Bill, I am definitely in for one with the light. Sure will help to diagnose issues without taking the car apart, unless it needs it.
I do have occasional problems with temp gages reading too high but it has always been obvious when measuring the temperature of the radiator using an infrared heat gun that the sending unit is at fault. Used or NOS original sending units have always fixed that problem.
I have never found a defective instrument voltage regulator despite having owned 70+ 1967 - 68 Cougars and a few 65 - 68 Mustangs and several 64 - 68 Falcons. Occasionally they have been found unplugged. Then they donāt work.
Recently I have come across lots of gages that stick in one position even with power removed. I think some of them just reach their ultimate age limit and fail.
Recently I had an oil gage that would peg with the engine off / key on but go to zero when the engine was started. Guess what was wrong.
If you build it they will buy.
Once you get the Cougar one sorted out, you could verify it is same or different for Mustang and market it for that as well.
A little more testing, you could probably make a version / confirm it as usable for other Ford products.
ā¦then branch out to all the other manufacturers. Iām sure the GM and Mopar guys would love to have something like this too!
Pure gold, Bill. Pure gold!
Recently I had an oil gage that would peg with the engine off / key on but go to zero when the engine was started. Guess what was wrong.
Sounds like they used the sender for an oil pressure idiot light. On with no pressure, off with anything over about 6 psi.
The way I got into to this was by rebuilding the fuel sending units. I could calibrate the unit and know if was dead on but some fuel gauges were still showing inaccurate readings. The gauges are internally adjustable so once I had the known correct signal then I could very very carefully adjust the gauge to make it read correctly.
Interested! Keep us posted
This should work for other Ford products of the same vintage. I know it will work for Mustang and Cougar. I am researching the other cars.
So if our fuel gauge is not accurate after replacing the fuel sending unit, an adjustment to the gauge itself is probably required?
Count me in tooā¦and I am fine with either versionā¦I have waited this long I can wait another month - also trouble shooting electrical on multiple cars it seems likeā¦Thanks Billā¦trying to round up a few sending units to send to youā¦now where did I put those things - LOL
So if our fuel gauge is not accurate after replacing the fuel sending unit, an adjustment to the gauge itself is probably required?
Yes. Or a replacement gauge. The adjustment is pretty touchy. I will shoot some pics to show what is involved. Once you have the tester and an IVR you can test the gauge pretty easily. You power the the IVR with 12 volts, then run the 5 volt out to the gauge then hook the tester up to the sender post on the back of the gauge, and to ground.
Can I buy one of these yet? 
There can be other issues than that for fuel. The sending unit float can be bad, the arm may need adjusting (thatās typical for reproductions), the sending unit can be installed at an angle so that the system reads lower than it should, etc. In my experience, gauges rarely go bad or need adjusting. The IVR if old-style, can be adjusted so that the fuel tank reads correct, but that will affect your oil and water gauges as well. I routinely adjust the IVR so that a 1/4 tank of gas gives a 1/4 gauge reading.
This sounds like a nice little diagnostic tool. I am going to add a vacuum gauge and was thinking to supplement my oil and temp with aftermarket gauges. I could save the expense and space if I have more confidence in the factory gauges. IM me if/when they become available.
Wow! I was changing my cluster to LED lights and generally cleaning up the connections on it and trying to figure out why my gauges were reading low.
While burnishing the contacts on the circuit board and the plug I was thinking about building one of these. Iām an electrical engineer so conceptually this isnāt difficult.
What if you put a capacitor across the light bulb with terminals for volt meter probes so you could measure the regulated voltage for the mechanical IVR as well as the newer style 5V IVR?
I like that a lot. Need to scope a couple of samples and see what they look like
The less the current draw on the bulb the smaller the cap. Use an LED instead of a bulb for minimal draw. They are easier to mount than a bulb too.
Are you using the sender model of 10 ohms for full scale, 73 ohms for 0 scale (Empty, cold, low pressure)?
I could use one of these testers let me know when I can get one.
Definitely 10 on the high end.
Will do.
Bill, it sounds like you have enough interest to start a KickStarter campaign, if you need some up front capital for tooling up. Ideally this would also work with other similar cars from the era but this community might be enough to get you off the ground.