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New sending unit and shower head pick up, The gauges on the dash are still way off. When the tank is full, the needle is WAY past FULL. When the gauge is at 1/2, your probably out of diesel.
Is there a way to calibrate those gauges? It's not that I'm THAT OCD, but my wife is.
Actually, most of the gauges are off. VOLTAGE gauge is off, TEMP gauge is, too. Oil pressure gauge is good.
The sender on the 80-86 trucks are opposite of the 87+ trucks. They both work on circuit resistance. One reads full at high resistance and the other reads full at low resistance. I can't remember which is which but if you search this site it's been posted many times how your 89 works. Sounds like you are off range a bit. Could be caused by a poor connection adding resistance to the circuit. It's been a long time since I last messed with mine but before installing a sender it's good practice to ohm them out. Checking the reading from float up to float down noting the high and low resistance readings and while swinging the float seeing a smooth transition in the ohm reading. Then you can ohm out your circuit to make sure when added to the sender ohm readings you are within spec. It's not unusual for the guage to read past full a bit when the tank is full. It comes down pretty fast from that reading. Does your guage go to half full in about the right amount for a half tank used but then stays at half for an unusually long time? Could be the float is hung up at half way on something? I know of no way to calibrate these guages. They are supposed to be built correct. Then after many decades they do go crazy, wear out.
my 88 is off by at least 1/2 tank. it will read full for ever. then go from full to 1/2 in 10 miles. then to empty in another 25 or so. stop and "fill" when it hits empty and you only put 6-7 gallons in the tank. so i learned a long long time ago to always keep one full tank, and fill the in use tank at 250 miles
It's not surprising that your fuel gauge reads inaccurately. If you remove the fuel sending units to double check their ohm range you need to make sure the shower head is present. Either that, or extend the pickup about 2 or 3 inches with compression fittings and a fuel strainer.
On my 1990, the higher the resistance, the higher the guage. So if there is any resistance between the sender and the guage, it will read too high. In my case, it was the tank selector valve having dirty contacts inside. It was one of those gray ones from Bezos. Tank must be well grounded, connectors at tank, tank selector valve, and big plug on back of cluster must be perfectly clean.
Measuring to ground:
Empty Tank: ApproxiApproxi 16 - 22 Ohms
Full Tank: Approximately 155 - 160 Ohms
Mine is exactly 16 & 160
This 16–158 Ohm range applies to most Ford F-Series trucks from 1987 and newer.
One of mine was so old it, had apparently rusted off, so someone in the distant past had put a compression fitting on the stub, extended the tube with a length of steel line.
I like how my Facet fuel pump has the removable screen. Personally, I prefer the screen outside of the tank for obvious reasons.
I opened up the cheap tank valve, and I wasn't surprised to have fuel run out of the electrical area....
motor in rear turns a black thing on screw threads that moves port valve fore & aft to select tank lines.
Slider also has 2 sets of contacts:
1 selects sending unit to guage (rectangular contact)
2 switches power to motor (triangular contact)
Possibly, very carefully, drill a small approx 2mm hole just deep enough to get inside at location marked in red, and spray contact cleaner inside.
In my case cheap design filled with fuel.
I ended up drilling rivets on 36yo valve, polishing, greasing contacts (it had old hard yellow grease) and reassembling.....works like new! Guage circuit fixed.
I opened up the cheap tank valve, and I wasn't surprised to have fuel run out of the electrical area....
motor in rear turns a black thing on screw threads that moves port valve fore & aft to select tank lines.
Slider also has 2 sets of contacts:
1 selects sending unit to guage (rectangular contact)
2 switches power to motor (triangular contact)
Possibly, very carefully, drill a small approx 2mm hole just deep enough to get inside at location marked in red, and spray contact cleaner inside.
In my case cheap design filled with fuel.
I ended up drilling rivets on 36yo valve, polishing, greasing contacts (it had old hard yellow grease) and reassembling.....works like new! Guage circuit fixed.
That's excellent that you were able to repair the FSV!
Mine went bad years ago so I replaced it with an aftermarket one (which I recently had to replace). I think you can get OE style FSVs from LMC now. That wasn't an option when mine originally died.
There are a plethora of those gray plastic valves online, all appear the same. The pics even show the same orange & yellow caps on the fittings. I'll avoid them in the future. Possibly grab a couple of OEM ones at PYP and figure out how to replace whatever seals are inside the valve portion as well.
Last edited by gespeter; May 14, 2026 at 08:48 AM.
As a followup question, and mine is an 86 F-250 6.9L IDI, since I had a dual switch fuel selector mounted in the inside frame roughly close to the front tank. and the mechanic told me at the time that it might not be showing exactly correct, was that because he didn't also replace the sending unit in the rear tank? In other words, should both the front and rear in-tank sending units be replaced along with the selector switch? The dash gauge reads accurately on the front tank, but the rear goes from full to 1/4 suddenly and I know it's not accurate, not from reading OHMs resistance but from driving it far past the 1/4 tank showing on the dash gauge, numerous times
Prior to the replacement it was reading very much the same as after the sending unit was replaced
Replacing the fuel sending units won't change the ability of your fuel gauge to read more accurately. Unless you happen to use the wrong year of FSU in your truck.
If the gauge reads accurately for your front tank and then goes to Empty quickly for the rear tank I'd call that a win.
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