With the wires disconnected from the sending unit, the sending unit is out of the picture and can't affect anything.

Basically, what happens is.... power comes from the battery to the ICVR (a voltage regulator that provides reduced voltage to the gauges)... from the ICVR to the gauges.... from the gauges to the sending units, the sending units provide a variable resistance to ground, which allows variance in how high or low the dash gauge needles read.

I should probably go and re-read this entire thread, just to be clear on the problem and what has been discovered thus far....

About your gauges, I was wanting to know if they all appear to be reading "too high" or "too low" together. When the ICVR wigs out, those three gauges will occasionally (and somewhat suddenly) start reading lots higher or lower than (then they normally do) together.

 

What setting do you have your meter on? It is possible that you are getting a weird reading because the IVR, instrument voltage regulator, which is chopping the direct current into pulsating current, is creating a frequency that your meter doesn't like.

But, the fact that you have some voltage there says the wiring and the tank switch are good.

 

It's the EVTM (Electrical and Vacuum Troubleshooting Manual). It is published by Ford and part of their Shop manuals.

If you just do a search on 1986 F150 EVTM you will find it out there somewhere. It covers more then just the F150 btw. Cover up the the 350

 

I found out that the back tank actually works the gauge, I just didn't put enough gas in it to really tell until I slapped the dash to make it move. I had it set on 20V and on 200V and I tried other settings too but they all gave me the same thing. Oil pressure gauge works fine but it goes all the way up to high (engine has a high pressure pump) but it does go down slightly when your running down the road when the engine gets hot like it should. Amp meter works like it should. And temperature gauge works normally. Oh and the tank switch in the dash does switch the tanks like it should. I really need to put more gas in the second tank to make sure it's not just some fluke thing with the gauge moving by hitting the dash. But every time I switch tanks and make the gauge go up and down it always goes to the same spot when I select the rear tank. Makes sense because I only put 2.8 gallons in (12 bucks worth)

 

That's a good point, I forgot about the pulse behavior of that thing.

 

Where is this regulator at?

 

The regulator is a rectangular lump on the back of the instruments, on the printed circuit. It is supposed to modulate the voltage such that it looks to the instruments like it is 5 volts. But, each IVR is different, and they tend to change voltage every once in a while, which is noticed by all the instruments changing together.

 

https://www.ford-trucks.com/forums/1...l#post11335944

 

I just had my dash cluster half torn out last night and never even noticed that thing. I doubt that's the problem since every other gauge works perfectly fine.

 

At 16 you are good at deductive reasoning. Good job! If the other gauges work it isn't the regulator. If the fuel gauge goes to full if you ground the wire then the gauge is good. And, if you are seeing something showing on your DVM on the wire to the tank you have connectivity to there. In other words, you have a bad fuel sender.

 

Thanks! I actually have 4 1/2 months to go until I'm 16,(just got my permit recently) I guess I'll remove the gas tank someday soon and pull it out.

Thanks all for your time and help I think I'm going to drop the tank this weekend. I'll let you all know what I find when I drop it.

 

You don't actually have to drop it. The all-threads holding the ends of the straps together usually have enough length to allow you to loosen them enough to get to the sender. You'll need something to tap the wedge to free it up, and since when re-installing it you'll have gasoline vapors you should use a non-sparking drift such as brass.

And, if yours has rubber fuel line you should replace it at the same time. I've seen some absolutely awful stuff and they sometimes leak gas right through the walls of the hose.

 

Chris, Ford trucks prior to 1989 have a 5 volt regulator for the gauges, as a result the voltage pulses from 0 - 12V and back to maintain 5 volts.

FordFETruck, many of the replacement senders are not grounded at the sender, they require the tank be grounded. Try a ground from the lip of the tank outside the weld to the frame and see if it starts working. FWIW, open circuit on the early system = empty.

 

I think I'll try that, I can actually get both of my hands up in there right now as it is. My tank is a rubber line to a certain point but the hose feels really soft and looks fairly new actually. I'll double check it again to make sure though lol

 

Alright so I dropped my midship gas tank last night, in the rain. Crawling all over puddles of water in my driveway for almost 3 hours . A royal pain that was. I took the sender out today, my first question is why the he** are they so expensive for something that simple? also the part number for mine shows up as a slightly different sender than what's in my tank.