Gas Gauge Regulator

Another weekend, another project.

This weeks episode, "The Wonderful World of Wiring", brought to you by Mutual Of Omaha.

New gas sending unit. It tests out fine. Hook it up to the existing gauge, a slight movement and it races to half. Uh oh. Wire from the gauge is 12 volts. Not good.

Search, read, search, read. I learn a lot. The gas gauge has an internal resistor that isn't actually a resistor, it's a bi-metal regulator that opens and closes rapidly and digital volt meters will register this as 5 volts. A test light should show the open and closing. Mine is a nice, solid, very bright 12 volts.

Pull the cluster out. Clean up some PO's wiring mods while I have the dash off. With gauges apart and in my hands, I test with ohm meter. John Strenk has a test guide linked on another inferior site, but seems well put together. My readings match his exactly.

The regulator heater looks like the insulation has burned off, but the test readings show it isolated from the bi-metal strip. I run 12 volts in, test light to ground out and it doesn't heat up. The gauge arms on both the gas and temp gauges work fine- I can feel them heating up as the needle sweeps when I do the same test.

The regulator arm is quite rigid. Might it have been "adjusted" by a PO? The contacts are clean and not pitted.

After this long, babbling intro, here is my question. Why did the chicken... nevermind. Do I need a new gas gauge and where can I find one?

I'll post a picture. For those that know what I'm rambling about, it may help. For those that don't... be glad.

 

It's a 1966. I need to add a signature, I guess.

 

This place has gages and the whole spedo if needed.
http://www.cj-jeep.com/cj5-6/cj5-6_parts_and_accessories.htm

 

The internal regulator is is fried, as you have observed with your volt readings and test light probing. If it was mine, I would shunt across the internal regulator and install an external one from a later model. If you really want to tinker with it you may have to use a variable resister with the external regulator so you could calibrate the gauge.

 

an internal resistor that isn't actually a resistor, it's a bi-metal regulator ?
when i 1st got the slug
i spent about 5 minutes reading up on my gas gauge.
then i went and found myself a nice stick to measure with.
p.s.
i ran out of gas yesterday.....

 

I went through something like what you are doing and found I had 2 choices.
You can replace the gauge with a new one. They are available as repro from places like Walcks. They also read opposite from originals but my 66 now has one that is working great. I ended up replacing both temp and fuel as a set along with the sender when the old one rotted.
Second choice is to make a 5 volt regulator from a regulator chip that you can get at an electronics place. In searching it turns out the Mustang guys go through the same problem with the bi-metal regulator that burns up. Take the 12 volts as the input and tie the 5 volts to where the output of the bi-metal should be showing about 5v. This way you keep the old gauge. I havn't searched out the parts but it seems doable.

You might also want to be sure there is a separate ground running from the sender to the body sheetmetal. The original tank used chance and friction to make a connection to the body..not that great.

 

Solved. A spare XM adapter that puts out 5 volts, a quick bracket and the gauges work.

 

Nice innovation!

 

An adapter like that likely contains a 7805-type device http://www.radioshack.com/product/index.jsp?productId=2062599 Of course, the RS part is about ten times what you'd pay from a real electronics supplier (Mouser, Digikey, Allied, others) but you can buy it from a store front off a peg.

Easy to wire up if you have a little electronics experience. App notes in the data sheet typically have a lot of useful info http://www.st.com/stonline/books/pdf/docs/2143.pdf