new sending unit, gauge pegs

[bigtex]

I got a 1988 f-250 with the 6.9 idi. Well the fuel gauge starting waving at me and then went out. I swapped a new sending unit and now the gauge pegs out way past the full mark. Tank is stock and the sending unit was a direct replacement. Whats stumping me is when the unit is out of the tank, it works fine. When it is half way covered in diesel, the gauge pegs out. I've tested the gauge cluster, fuel tank switch, the switching valve and checked for bad grounds and even regrounded the ground wire from the sending unit to the frame. This is a new sending unit and I'm at a loss at what to do next.
Btw, the gauge cluster tested positive continuity to the sending unit.
Any suggestions before I go bat **** crazy?

 

[hesutton]

How does it work with less than a full tank? I've had to "adjust" the float arm on new sending units before. I really want to know when I' have 1/4 tank left (about 5 gallons). I empty the tank, put in 5 gallons, put the new sender in the tank and see what my gauge reads in the cab. If I see 1/4 tank. I leave it alone, even if it is pegged past full at a less than full. If it shows more than 1/4 tank, I bend the arm up to raise the float in the tank and retest. If it shows less than 1/4 tank, I bend the arm down to lower the float............. and retest.

To me the sender had better be accurate at telling me I'm getting close to an empty tank, and I start looking for a truck stop at around 1/4 tank.


That's just my personal preference, and that my not be helpful to you, but, new senders I've had are rarely on the money out of the box.


Heath

 

[KyleQ]

^^^

Good points on checking where the damn thing actually reads with a specific amount of fuel. Mine are terrible, I can drive nearly 100 miles past the gauge ******* out on my front tank before I run out of fuel...

 

[sle2115]

Mine started acting up as well, but I didn't change anything...fuel gauge just stays at like 3/8 of a tank. I put new senders in sometime ago, but this happened way after that...so I'm curious to see what you find.

 

[jim x 3]

I got a 1988 f-250 with the 6.9 idi. Well the fuel gauge starting waving at me and then went out. I swapped a new sending unit and now the gauge pegs out way past the full mark. Tank is stock and the sending unit was a direct replacement. Whats stumping me is when the unit is out of the tank, it works fine. When it is half way covered in diesel, the gauge pegs out. I've tested the gauge cluster, fuel tank switch, the switching valve and checked for bad grounds and even regrounded the ground wire from the sending unit to the frame. This is a new sending unit and I'm at a loss at what to do next.
Btw, the gauge cluster tested positive continuity to the sending unit.
Any suggestions before I go bat **** crazy?

Tex:

If the gauge reads way more than full, and your sender is good (sounds like it is), then you have an open in the wire between the sender and the gauge. If the system tests well in your hand but not in place in the tank, you may have a broken spot in the wire that won't make contact when the cabling is in its normal position at the top of the tank - or it might be the connector.

Good luck.

Regards,

 

[OLDBULL8]

The sender has a resistance to ground, there is no hot VDC to the tank. When the tank is/reads full, the ground side (Neg.) from the guage thru the sender is a dead short to ground, as the tank/fuel empties, then you have resistance to ground, the less fuel the higher the resistance to ground.

 

[bigtex]

Its like 73 ohms at empty and 16-20 at full.
Hesutton,
The gauge works as Im dipping it into the tank. When the reheostat is half way covered in fuel, it pegs. I have an equis after market gauge (pn 7361) imma try to wire in and bypass the all the wiring to the cluster, since the factory cluster blows. I don't use the back tank anyway so whats the harm. Wish me luck.
Btw, I detest electrical work. I hate that **** like no other.

 

[jim x 3]

The sender has a resistance to ground, there is no hot VDC to the tank. When the tank is/reads full, the ground side (Neg.) from the guage thru the sender is a dead short to ground, as the tank/fuel empties, then you have resistance to ground, the less fuel the higher the resistance to ground.

Oldbull:

While I think the older trucks had an inverse relationship between resistance and tank level as you say, my '88 has a direct relationship (i.e. higher tank level = higher sender resistance in ohms). Since I needed to replace the sending unit in my rear tank, here are the electrical values I determined empiracally when designing the replacement:

fuel sender resistance, KOEO	gauge reading	sender supplied resistance	current
		(ohms)	(milliamps)
rear tank	(touching) low side of red E	21.9	89.6
measured 12/2009	E	31.8	79.3
	1/4	52.4	65.9
	1/2	94.2	48.9
	3/4	123.6	41.2
	(touching) low side of F	149.8	36.7
	F	181.5	34.5

[tr]

Regards,

 

[bigtex]

Welp, I wired an after market equis gauge and still the same thing, except it pegs in the opposite direction. Im wondering if the tank is grounding to the straps which tie into the frame. God this has me frustrated like no other. I ran new wires and everything is the same except the gauge drops way past empty on the new gauge. The tank grounding out is the last thing I can think of.

 

[OLDBULL8]

There are so many different fuel guages that require different resistance's to make them indicate correctly. The guage you bought should have noted what resistance the sender should be. Google "Fuel tank senders"

In have an Equuis after market fuel guage on my Ford tractor, and it has about the same problem your having. My tank is 17 gallons, and I know it's only 1/4 full but registers 1/2, see pic. Equuis is made by Autometer. Autozone sells them but by catalog order only.

 

[bigtex]

I did. The gauge reads 73 ohms at empty and 10 ohms at full. That exactly what the ford unit puts out, maybe I grabbed a crap unit from the crapyard? This is getting frustrating, I might just keep playing "guess the miles" until I figure something out. But I need to fix this asap because I pcs to Alaska in august.

 

[OLDBULL8]

I spelled Equus wrong, added an I.

 

[bigtex]

Just for ***** and giggles I put a good spare rear tank reheostat on the front sending unit and lo and behold, it works. Ohm reading for the rear was 168 ohms empty and 19 full. It reads 3/4 tank when I know it has alittle less than half in the tank. WTFE it works (What THE F*** EVER). But I have a replacement for a FI gasser that reads the same as a diesel unit so Imma do some modding and see what happens tomorrow. Thanks everyone for guiding me through this retardedly easy repair.
Will update tomorrow.

OLDBULL- I love the cluster on the tractor, thats cool as hell

 

[datkinsonsr]

Usually the sender is a variable resistor in the ground side of the circuit. If there is a direct short to ground the gauge will read full. An open in the circuit usually indicates empty.