Fuel Level Gauge No Longer Working

[The Warden]

Hello!

I know that it's a pretty common problem for fuel tank sending units on these trucks to not be working properly. So, it's been a bit of a pleasant surprise that, over the last 12+ years I've had the F-250, both senders have worked flawlessly. I was therefore surprised to find my fuel level gauge not responding when I drove the truck last week. The gauge is sitting at the lowest point the needle can travel to (i.e. the same spot where it sits when the key's out of the switch), and the gauge doesn't move when I switch tanks using the factory switchover valve. I checked the fuse and cycled the valve a few times with everything else off, and I heard it moving, so I think the valve is okay. Certainly, the truck's running just fine.

Given that the gauge won't respond with the valve in either position, and given the suddenness of it all, I'm having trouble believing that the senders failed...it seems to me that fuel level senders generally fail more gradually than this, and why would they both fail at the same time? I pulled the plastic cover off the gauge cluster and tried moving the needle by hand to make sure the needle wasn't physically stuck...it seems to be moving just fine.

So, what else can I check? I should note that none of the other electrical gauges in the cluster have ever worked; they've been disconnected in favor of mechanical gauges since before I inherited the truck...so, it's a possibility that the IVR failed, if this truck has one (I want to say it does, but it's been so long that I can't even remember if one exists, let alone where it's located). Alternately, what fuse operates the instrument cluster gauges? FWIW the gauge illumination works properly, and I couldn't find another fuse that might control anything in the cluster. I'm sure I'm missing something simple...if necessary, I can go back to switching tanks at a set mileage and refueling at another set mileage like I used to do with my old van, but I'd love to get the gauge working again if it's feasible.

Any thoughts would be appreciated...thank you in advance

 

[gatorman21218]

When you disconnect the sender on a 91, the needle reads full. So I think you are on the right track about your cluster.

I now have aftermarket stewart warner gauges for fuel. The front tank is pretty damn accurate. Dunno about my rear tank because the FSV blew up about a week after I installed the gauge

 

[gandalf]

Now you know why Ford put a trip odometer on these trucks. They're not as dumb as we thought.


My gauge runs the opposite. The needle reads off the scale to the right, way above full.


I guess that means that my gauge is an optimist and your's is a pessimist.

 

[franklin2]

Put the switch to the front tank. I believe you can stick your head up in there enough to get to the front sending unit on top of the tank, and unplug the wiring. Once you unplug the wiring, get a scrap piece of wire and bolt it to a good ground somewhere, you could go all the way to one of the battery neg posts if your wire is long enough.

Turn the key to run but do not start the engine. Get someone to watch the fuel guage while you get back under the truck, take your good ground wire, and touch it to the yellow/blue or the blue/yellow(I forget which one goes to the front sending unit). When you touch the ground wire to the correct sending unit wire, the gauge should swing full scale empty or full, depending on if you have a 87-down truck or a 88-up. Once it makes it full scale, take the ground wire off the sending unit wire, and let the plug just sit there and touch nothing. The gauge should swing full scale the opposite way.

If it passes this test, the wiring, the switch/valve on the frame is good, and the wiring to the gauge and the guage itself is good, and your sending unit is bad. There is the slight possibility the factory ground to the sending unit is bad, but to check this you can use that ground instead of the ground in the first test and see if the gauge still swings back and forth.

 

[OLDBULL8]

Search: I posted the FSV and guage circuit several times, not too long ago.
Lost all my several hundred pic's and info, virus hit me badly. There still on my puter, but don't know how to retrieve them.

 

[The Warden]

When you disconnect the sender on a 91, the needle reads full. So I think you are on the right track about your cluster.

That's true on brick-nose and OBS trucks, but the slant-nose trucks go the opposite way for resistance. Brick and OBS trucks are 22.5 ohms empty and 145 ohms full; slant-nose trucks are 70 ohms empty, 10 ohms full. Also...IIRC on all brick and OBS trucks, the gauge freezes in its last known position when you take the key out; on slant-nose trucks, the gauge goes to the "stop" on the E side as soon as the key comes out, then is supposed to go to the position the sender tells it to be at once the key's turned back on.

Now you know why Ford put a trip odometer on these trucks. They're not as dumb as we thought.

NIIIIIIIIIIICE...unfortunately, trip odometers were still only an option in '84, and even though my truck came from the factory loaded with every other option available except for a slushbox, I don't have a trip odometer on mine

My gauge runs the opposite. The needle reads off the scale to the right, way above full.


I guess that means that my gauge is an optimist and your's is a pessimist.

I like your way of thinking

Put the switch to the front tank. I believe you can stick your head up in there enough to get to the front sending unit on top of the tank, and unplug the wiring. Once you unplug the wiring, get a scrap piece of wire and bolt it to a good ground somewhere, you could go all the way to one of the battery neg posts if your wire is long enough.

Turn the key to run but do not start the engine. Get someone to watch the fuel guage while you get back under the truck, take your good ground wire, and touch it to the yellow/blue or the blue/yellow(I forget which one goes to the front sending unit). When you touch the ground wire to the correct sending unit wire, the gauge should swing full scale empty or full, depending on if you have a 87-down truck or a 88-up. Once it makes it full scale, take the ground wire off the sending unit wire, and let the plug just sit there and touch nothing. The gauge should swing full scale the opposite way.

If it passes this test, the wiring, the switch/valve on the frame is good, and the wiring to the gauge and the guage itself is good, and your sending unit is bad. There is the slight possibility the factory ground to the sending unit is bad, but to check this you can use that ground instead of the ground in the first test and see if the gauge still swings back and forth.

Thank you for that! I'll give it a shot tomorrow, although I don't have anyone to help me...but I can get creative with alligator clips

Search: I posted the FSV and guage circuit several times, not too long ago.
Lost all my several hundred pic's and info, virus hit me badly. There still on my puter, but don't know how to retrieve them.

Any chance you remember the title of one of the threads you posted it in? I did a search but haven't found the circuit diagram you mentioned yet...I'll look some more in the "morning" (it's 05:00 here and i'm about to go to bed).

Thank you guys again I'll look into it again tomorrow...supposed to be towing a boat up the Delta this weekend, so hopefully I can get it figured out tomorrow...

 

[gandalf]

I think, and it's only a maybe, that I have that wiring diagram that Bill posted. It's in a pdf format, so I'll have to figure out how to insert it here. Maybe it goes in like a picture. I have it labeled for an '88, but that may not necessarily be true.


Nope, it will not upload. The system claims it's too large for that type of file. Send me a PM with an email addy. It's 2.54 MB, and the limit is ~244Kb.

 

[sle2115]

I think, and it's only a maybe, that I have that wiring diagram that Bill posted. It's in a pdf format, so I'll have to figure out how to insert it here. Maybe it goes in like a picture. I have it labeled for an '88, but that may not necessarily be true.


Nope, it will not upload. The system claims it's too large for that type of file. Send me a PM with an email addy. It's 2.54 MB, and the limit is ~244Kb.

Send it to me and I'll see if I can get it on [email protected]

 

[riphip]

What I've seen on my senders in the past, is the float fills with fuel & will not register. I have replaced the floats with originals but they end up leaking also. Am presently using a nylon bottle with screw-on cap (sealed) and have not have any problems in the past year. This is an '86 I've had for a few years. Rick

 

[icanfixall]

I was thinking the float finally gave up and drank some diesel. Might pull the front tank sender and see if thats an issue. I didn't read what tank or maybe both tanks have this issue in your first post...

 

[bookite03]

Good timing on the diagnostic info...my front tank reads empty all the time. I discovered this when I tried filling it up after I bought the truck only to find myself with a filler-neck full of fuel! I checked around for availability on a sender, is it a dealer only part?

 

[sle2115]

Here ya go

You must be registered for see images attach

You must be registered for see images attach

 

[The Warden]

Sorry for my delay in replying; got busy with work and a few other things and didn't have a chance to get back to it until this evening.

And...I think I'm even more stumped than I was. Given that the gauge was already as far towards "E" as it would go, and since my '84 wouldn't peg at "F" when I shorted the wire to ground, I thought to try shorting to +12v (used a plug going to one of the running lights so that it wouldn't have too much amperage, and I could switch it on and off from the dash) to see if the needle would move to "F". Since I wasn't sure which of the terminals at the sending unit I should short to, I went to the FSV plug instead and shorted to the #4 wire that goes to the gauge. And, the needle started moving! So, it looks like whatever problem exists is either at the FSV or at the sending units.

I tried unplugging the FSV with the switch set for the forward tank, then moved the switch to the aft tank with the key on, then got under the truck and plugged the FSV back in...and, when I did, the FSV clicked like it's switching over properly. So, I THINK the FSV is okay...certainly I hope so; I'm towing a boat tomorrow and would really like to not have any fuel starvation problems. (I had "fun" redoing some of the trailer wiring; that's another story :rolleyes )

Also, I was able to pull the plug going to the forward tank sending unit, and did a resistance test on the terminals in the sending unit...and, the voltmeter showed 70 ohms, which is consistent with an "E" reading on the gauge. So, maybe the sending unit really did fail...but, again, it strikes me as very odd that both sending units would fail at exactly the same time, and with no warning...unless it's like Rick said and both floats happened to get holes in them between the last time I drove the truck and two weeks ago. Again, this problem started happening with both tanks, at exactly the same time.

I'm about to collapse...thank you guys again for all of your help I really appreciate it...and, any more thoughts would be greatly appreciated

BTW, does anyone know what model year that diagram's from? If this would benefit anyone, I can take the full-size PDF and throw it up on my webpage...Ken, I'll PM you my E-mail address...

 

[franklin2]

It says in the upper left corner "88 F series". I think you need a redo on the diagram before you put it on your website, somewhere during the process it got a little too small and fuzzy to read, for me anyway.

Don't put voltage on the sending unit wire going to the gauge. The gauge is fed voltage from the cluster, the sending units are actually the ground for the gauge. More or less ground(more or less resistance in the sending unit) makes the gauge read across the scale. The older trucks(around 87-dwn) actually run the gauge on a pulsing voltage, and you can see this with your meter if you take the sending unit loose and put the meter on the plug coming from the FSV. I am not sure what you would read on the newer 88-up trucks as far as voltage from the gauge. The 88-up clusters are really more of a copy of the system GM used for years.

What ground did you use for your test? I believe you are correct, 70 ohms is giving you the "E" reading on the gauge, 0 ohms(shorted to ground) should have given you a full reading. make sure you wait a little bit, these gauges can be slow sometimes.

 

[The Warden]

Yeah, in retrospect, I don't know what I was thinking when I put voltage to the line...I was tired and a bit frustrated last night. Still strange that the gauge responded to that and only to that.

For the tests, I grounded directly to the frame...I had to take some steel wool to get a decent ground point, but I did a continuity check and made certain that I had a good ground. I left the key on for a solid minute with no change; certainly, the gauge reacted more quickly in the past!

And, yeah, I missed the URL on the top that said 88... that's what I get for posting while tired :rolleyes My 1984 shop manual actually made reference to an instrument voltage regulator, so I know for a fact that one exists...maybe my next step is to look at that? Unfortunately, that also means that there's at least one difference between the '88 diagram and an '86 and older truck.

Let's just hope that the FSV is playing nice...I'm about to hit the road

Going back to the diagram, I spent some quality time working with the file...here's what I came up with:

You must be registered for see images attach

I also made a super-high-resolution version that's too big to attach as a file; you can find it here for the moment: http://dieselwarden.net/temp/1988 Ford truck fuel tank selector wiring (highest quality).jpg

How does it look on other computers? It came out pretty good on mine, FWIW...