings indicate the voltage limiter is not functioning and the fuel gauge should be replaced.

C-1 = .060 mfd. 330 VAC Q-1 = Bendix PS-26 R-1 = 2-Ohm, 5-Watt R-2 = 50-Ohm, 5-Watt R-3 = 1.5k-Ohm

Readings for T-1 Between terminals 1 and 2 = .4 Ohms Between terminals 2 and 3 = 5.7 Ohms Between terminals 4 and 5 = 99 Ohms

Special notes: If replcing Q-1, use MJ2955. Radio Shack number 276-2043, and rebias it by changing R-2 to 120 Ohms (per Dick Benjamin, 6/99). If using the above modification: Emitter of Q-1 = 10.4 Volts DC Base of Q-1 = 13.2 Volts DC Output at the white wire is 190 Volts A.C. All voltage readings taken with engine off, parking lights on, dimming rheostat at full dash brightness (light knob turned fully counter-clockwise).

The thing that powers the electrolumiscent dash lighting is called a "power pack." It is found behind the dash under the glove box. It should be held in place to the lower part of the dash with two screws. The wires on the power pack are orange and white. The orange wire gets +12 volts when the headlight switch is pulled out. the white wire is the output of the power pack and that white wire should be unplugged from the connector when checking the voltage on it. That white wire should have 230 volts AC on it. If you don't have an analog meter to test it with you can use your finger and touch the white wire. You will need to get a meter to check that part of the circuit. Put the negative lead on frame ground and probe the white wire with the headlight switch turned on. The reason the plug should be unplugged is that if any guage has a fault or short, that one guage will kill the entire electroluminence of the guages, which includes the needles and on a 66 I believe the clock and radio. the power pack consists oc circuitry that makes it an oscillator that has a frequency of 250 cycles per second. It is driven by a transistor in the power pack, that beefs up the output. The output is part of the oscillator and if the output is subjected to a short in any guage,then the oscillator will stop working and there goes the 230 volts. The test light you have will probably load down the circuit and stop the oscillator. In any case the blub on the test light is for 12 volts DC, and you are putting it on 230 volts AC which can burn out the blub if the oscillator keeps on running. You will need to take care of this part the best way you can come up with.

The El power pak is under the glove box and you will need to check the white wire output with an analog ac meter for 230 volts AC at 250 cycles per second. As long as you have around 230 volts AC the frequency will be OK. The orange wire at the power pack is where you should have +12volts dc when the headlight switch is turned on. I would disconnect the white wire plug at the power pack and check the AC voltage to see if the power pack is good. If any of the guages has a short, then the power pack oscillator will not run, and there will be no AC voltage generated if the oscillator does not run. You will be able to isolate where the problem is if you keep this fact in mind. Might be just one guage causing all of the guages EL not to work.

Ralph, the markings on the top only indentify the transistor manufacturer as Bendix type 1859-29. The transistor is a PNP transistor and current flow is against the arrow, down through R3 resistor and then through the primary of the transformer setting up a field in the transformer that is induced into the secondary that also chargers the capacitor. This field of the transformer and the capacitor starts the oscillations that creates the frequency of 250 cycles per second. The transformer is not replacable but the capacitor can be replaced. Just ask for a .05 micro farad 600 volt ac capacitor. Do not accept an electolytic capacitor as they will not work. The purpose of the transistor is to excite the primary of the transformer to make it produce a more intense field thereby proc\ducing a more healty output from the seconday of the transformer. After all you are changing 12 volts DC to 200 volts AC and something has to be there working away to make this change, and the power transistor is the dude doing the work. This is a simple circuit so anyone worth their reputation should be able to trouble shoot the circuit if yours does not work.

I looked the data over in the discussion group and this is my conclusion after going over the schematic of the circuit. The orange wire is where the +12 volts is applied and this creates current flow through the T1 tansformwer and r2 and r3 resistors. This current flow starts the transformer T1 to develop a negative potential at the base and a positive potential at the top of T1 while developing a postive potential on the base of the transistor to shut the transistor off. This starts the oscillaiting effect of T1 and C1 and these oscillations cause the transistor to conduct and shut off which generates the frequency and power level of the output which is observed on the white wire out. If the white wire out is shorted to ground this would put ground on both legs of the secondary of the transformer which would stop the oscillator and the lights would go out of course. The oscillator frequency which is determined by the values of T1 and C1 and the action of Q1 (transistor) would determine the output voltage. From what I have been exposed to the output is 200 volts ac, but I don't have a frequency meter to determine the frequency. I'll just leave that to the engineers who designed the thing. Very seldom do transformers give trouble, and as for the capacitor, it can not be an electrolytic type capacitor, because it is in an oscillator circuit. Resistors and transistors and capacitors can be obtained at numerous electronic places.

With a multimeter, you can check the load at the orange wire to the el pak for instance. Say it reads 12 volts. Now you can test the pak by switching to ac test and see if you are getting the minimum of 200 acv from the white wire. Each power pak will get a slightly different read. ie., mine reads about 260acv.

Published here with kind permission from Don Dodson

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