Re: how do you engage the overdrive?

Posted by HH56 on 2021/4/21 13:24:57
An interesting question. Logic would say it would burn out yet the 54 6v regulator has the same part number as the 12v item used in 55-6. There have been any number of 51-3 models which have a different part number regulator that have been converted to 12v. I don't recall anyone mentioning a problem after the conversions.

The regulator uses a heater to bend a bimetal strip to open and close a contact. That action provides brief pulses of full battery voltage to the gauges. The regulated voltage value is calculated between the on and off times to be the average value of 5v the 51-6 gauges use. It could be the action is more rapid at 12v so the regulator heater or gauge heaters never see a long enough on time of 12v to get damaged.

Prior to 51, instrument regulators were not used so those earlier cars definitely need something to lower voltage to the gauges. The Runtz solid state device is one option but seem to be sold for negative ground only. Resistors are the fall back and probably the most common solution.

Here is the regulator internals and an oscilloscope view of the action from a 54-56 regulator at 12.6v. In this example on time is just over 100ms and off time just under 500ms. Don't remember if I had 2 or 3 gauges in the circuit when taking the reading but the gauge load would vary the timing of the pulses.

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