Gas Gauge problem

[JayEstes 172]

I have a gas gauge that is not working. I first tried to verify the gauge by disconnecting at the sender (at the tank). I looked for gauge to go "hi" - but it did not. Next I went to the fuse block. After replacing the fuse, I notice the gauge barely rises above E when the key is on, but even with a filled tank, no higher. I am believing the gauge is ok, and I have mech/elec problem since it moves a small amount now.

 

So, I think I next need to try disconnecting at the tank and seeing if I can get the gauge to go "hi"? If it doesn't, what does that mean? Whats my next step? go on a long drive to empty the tank (more fun than just draining into a bucket right?) and check the float?

 

Any other advice here?

 

Thanks in advance - J

[69RavenConv 286]

Disconnect the wire at the tank and short it to chassis ground. The gauge should go to full. If it does, you've got a bad sender or float. If it doesn't, you've got an electrical problem. Don't leave it shorted for any length of time or you could do damage.

[JayEstes 172]

That's what I needed to know thanks. I went ahead and disconnected the plug again with the new fuse, and (as one should expect after reading your post) nada.

 

So in the "shorting to ground" process, is this the same as shorting one plug to the other, OR do I need to short one specific wire to bare metal on the chassis?

 

Sorry, but while I like to think I am a somewhat capable mechanic - even simple DC circuits can just baffle me sometimes. And thanks for the "don't short for too long" comment - I probably would have screwed that up and broken something.

[Rcodenewf 44]

Take the wire off the sending unit at the tank and ground it by using a small test lead or something that will make good contact inside the plug to a good clean chassis ground.

 

Have someone watch the fuel gauge while you are doing this as you may have to reposition the connection to ground. This is with the key in the 'on' position. No need to touch any fuses.

 

As mentioned previously, if the gauge goes to full then your gauge is fine and the issue lies in the sending unit.

 

John

[JayEstes 172]

I'm assuming from that it doesn't matter which contact in the plug I take to ground? (There are 2 wires in that plug)

[69RavenConv 286]

I'm assuming from that it doesn't matter which contact in the plug I take to ground? (There are 2 wires in that plug)

 

There should be only one -- that might be part of the problem. It's a yellow wire with a black right-angle boot connector, Here's a pic an a link from Mustang Monthly.

 

http://www.mustangmonthly.com/howto/mump_0606_ford_mustang_fuel_sending_unit/photo_04.html

 

[JayEstes 172]

OK. confession time. I am working with a 71 coupe in this case (although my car is a 69 coupe). And, I didn't even go back and do a direct compare my harness (currently out of the vehicle), because it all looked so familiar.

 

I am surprised the 69 has 1 wire and the 71 has 2, but I guess there must be some reason for that. It's amazingly similar other than that.

 

Good news is that I was able to try both pins, and one of them - when shorted to ground - caused the gauge to go "hi".

 

So, much to my relief, I don't need a new gauge, and I've verified the vehicle harness is good too.

 

I now know it's down to either the sender or the float. I'm suspecting the float because I get a gauge needle movement up of about 3 needle widths just powering on the car. I also hear todays gasoline is bad for these old floats, so that's a likely candidate.

 

So, now I'm looking forward to a nice long drive to empty the tank, and getting in to replace the filter, the float, and clean things up a bit.

 

Thanks so much to everyone on this forum - I find this is an extremely helpful place to work my issues!...

 

Jay

[69RavenConv 286]

Glad you solved it. Those floats are a common failure, as they don't (float) a lot of the time.