Gas gauge not working properly

[Parsa Bagheri]

I have a 1988 ford ranger, 2.9L V6 with a dual tank system. Right now the gauge somewhat works but its as if there is a gray zone berween the 1/4 and 3/4 fill lines that the gauge cant go over/under. When we tried testing the ground on the pump we found that it may be a grounding issue since the gauge worked and showed what seemed to be an accurate amount of gas on the gauge when we used a test light on it, however when we removed the test light it went back down to the 1/4 fill line when tank is nearly full.

Even with the apparent grounding issue the fuel pump works just fine so thats where I’m a little lost. We recently busted open and repaired the electrical on the tank seitch valve since the rebuilds were completely wrong for my fuel lines, but I don’t think that has anything to do with it since it was doing this same thing before.

Any ideas/advice is appreciated.

 

[franklin2]

The sending unit is the ground for the dash gauge. The cluster supplies regulated power all the time to the gauge (when the key is on) but there is no ground, the ground is the sending unit wire and the sending unit is a resistor that varies that ground. It sounds like you have a bad sending unit with a dead spot in it.

If you want to test it some more, you can take the plug off the sending unit, find the wire that comes from the gauge (sometimes Ford used a yellow/white) and take this wire and ground it to a good ground. That will put the wire at zero ohms to ground, and the gauge should swing full scale empty or full (I can never remember which) when you turn the key on. If you then take this sending unit wire off ground and just let it hang in the air, this will be infinite ohms or very high ohms, and the gauge will swing full scale the opposite way.

If it passes that test, the wire and the gauge are good, your sending unit has a problem.

 

[Parsa Bagheri]

I’ll definetly give that a try, wheres the sending unit located?

 

[ShhhhhDougsHere]

I don't know for ever vehicle but I know in my 94 Ranger the sending unit is part of the fuel pump assembly. I. E. It is attached as part of the fuel pump assembly. Don't get me wrong the fuel pump is totally different. However it comes as one unit.

In these 2 pictures you Will see there are 2 pipes that go into the fuel tank.
1 of those pipes has the fuel pump the other pipe has the fuel sender unit on it.
The pipe you See that also has the black hose is the fuel pump pipe. The other pipe has the sender unit on it.
However if I remember correctly you said you have dual tanks so I don't know if it's the same for you. Just giving you a general idea.

Having 2 tanks you may have 2 fuel pump assemblys. I truly don't know for sure.

I hope this helps you because that is my intentions, is to help.

 

[Parsa Bagheri]

Alright I think my truck uses the same fuel pumps if not similar, I’ll check it out.

You’ve been very helpful, thank you!

 

[ShhhhhDougsHere]

Here 2 more pictures of my fuel pump assembly. You will see what I found wrong with mine. I'm still playing around trying to get my gas gauge to work right.

In that picture you see 2 wires for the fuel sender. Note the black wire goes from the sender unit to the plate the unit is attached to.

In this picture you see the in tank side of the point where the wiring harness plugs onto the fuel pump assembly.
The manufacturer of the fuel pump assembly thought the ground voltage or whatever it is would go through the metal of the pipe and the plate that the Sender unit is attached to, to the ground plug.
However the pen that goes through the upper plate that you see from the outside of the fuel pump assembly when it's installed has plastic around it and therefore the signal can't go through it.
I've also figured out that the metal that the pipe and both plates, the 1st one that the Sender is attached to and the 2nd one at the top that you can see from the outside when the assembly is in place are all 3 bad conductors.

This is what I've found wrong with mine.
I don't know what yours is got going on.
I'm just trying to help once again.

I hope you figure it out and wish you luck.

 

[corerftech]

I would investigate FIRST before the sending unit, the slosh filter board in the dash.

It is located next to the gas gauge, must remove the instrument cluster, a small circuit board about 1.5 inch by maybe an inch with a simple PC board. On it will be an electrolytic capacitor of a value, likely has exploded. Replacement will fix, typically. It snaps in with a brown plastic spring clip that is part of the connector.

The better solution is to bypass the board. Multiple ways to do. The easiest is to cut the traces and solder a shunt into the circuit or the super cheapy is tape over connectors . It will simply redirect the sending unit signal past the board and into the gauge direct. The gauge will work great, bob a bit but will be outstanding.

There is a group of 3 pads and group of 2. Insulate in any way the THREE and BOND/SHUNT the TWO.

I literally severed the spring pads at the board on the THREE and the TWO, then soldered a bridge wire at the base of the TWO to short them.

make sure the THREE are not making any contact to the board when done. The TWO can contact no issues, the short you create will bypass the board.

You can also cut the two wires and splice in any form your comfortable with, in the harness. Then remove the module completely. It is better, IHMO to leave module in place and bypass on the board.

there is at least 403.2 more ways to carry out the bypass. The gist is, short the cluster of two pads and defeat the three......

Lastly, oxidation and/or the filter cap could be your issue. Clean the contact pads well and test. That may be the only issue. My gauge was dead in 2020 on Frankentruck. I replaced the cap with a sundry test bench cap and that fixed it for a while. A year later it failed again and I said never again. Bypassed and trouble free for 3 years. Cap obviously vented in the case of the OE cap. Second failure was not diagnosed. I was pissed about taking cluster out AGAIN for the 900th time on all sorts of other issues....

 

[corerftech]

missing is the axial cap that lives above the op amp chip.

 

[ShhhhhDougsHere]

The technical name is SLOSH MODULE.
Front and back.

Dollar bills to give size perspective.
A SLOSH MODULE that's been bypassed.

Once again dollars for size perspective.
They made more than one kind.
I've seen another one that had 6 of the little metal tabs on it instead of 5.

 

[ShhhhhDougsHere]

I enjoyed reading your posts, " corerftech ".
Another perspective is always a good thing in my opinion.

 

[franklin2]

Would a faulty module still pass the ground/unground test at the sending unit?

P.S. Did 1988 have that module?

 

[ShhhhhDougsHere]

Parsa Bagheri,
I've found for me getting my instrument cluster out was not very easy.
Getting to the instrument cluster was easy peasy.

However getting the speedometer cable and the wiring harnesses unplugged from the instrument cluster was very hard for me.
I have rather fat fingers and hands, because I'm a short, fat guy.

I had to unhook my speedometer cable at the transmission so I could push the cable in through the firewall to get my cluster out farther so I could get my hands behind it. And that only helped a little bit.
I ended up using a flat head screwdriver push the clips on the wiring harnesses. And push up from below on the speedometer cable as well.
Hooking my speedometer cable back up was a whole nother job in itself.

Please be very careful if you choose to go this route.
I myself watched a few YouTube videos and they somewhat helped but not totally.
I find that sometimes hands-on experience is just the best teacher of all.

Once again I hope this helps you cuz that's my true intentions is to help.
Good luck.

 

[ShhhhhDougsHere]

I can not answer those 2 questions because I personally don't know.
"Curious Hound" or "Brain75" would be two guys I would ask those questions.

 

[Brain75]

You have a little different setup with dual tanks, but not terribly... I think only the very first few years (83-85? 86?) had a (gauge) sender only with no pump in tank. So you should still have a combo sender unit with gauge and pump in tank - everything after those first few years all had it all the way to the end.

That said with the dual, you have 2 sending units that both send signal up to the dash (or as pointed out more correctly provide resistance to ground). And a switch that switches which sender is completing the connection.
The pump ground is shared with the sender, so if you have a working pump, then you should have a good ground - unless a wire has crapped out inside the unit (like Doug's did - so he got to break out the soldering iron).

DC power has to be a closed loop and here is the full skinny.

Batt+ goes to the instrument cluster (one lead of the fuel gauge) then exits the 2nd leg of the fuel gauge on a Y/W wire heading to a switch (to toggle tanks) then to the fuel sender in the tank, then ground (and ground back to battery -)

I don't know for sure when the anti-slosh module was first added.... of the top of my head, I would guess before Rangers existed, so it is in all Rangers - but that is a guess.

edit add: the slosh module is either before the gauge or after the gauge don't know which for sure but the Y/W wire coming out of the dash to the fuel sender has already gone through it.

 

[Brain75]

the 87-89 mustang guys bemoan the slosh module too and indicate theirs is a different part number than 90+, so a direct answer... Ford was using anti-slosh modules by 88 for sure, and almost certainly the 88 Ranger does, have to pull the cluster out to see.