How do fuel efficiency lights work ('75 Eldorado)

[benji808]

I think I found the switch/relay on the upper-middle of the firewall. It has a line running to manifold vacuum, and two plugs going into two wires. My assumption is that the switch does something (perhaps grounds one or both of the wires?) based on the vacuum reading, causing the orange or green dash light to shine (my '75 has two separate lights; I think the '76 just had one that changed colors). Currently I get full power off both plugs with the car running, with or without vacuum (when I unplug the hose).

Am I correct in thinking the switch is busted? Any other ideas?

Thanks! My car is running pretty nicely now, so I'm moving on to more nice-to-have and novelty items

[TJ Hopland]

You are correct in the concept.  I don't have a good enough memory or a manual handy at the moment to confirm or deny what electrically or what vacuum reading is supposed to be happening but yes its just a switch triggered by vacuum.   High vacuum means things are not working that hard so green light.  Low vacuum means hard work so they picked a sweet spot that would change the light to amber or red.

[Steve Lomas]

I'm just learning of this now?  I had no idea my car had this feature (if it was standard) where are the lights?

I had a 76 also and never heard tell of such a thing..

[benji808]

The '76 Eldo I owned previously had it. It's one of the dash dummy lights- just a green or yellow/orange dot that lit up for "good" mpg (green light) or "worse" (yellow/orange).

[TJ Hopland]

I can't remember if you can see that the light(s) are there if they are not lit?  I don't think it was a standard item. I have been in Cads that had it but none of mine have that I know of or remember.

[KOKNEYELDO]

I do not think they were standard (only optional), although my Caddy mechanic handed me one a few years ago and was quick and easily installed and works great.

Shown on the right side of the attached photo.

[J. Skelly]

yes, it was optional

[benji808]

To put a bow on this - I took mine off, sprayed contact cleaner, messed around with the screws and attempted to bench test it, put it back on and it works now!! If I had to put money on it, I would bet that the ground nut onto the firewall bolt was just a bit lose. lol. I'll take the win for now!

[eldofever58]

As mentioned previously, it was optional, but all the wiring is present even if you didn't order it. These can be handy for monitoring other systems in an unobtrusive way. On my '76, I populated the sockets and tied them to a relay that would indicate high & low stall of the switch pitch torque converter. Or add a vacuum switch and bulbs and enjoy the "free" upgrade.

[TJ Hopland]

What year did that become an option?  I assume it came with the new dash which was 74?

[amarzano]

Is this item available to retrofit a car that did not come with it standard? Any one have a source?