Progress Thread Goofy fuel gauge on slow progress

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Kearney, NE
After checking 'the magic book' it refers to using a ford tool that likely nobody has, I really want to collect some of these test tools, they are out there someplace but the book gives resistance readings for the sender so a VOM can be used for that:
145 ohms at full
22.5 at empty
Question #1 (GT model only) does the low fuel light work?
Note: GT model only, not sure if this is all GTs, mine is an 89.
#2 does the gauge stay in place when key is switched off?
#2. Pretty much. It stays where it is as long as I want to sit there and watch. It might not read right next time I get in though. Is that good or bad?

The gauge was in the middle when I got in the car this AM (not right, I had 3/4 of a tank). I tuned the key to the on position and the needle did not move. After it ran a bit, I turned the ignition switch on and off twice and the needle moved the second time.
And this afternoon the needle stays in place when off.
The reading drops a bit when the ac is blowing and the engine is off, and a bit more when the starter is hit, vs. when the when the engine is running. So now it’s moving when on instead of stuck.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
39,160
15,053
224
Massachusetts
You should be able to access the wiring at the rear of the car without dropping the tank. There is a connection for the fuel pump and fuel level sender that you can reach and unplug. You can then read the resistance off the sender and ground out the wire to the cluster to see what the needle does.

I’ll have to get the wiring diagram for 91 but it’s similar to this. Remind me later
 

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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
39,160
15,053
224
Massachusetts
Here’s the 91 diagram

18B44E2B-7FDC-4F10-B332-EE1EBA9A5095.jpeg


I think C400 is the connector at the rear that you can access that does fuel pump and sender. Black and yellow/white wires are what you want. If you check them from the sender, you should have 22.5-145 ohms depending on how much gas.

If you ground the yellow/white, it should max out the fuel guage.

Bad slosh module also a possibility
 
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7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,373
3,078
194
Kearney, NE
Here’s the 91 diagram

18B44E2B-7FDC-4F10-B332-EE1EBA9A5095.jpeg


I think C400 is the connector at the rear that you can access that does fuel pump and sender. Black and yellow/white wires are what you want. If you check them from the sender, you should have 22.5-145 ohms depending on how much gas.

If you ground the yellow/white, it should max out the fuel guage.

Bad slosh module also a possibility
Thanks!!!
This is next- after the yard projects and recovery. Help finally showed up, but this is likely to get in progress sooner than that anyway. I should have some resistors or potentiometers in my electronics cabinet to set up a test with.
 
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