I have a 68 convertible with Ralley gauges and also have a two year electronic degree. I worked in San Antonio a few weeks ago for 8 day too:)
Anyway, I can't seem to find any of my wiring notes, but from memory none of the factory wiring diagrams available will show a rally gauge setup, however many of the colors will be the same.
My car did not come factory with rally gauges. I bought a set off ebay and bought all new wiring harness from M&H for my whole car and a new printed circuit board.. Even with new wires I had a bunch of issues with my gauges. Mostly because I painted the metal gauge pod and none of the grounds worked.
Since it sounds like you have power at the plug I would new focus on the gauges. What I did with mine was trace out the printed circuit on paper along with what gauge it went to. I also used a 9 volt batter, a 100 ohm pot and some alligator clips to test the gas, oil gauge and lights. DO NOT USE IT TO TEST THE AMP METER.
Tracing out the circuit on paper will make the issue clear of what might be going on.
If I remember correctly the 12V feed everything. The functions are based on grounding. So for example 12V goes out to the gas tank sending unit on the Tan wire through the 90 ohm wire resister and then grounded.
Sounds to me like you may have a damaged printed circuit board.
Rorht, your right. I have a 68 Service Manual and one of those tri-fold wiring diagrams. Both are for the wiring for the idiot light setup. I have not see a wiring diagram that includes the wiring breakout and colors for Rally gauges. Oversight by GM?
I bought an engine wiring harness for my 73 Trans Am from M&H. They do really good work. I'm having some issues with Ames, so even though I ordered a new PCB last Thursday I still don't have a new one. Trying to get that worked out.
I'll take your advice on tracing the gauges. I went to radio shack to get a battery and potentiometer. When I read 9V battery here I was thinking of those small squareish batteries you could buy in the past that were about the size of a coffee can? When I got there I asked about 9V batteries and he brought me the little square 9V batteries you put in a smoke alarm for the house. I felt pretty dumb. Is that what you used? I'm going to try that first. Radio Shack did have a very small square lead acid 12V battery for $22. I may go get one of those. Would be perfect for testing these kinds of things albeit a little heavy.
I couldn't remember what you got for a Pot so I got one of the smallest ones they had, that had a post you could roll with your fingers. Its 5k Ohm, 500VDC and .5W. I'm guessing its just overkill for this application but should still work. If you got a 100ohm Pot for a reason, please let me know. I also picked up some alligator clips.
I had some questions on how you did this testing. I am assuming here the Pot is to vary the amount of voltage fed to the gauge. So completely counter-clockwise the Pot is sending 0V and completely clockwise it is sending the full 12V.
If that's correct, I should be good to test but I am curious what varies the voltage for each gauge. Say the gas gauge. If there is 12V coming in on Pin 10, what is controlling the variation of voltage fed to the gauge? The other wire coming from the Sending Unit? So for the gas gauge you have 12V coming in on Pin 10 and then what the "Control Signal" from the gas tank sending unit on Pin 3?
I'd like to better understand how the 12V and the sending unit interact to put the needle where it is supposed to be on the gauge. You also mentioned a 90 ohm wire resistor. Where is that located? In the gauge? Sending unit? I know the purpose of a resistor is to introduce a certain amount of resistance into the circuit. Is this the control signal? Resistors are usually a static value that does not change so I am not sure how that would be used to vary the voltage.
