Non- tractor. Automotive electrical question

[6wheeler 603]

Hi all. I need to pick an automotive electrical engineer gurus' mind. The patient is a 1988 Ford E-350 Cutaway with a Coachmen Leprechaun Motorhome body on it. The RV has 14,000 act miles on it. The RV is in excellent condition. However. Houston, We have a problem. The Motorhome has a single 42 gallon fuel tank, no switch. This unit has 2 fuel pumps. 1, low pressure pump with the sender in the fuel tank. And, 1, high pressure pump inside  the drivers side frame rail closer to the engine. The rear pump was failing and the sender quit "sending". I replaced the both pumps. While the rig runs and drives perfectly now? We have a problem. When the tank is full? It reads empty. And, Visa versa. I tried using jumper wires to reverse the polarity. That didn't work. I thought about switching the wire on the resistor to the opposite end. I don't know if that would work. Or, should I try to change the wires on the gauge in the cab itself? Everything works on this unit. It has never sat outside if it wasn't being used. The paint is in great shape. I can still read all of the numbers on the frame rails. The only things I have done so far to it? Fuel pumps. Servicing. Replace the original 35 year old Michelins with new Hankook RV tires. All 4 shock absorbers were replaced with Monroe heavy duty RV shocks. I did put a heavy duty rear stabilizer bar on it as well. Plus? Converted the vehicle AC unit to R134. Outside of this gauge issue? It is a good unit. Any help will be appreciated.   Thanks  Pat

Edited March 23 by 6wheeler
single fuel tank

[ri702bill 8,327]

Single tank or dual tanks with a dash selector switch??

[Lee1977 6,663]

Either the fuel pump sending unit is bad, or the selector switch is. 

[JoeM 7,874]

Nothing fancy about those sending units, just a variable resistor. Might be the way the linkage is setup for travel.

 

Being you don't drive it much and it does work but opposite. Perform a backwoods fix with a couple of stickers "E and F" over the fuel gauge letters.

If your stuck on fixing it, just have to pull the tank and check out why or get the right combo. As you know lots of work. 

 

Figuring the sending unit is part of the pump. One thing for me was I don't go cheap on these any more. To much work to replace.

My last two repairs on ATV's I used these guys. Nice quality and fit.

[squonk 41,137]

I would say the sending unit was put together wrong from the factory. Changing the wires on the gauge would probably fix the issue.

 

Many years ago whilst at my  Napa days we had a co. that build seaweed harvesters. United Marine International. The shop was within sight of my house. They bought fuel gauges and sending units from us. They would get Stewart Warner gauges and a COMPATIBLE sending unit. Well Napa (Balkamp) discontinued the gauge they bought. So UMI picked out a VDO brand gauge. Built the harvester, put 250 gal of diesel fuel in it and the gauge read empty!! Took several phone calls on tech support to finally come up with a sending unit that would work with their set up. They had to drain about 200 gal of diesel out to change the sender! 

 

They looked similar to this:

 

[wh500special 2,184]

Usually the sending units are configured to  have a 240 ohm resistance when full and 33 when empty.  Linear between.  Other resistances are available but the 240-33 ohm is the US default.   Generally 12v power flows through the gauge wiring and tank sender to ground.  The variable resistance causes the current in the loop to vary.  I think older gauges had a tiny heater in them that caused a device to expand more (tank toward empty) and push the needle more toward E.  The heating is a function of the current, so if you short the input to the sender to ground rather than through the gauge it should pin the needle to E.  
 

anyway, my point is that changing the polarity shouldn’t affect the current loop or needle direction. 

 

9 hours ago, 6wheeler said:

…I tried using jumper wires to reverse the polarity. That didn't work. I thought about switching the wire on the resistor to the opposite end. I don't know if that would work. Or, should I try to change the wires on the gauge in the cab itself?…

 

what do you mean by switching the wire on the resistor to the opposite end?   Does it have three terminals?   If it does maybe their functions are mirrored perhaps by two resistors situated in parallel (mechanically).  

 

 

8 hours ago, squonk said:

I would say the sending unit was put together wrong from the factory…

Mike is probably right that the sender was assembled backwards.   Or it was intended for a vehicle that has its gauge internally configured opposite that of your Ford.  
 

You’re most likely looking at needing a new sender or figuring out how that one’s action is flipped upside down. 
 

Good luck. 
 

Steve

[6wheeler 603]

This is the only fuel pump I can find to fit this unit. I got this number from my local Ford dealer as they do not carry it any more. So, here are some pics of a new one I got in case the other one didn't work. Hard enough to find them. I wasn't taking any chances.

[6wheeler 603]

You can see where the blue wire is attached. Would it change if I moved it to the other end? Would I damage the wiper? Or, hurt the resistor?

[6wheeler 603]

Is this something I can do on the pump unit itself? Or, should I try something else.

[6wheeler 603]

Some more pics

[wh500special 2,184]

It sure looks like if you can move the blue wire to the other end that it would make it act opposite what it does now.

 

assumjng the black wire is connected to the arm,  you can try probing it if you have a voltmeter set on the resistance setting.  Probe between the black wire and the blue wire while moving the arm.  You should (probably) see it sweep from 240 ohms when the arm is far away to 33 ohms when the arm is close to the blue wire.  And vice versa if you instead probe between the black and the spot you’d attach the blue wire on the other end. 
 

I am guessing that as-mounted the arm is close to the blue wire when the tank is full since the resistance ought to be lower there and a low resistance would normally correspond to an empty tank. 
 

any chance it could be mounted in the tank upside down?  Or maybe the arm itself is upside down?

 

worth messing with on the bench before opening the tank again. 
 

Steve

[6wheeler 603]

Thanks for the info Steve. When the arm is closest to the blue wire as in the first picture. That is in the empty position. The second picture is in the full position. It has to mount in the tank the way it is. When the arm is in the empty position, I get 24 ohms. And, in the full position I get 164 ohms. That is why I was wondering about switching the wire to the top of the sweep. I cant flip the whole arm assembly upside down because of the way the mount is on the assembly. So, that is out of the question.