1st Gen Instrument Cluster Info

[Casey]

Putting this info here, in case it might assist others. I'm having fun reverse engineering the 1st Gen instrument cluster, having stumbled upon this lot on ebay, bagged for £35 delivered. A full cluster, with slight cosmetic damage, but working and a load of working spares:

[Casey]

I have ordered some "Molex" female pins, which look to be perfect to make up the two round, multi-pin connectors, unless anyone has a harness they are willing to donate to the cause of understanding the dash operation?

I'd love to work out how to install a slightly louder and better sounding redline alert for a start!

It's amazing to see how crude the gauges are, compared to the equivalents I have been playing with in an FD cluster.

Progress so far:

The fuel, temp and oil pressure gauges are all low impedance (~50ohm). Very crude, with resistance wire wrapped around bimetallic strips which, when heated by current flow, physically drive the gauge needle. All working fine.

I managed to drive the tacho using a simple, low frequency (~40Hz- 220Hz) square wave source and discovered the over-rev buzzer wasn't operating. The buzzer is driven directly from a PCB on the tacho. Measurements showed the buzzer output from the tacho was at near zero volts until 7000rpm was registered, then it changed to 12 volts. The other supply to the buzzer is a constant 12 volts, so the change to zero volts should activate the buzzer! Faulty buzzer? No, much simpler! The output form the tach is via a steel screw and washer to the copper PCB. 40 years of sitting in variable, often humid, conditions, had caused corrosion deposits between the copper track and the steel washer, leading to a 3 megohm resistance. The buzzer then worked fine when the faulty contact was bridged.


The washer corrosion:



Corrosion on the copper PCB track:



Now working by bridging the faulty contact:

[Casey]

If anyone has an issue with their fuel sender not working at all, then, apart from the obvious wiring break, a faulty sender (I'm just replacing mine!) or faulty gauge, there is one more component that is unique to the fuel level measurement. There is a small PCB, hidden inside the dash assembly which generates a stabilised supply of 7 volts to the gauge. I guess it's there to give better gauge accuracy, given the battery may go as low as 11 volts and when the alternator is charging, the voltage can be around 14.5 volts. That would cause the gauge to give wildly different readings when the gauge is checked with the ign key on "ACC", or with the alternator running.

For the record - maybe not of interest to many - here is the 7 volt PCB (removed from a small heatsink) and the circuit. As someone who grew up with 1970's electronics, it's a very neat circuit which, today, would simply be a 3 pin integrated voltage regulator. It would be easy to repair one of these, or replace with a modern equivalent. For those "into" electronics, D2 is a 5 volt zener diode, which give a clue to how the circuit works