Aftermarket gas gauge?

[dohebert]

My gas gauge doesn't move at all. I tested the sending unit by unplugging the connector at the tank and jumping the two sockets with a wire. (That should ground it, right?) [Editors note - that might be what blew the 20 amp fuse that my cigarette lighter shares.]

I replaced the voltage regulator and swapped the gauge itself (both gauges I have will pegged to "FULL" when I momentarily fed them 12V. The printed circuit looks good - no splits, no breaks, no other problems. Nothing obvious there.

So I'm open to suggestions - should I test the wires at the sending unit differently? (The instructions I read are, "Disconnect the wire at the sending unit and ground it - the gauge should go to full.) Hmmm . . . where does that circuit ground, anyway?

I'm bout at the point where I'm thinking I'd like to just start from scratch and put in a new sending unit, run new wires to the dash, and retrofit an aftermarket gauge. I worked aftermarket mechanical oil, temp, and ammeter gauges into the stock location on the console, and those turned out pretty well. Any suggestions for an aftermarket gas gauge with a similar diameter as our Ford gauges? Or any other suggestions that might get me back on the road to having a working gas gauge?

 

[TheRktmn]

If your gauge (Fuel, Oil or Temp) goes to full range with the key on then you have a bad instrument voltage regulator or a short to ground between the gauge and the sending unit.

These gauges are all the same internally - they use a bi-metal strip that heats up as 5VDC is passed through them to ground. The sending unit acts as a varistor to regulate how fast the current gets to ground, thus regulating how hot the bi-metal strip gets and how far the needle moves. They are made to read low scale at 73 ohms and full scale at 10 ohms. Above full scale usually means that the gauge is getting a full or almost full ground (~0 ohms). The gauge will also swing full scale if the voltage regulator that steps the 12VDC down to 5VDC - located on the back of the cluster - goes bad, but that will effect all three gauges (if you have Oil & Temp gauges as well).

Since you already swapped the instrument voltage regulator with a known good one the next step is to check the wire from the gauge to tank for grounding.

EDIT: You may want to check your body ground as well. If the body ground is bad then power is going to go whatever route it can find to ground.

 

[dohebert]

"Since you already swapped the instrument voltage regulator with a known good one the next step is to check the wire from the gauge to tank for grounding."

I could use a tip or two on this. I unplug the connector at the sending unit, turn the key on, and test each side of the connector with a voltmeter. I stuck the hot prong of the tester into the connector, and held the ground side against several parts of the body. I'm picking up very minimal voltage there - point 01 to point 03 volts on each side, whether the key is on or not. That doesn't sound right to me - voltage should be traveling to the sending unit on one of those wires, and the voltage should be higher, I think. Got the same result at the plug in the driver's kick panel, so I don't think the problem is a bad wire in the harness between the kick panel & the tank.

I'm not having any other electrical issues - but where is the body ground?

 

[TheRktmn]

Yes, you should have voltage coming from the gauge to the sender. A lack of voltage looks like a ground short to me.

By the way - don't put 12V into your gauges - they are made for 5 volts and will burn out VERY quickly with 12 volts going through them.

When you unplug the wire from the sending unit and turn the key on what does the gauge do?

If it goes up you have a short to ground somewhere between the gauge and the end of the wire.

If it does nothing there is no ground. That's good as far as shorts go, but now you need to check the sending unit.

You will need an ohmmeter to check the resistance of the sending unit:

First, disconnect the wire(s) from the sending unit.

Connect one ohmmeter lead to the post that the gauge wire (yellow/white?) hooks to.

If a you have a single wire going to the sender: Connect the other ohmmeter lead to a good ground.

If you do have two wires going to the sending unit the other post should have a black wire to it. Hook the other ohmmeter lead to the other post (instead of ground) so you can get a reading though the sender.

If you get a reading of 0 your sending unit is bad.

If you get a reading between 10 and 73 ohms the sending unit is not the culprit. If this is the case then you have to start chasing gremlins and that is pretty much trial and error.

I do not know off hand where the body grounds are on a 71-73. I believe that the 67-69 use a strap between the block and the firewall and the 70 had an additional heavy lead from the battery - side to one of the bolts that holds on the voltage regulator. Someone here should know for sure. When in doubt, add one. You can never have too many grounds.

At this point we have about exhausted my limited knowledge of accessory gauge wiring. Hopefully it is some help.

(editted to differentiate between 1 and 2 wires at sending unit)

 

[dohebert]

Thanks!!!!

The ohmmeter jumps around a little at first and settles at 87.5 ohms at the sending unit prongs.

When I disconnect the plug from the sending unit & turn the key, zilch, nada, nothing.

Coincidentally, there's a wire under my dash that isn't connected to anything. I call it the mystery wire. It has a plug on the end that is shaped like a two prong plug (like on the back of the tach), but instead of having two wires with a female and male side, there's only one wire to it and it has a male end. Which tells me it's a ground . . . because they don't make hot wires that would short if you unplugged the connector. This particular wire goes over to the passenger side in the dash and through the firewall into the area where the air intake hats are, and it doesn't seem to reappear anywhere. I'm going to take a wild guess that if I lay down with a flashlight under my dash and figure out where the other end of that plug is, I might get my ground back. That's a "first thing in the morning" job, before it gets too hot around here. Anybody know the area under the dash where I should be looking for the end of the gauge ground, or what color the wire is? It might not have its connector end still attached on the gauge side.

 

[dohebert]

No luck. :-(

I followed the mystery wire - it goes to the red box over on the pass. side firewall that would probably make a "buzz" if my seat belts weren't fastened, etc. - no wonder that isn't hooked up. I found two other unattached ground wires - one of them at the cigarette lighter and the other just a loose black wire behind the instruments; but neither made the gas gauge needle move when I hooked them up to a solid ground.

Since I'm not running any other factory electrical gauges, can I just run a new ground wire from one of the threaded prongs on the back of the gas gauge? The one on the passenger's side when the gauge is installed in the dash is the ground side?






Ah, I'm re-reading and I think I'm getting it -

If the gas gauge needle doesn't move when I unplug the connector from the sending unit & switch the key to "on," that's good, as it indicates that I don't have a short; my reading with the ohm meter is high, so even if everything were working properly, the needle would still read "empty."

But I have to figure out why I didn't read any voltage in the wire leading to the sending unit before I worry about replacing the sending unit.

Like I said, I'm about ready to start from scratch and retrofit a whole new sending unit and gauge that bypasses the original. If anyone has any more suggestions on either fixing the original or replacing everything, I'm all ears.