One thing I have found so far, it that the gauge lighting intensity varies a little depending on how far you push the bulb and its holder into the gauge socket in back. Now, if you are using a different sized bulb (length-wise) the possibility also exists that you can push the light assembly too far into the metal gauge housing and have the bulb actually resting up against the inner-workings of the gauge. I have confirmed this on the
oil and temperature gauges so far. I have not checked the AMP meter for this problem yet or any of the other gauges. Having the bulb against the instrument internals will affect the gauge reading by limiting or preventing needle movement.
If you are using the standard bulb and holder, this should not a problem.
Here is a quick overview of my findings so far:
All of the light entering the gauge needs to reflect around inside the instrument housing to reach the 4 small light windows located on the gauge face. The gauge face itself is set back from the front glass about ~3/16”. The ring of material that supports the face has 4 light windows (opaque white areas) on it. This is where all the illumination enters the face of the gauge. The area of the light windows is maximized and cannot be improved on. The real problem is that the outside diameter of the gauge face (and the 3/16 ring of material that sets back and supports the gauge face) has only ~1/16” of clearance all-around with the ID of the instruments metal enclosure. (in other words: The ID of metal enclosure is ~1/8 larger than OD of instrument face – so you have ~1/16” clearance all around)
So all the light needs to be directed into this ~1/16” wide and ~3/16” deep slot after finding its way around the instrument internals which block any straight path of the bulbs light beam, except to the top light window.
So clearly the solution is more light. But larger wattage bulbs mean more heat. And the gauge face is some kind of plastic which distorts and melts with more heat.
I have made 2 LED prototype light sockets so far. One is a single LED and one with Double LED’s. The biggest improvement so far is that the light emitted by the LED is white not yellow. That alone makes the gauge easier to read. I had originally hoped to be able to make a bulb fixture to aim 2,3 or 4 small LED’s directly at the 4 light windows on the gauge face, but that’s appears impossible to do.
As some of you know, some LED are not 12v and require a resistor on the “+” side to cut the voltage down. Which leads to another problem regarding just how much voltage is in the wiring at the instruments. So far I’ve seen between 10.2 volts and 12.2 volts. On paper you should be able to get 14 volts to the bulbs. Make a big difference when sizing the resistor…
The dash light dimmer is another problem. Many different styles with one thing in common – all cheap.
FYI: Sometimes bypassing the dimmer all-together make a big difference in the gauge lighting…
So in closing:
I have some new 12V LED’s coming in the mail. This should eliminate the resistor problem. And since I have no speedo, tach or
oil pressure or fuel gauge instruments to test yet, So I’m unaware of any unique internal lighting problems with these instruments. But I know the tach has a different lighting configuration that will require a different approach for sure. Thanks to those who have sent me the broken
oil& water temp instrument and the AMP meter. I still need the others if anyone has broken units laying around.
I’ll post more results as they become available…
Lighting Tests - (Long Post)
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