Hey all, I know this topic is a bit of a broken record but I am having some pretty weird problems with my fuel gauge. When I bought the truck the fuel gauge was working but the dash bulbs were burnt out, so I decided to upgrade and replace them with LEDs. Immediately after swapping in the LEDs the fuel gauge stopped working. On some fluke the gauge began working again (a couple weeks later) and I figured all was well, but when I switched from the front to the rear tank the gauge immediately pegged to empty... Then when I tried to switch back to the front tank the gauge still read empty and has been that way ever since. I have read that its usually a matter of a bad sending unit or a bad ground but I just installed a new sending unit on the rear tank and I also made sure the ground wire was making a good connection. I know the gauge cluster is controlled by a voltage regulator which makes me skeptical about the LEDs I installed but wouldn't the LEDs affect the voltage of the entire cluster and not just the fuel gauge? If anybody has any ideas I'd love to hear them!

 

Could be the tank selector switch is corroded. Try working it up and down quickly to make the contacts function. Might work.... Other wise try grounding the signal wire at the tank/tanks to see if the gauge moves.

 

I agree with OldStyle, investigate the tank selector switch and its connections, but because the most recent change was behind the cluster, ensure your connections & wiring back there are clean & tight.

With the key in RUN (but engine not running) remove the wire from the tank sending unit and ground it out, the gauge needle should swing fully back & forth as you ground/unground it.

the ICVR (Instrument Cluster Voltage Regulator) drops voltage to around 5.5V for the gas, temp & oil pressure gauges (if equipped) and has nothing to do with the cluster backlighting.

Power runs from the alternator/battery -> fuse panel -> ignition switch -> ICVR -> dash gauge -> tank switch -> sending unit -> ground. The sending units provide a variable-resistance to ground, resulting in the sweeping of the gauges as that resistance changes.

 

A couple minor corrections here. The fuel gauge is actually unfused. On trucks that have a FUEL or AUX FUEL slot in the fuse panel, this is for the tank switching solenoid. Also the ICVR is powered through an 8.5-ohm resistor wire to limit current going into the unit. The path is therefore:

Alternator/battery -> hot-in-ACC from ignition switch -> 8.5-ohm resistor wire -> radio suppression choke (where required) -> ICVR -> gauge -> switch -> sending unit -> ground.

To the OP: I agree to check the switch. Also, what did you do to verify the ground out at the sending unit?

 

Thanks, man! I figure if I'm wrong, somebody such as you or Mike0000 or similar will correct me.

If there isn't a fuse for these circuits in the fuse panel, won't there be a fusible link near the starter solenoid? As I recall, Ford fused *everything* somehow....

 

No worries. You're correct that a fusible link is upstream at the starter solenoid, however it's too large to protect the smaller fuel gauge wiring, so the fuel gauge circuit technically has no protection. However, since it's powered through a current-limited resistor wire, even if there were a short to ground somewhere where the wiring is exposed (along the frame, etc), the current flow is low enough that the wiring won't burn up. The only case where there would be an issue is if the actual resistor wire that's bundled in the harness were shorted near the source, and unfortunately the fusible link wouldn't burn. However, that scenario is almost impossible unless you're really trying to make that happen.

Most circuits are fused, but the fusible links at the starter solenoid cannot protect smaller wiring downstream in the event that the fuse panel is circumvented (like when people wrap foil around fuses, etc). 18-gauge wire will cook itself completely and the fusible link will be none the wiser. The fusible link is primarly meant to protect the heavy-gauge wire coming in and out of the ignition switch.

The ignition coil is unfused too, and this circuit is one that can actually be dangerous if messed with since the resistance is only 1.4 ohms. There are many cases of people frying their harness because they've somehow permanently grounded the coil, or they try to tie loads (like an electric choke) off the coil. The reason for not fusing it is primarly because it needs to be a fail-safe circuit, and in general there's not a problem unless someone tries doing something they shouldn't (which is the main time fuses and links come into play anyway).

On some earlier model Fords, the turn signals are not fused at all. Instead, the thermal flasher technically acts as a fuse.

 

After installing my new sending unit I took a dremel with a sanding attachment and cleaned up the location where the ground wire contacts the frame. I'll have to play around with the switch some more and see if I can't get something to work. I still haven't tried the method of grounding the signal wire to the sending unit but it sounds like a good start. Thanks for the advice

 

I have just installed a new side mount (mid-ship) tank and I do not remember how to connect the tanks. The selector I have seems to be different than the one shown in thebook. Can someone tell me how to connect the lines? I'm not sure how to post photos of my connector but it is an original. I have a 1977 F150 Ranger XLT.
Thanks,

 

Hey, Rick,

Best to start your own thread with your own problems instead of hijacking some other guy's.

Click on at the top of the forum's first page to do it.

For pics, upload 'em to ain image-hosting site such as imageshack.us or photobucket.com or whatever, then use the IMG code to source your images in your posts.

This should help:

User Gallery & Picture Display Help - Ford Truck Enthusiasts Forums

 

Thanks, Did not mean to hijack this one. New user but I will suely learn my way around. Thanks so much for your help.