Fuel gauge troubles - 94 B4000

[curtis73]

My fuel gauge is funky. It doesn't seem to act any specific way. One minute I'll look down and it says 3/4, then I look 10 miles later and it says 1/4. Another while and it might say full. I don't see it moving (but the slosh circuit probably just makes it move very slowly). When I first turn the truck on, the gauge will go somewhere. Right now I was just out playing with it and it said 3/4 when I turned the key on (which I know is false since I've been driving all week) and then after I messed around with airing up a tire it said 1/8th.

I separated the grey connector under the brake booster and the needle started to swing to full. I verified by turning the switch off and then on and it immediately went to past full. I grounded the yel/wh wire and the gauge read empty. I tested on the male side of the connector on the yel/wh and blk/yel wires and got 102.6 ohms. That sounds about right for 3/4 tank. I tested the female side of the blk/yel to ground and got 0.2 ohms, so I know the ground is good.

Before I go uninstall the bedliner and take the bed off, does it sound to you like the sending unit is bad? Like maybe its getting bounced around or intermittently losing ground? I checked the ground at G103 and it looks flawless so I don't think it's intermittent grounding.

I'm tempted to backprobe the connector with my multimeter on the windshield and drive around to see what the sender resistance is while driving; see if it drops to zero or shows an open every once in a while. Or, if it's showing 100 ohms but the gauge says empty, then I know I have a bad gauge?

Halp.

 

[RonD]

Reads like you have tested the sender wiring very well and it is working as it should.
You can rock the truck when OHMing the Yellow wire and you should see OHMs change as the Float goes up and down as gasoline sloshes in the tank.

My bet would be on anti-slosh module, it has many failure modes, but yes it could be sender has some dead or shorted out spots

 

[curtis73]

More on the story:

I took it out to run some errands and fill it up to see how much it took. Earlier today it said 102.6 ohms. As I pulled into the station, I noticed it said about 1/4 tank. So I pulled the connector and tested again and got 98 ohms. Consistent with driving it around a few miles. But to my dismay when I turned the key back on, the gauge read correctly at just under 3/4. So is it a faulty gauge, or did the faulty sender correct itself in that 30 seconds? I filled the tank and it only took about 4 gallons which confirms that it really was at 3/4. Then when I got home the same exact thing happened; it read about 1/8, I pulled the connector and it tested 155 ohms (consistent with just filling it) and when I plugged it back in the gauge read correct at full.

I suspected the connector itself, but backprobing both sides showed zero ohms no matter how I twisted or shook it, and the connector itself looks pristine.

Grrr. So frustrating.

I think maybe I should get a junkyard cluster and swap out the gauge and slosh circuit to see if anything changes. $25 and pulling the dash is a lot easier than pulling the bed.

 

[RonD]

Yes, problem is in the cluster, IMO

 

[curtis73]

How do I get the speedo cable off of the gauge panel?

Got the slosh circuit out. Looks like this. How do I test it/repair it?

 

[RonD]

Read here for slosh module by-pass: http://www.therangerstation.com/forums/showthread.php?t=140669

Or get another one from a wrecking yard, Ford used same module in many models in similar years

There should be a white plastic Clip on the speedometer cable, push up on the clip at about the 6:00 position and pull it back

Pictures here, towards the bottom of the article: http://www.therangerstation.com/tech_library/speedometer.shtml

 

[curtis73]

Thank you for those links. Found a guy selling refurbished and tested sloshies for $30. Figured I'd give it a try since a junkyard unit might not be good either.