fuel gage reading backwards

[comaaron]

darn gauges i changed my 92 sending unit to a 87 now it reads wrong backwards can i use the gas guage from the 87 to fix the issue since its a polarity issue ior can it be grounded beter or something

[HOrnbrod]

darn gauges i changed my 92 sending unit to a 87 now it reads wrong backwards can i use the gas guage from the 87 to fix the issue since its a polarity issue ior can it be grounded beter or something

 

The clusters from 1990 and below are not compatible with the 1991 and above for several reasons. One is because of the different polarity on the fuel sending unit that you have discovered, and several of the gauges are different too, the fuel gauge is one of them. You can either go to the fuel tank and swap the two sending unit wires (but the gauge will still read wrong as the sender pots are different values), or better yet, get the correct cluster from a 91-96 MJ/XJ so everything will read correctly and you'll have a working electronic speedo too. :D

[jeepdoggydogB]

darn gauges i changed my 92 sending unit to a 87 now it reads wrong backwards can i use the gas guage from the 87 to fix the issue since its a polarity issue ior can it be grounded beter or something

 

The clusters from 1990 and below are not compatible with the 1991 and above for several reasons. One is because of the different polarity on the fuel sending unit that you have discovered, and several of the gauges are different too, the fuel gauge is one of them. You can either go to the fuel tank and swap the two sending unit wires (but the gauge will still read wrong as the sender pots are different values), or better yet, get the correct cluster from a 91-96 MJ/XJ so everything will read correctly and you'll have a working electronic speedo too. :D

Don

If I read his post right he has a 92 and used an 87 sending unit? If that is the case I am in the same boat. JW wreckers quoted me a 1991 sending unit, but when I installed it I noticed that the fuel guage worked backwards. When I can locate a 1991 assembly, I intend to send it to this company to be rebuilt.

 

http://www.tristarrradiator.com/

 

If you still have the 1992 assembly you might want to send it to the above company? I have terrible luck with the fuel pump assembly from JY. They can rebuild it for some coin get it back to you and you can put it back in when you can afford some down time.

[HOrnbrod]

Yes Joe, you read it right; I read it wrong. :nuts: BTW, did you try reversing the sending unit pot wires to change the polarity input to your gauge? Don

[jeepdoggydogB]

Yes Joe, you read it right; I read it wrong. :nuts: BTW, did you try reversing the sending unit pot wires to change the polarity input to your gauge? Don

Don,

I haven't had a chance to get back in there, but I think what you posted is correct. If I reverse the wires it will be moving in the right direction but it will be very inaccurate. Maybe when I get to my balljoints I will take a peak, but from reading all the horror stories they might take up the whole weekend.

[mvusse]

With an 87 sender, you need a gauge from a 84 through 90 Cherokee, or 86 through 90 Comanche.

 

Conversely, you can keep the gauge and find a sender from an 91/92 Comanche, but that would be more difficult to find.

 

I have yet to sit down with a Cherokee sending unit to see if it is feasible or even possible to make it work in a Comanche. Yes I know they are mirror image. And no I have never looked at one in person in 3D yet.

[Eagle]

I'm having trouble keeping this one straight. What year is the truck? If I understand your other thread correctly, you have a 91 or 92 MJ in which you have installed an 87 fuel level sending unit. The polarity on the gauges did change starting in 1991, but I don't know if the terminals on the gauge are reversed, or if the printed circuit board for the cluster leads to the opposite sides of the gauge. And I don't have any post-1990 gauge clusters I could check.

 

You may need to figure out which two wires on the smaller bundle that attaches to the left side of the cluster (when viewed from the driver's seat) go to the fuel gauge, and see if you can swap them. But ... if the new style panel uses a common ground for all gauges, that won't work.

 

Someone suggested swapping the two wires where they attach to the sender (in the tank), and I was going to suggest that also. But when I think about it, I don't believe that will change the polarity. The gauge is really nothing but a variable resistor, so I believe it'll read the same regardless of which wire goes to which terminal.

 

The cluster that would match up with your sender is from an 87 thru 90 XJ or MJ ... but that won't work, because they used mechanical speedometers. Your speedo is electronic.

[702_MJ]

LOL mine does that too, i just got used to it. Orange light means I'm full of gas.

 

They had this whole situation explained when JP put the NSG-370 into their comanche. Its because early comanches read full at 80ohms and empty at 0ohms. And the newer ones are backwards. I believe thats what they said, can't tell you exactly cuz all my magazines were thrown away in the move :(

[HOrnbrod]

LOL mine does that too, i just got used to it. Orange light means I'm full of gas.

 

They had this whole situation explained when JP put the NSG-370 into their comanche. Its because early comanches read full at 80 ohms and empty at 0 ohms. And the newer ones are backwards. I believe thats what they said, can't tell you exactly cuz all my magazines were thrown away in the move :(

 

The HOs fuel gauges will read full at 5 ohms and empty at 105 ohms, so even if you reversed polarity at the sender the gauge will be still very inaccurate, but at least it will be moving in the correct direction. :D

[702_MJ]

well thanks hornbrod for correcting me i just thought i would throw my 2 cents in.

 

Thats why i just deal with it. And think if some one steals the truck on the F mark, well buddy you aint got a full tank after all :brows:

[Eagle]

well thanks hornbrod for correcting me i just thought i would throw my 2 cents in.

 

Thats why i just deal with it. And think if some one steals the truck on the F mark, well buddy you aint got a full tank after all :brows:

E = Energy and F = Fail, eh?

[702_MJ]

E = Energy and F = Fail, eh?

 

Nah E is Everythings good and F is Find a gas station quick. My gauge flops around pass the half mark anyway, just got used to it and worked around it.

[HOrnbrod]

well thanks hornbrod for correcting me i just thought i would throw my 2 cents in.

 

Thats why i just deal with it. And think if some one steals the truck on the F mark, well buddy you aint got a full tank after all :brows:

 

No, I was not correcting you; you're exactly right. I haven't bench tested how much the difference in the sender pot's resistance would affect the accuracy of the gauge, just theory guessing if polarity was reversed at the sender connection above the tank. You have the ideal platform to do a real world test; maybe the accuracy of the fuel gauge might not as bad as I think? :cheers:

 

BTW, where you at in HI? Hono?

[702_MJ]

No, I was not correcting you; you're exactly right. I haven't bench tested how much the difference in the sender pot's resistance would affect the accuracy of the gauge, just theory guessing if polarity was reversed at the sender connection above the tank. You have the ideal platform to do a real world test; maybe the accuracy of the fuel gauge might not as bad as I think? :cheers:

 

BTW, where you at in HI? Hono?

 

Oh my bad. yeah i would test but I'm perfectly content with the way it is, its been this way since ive gotten it from my dad so no complaints.

 

I'm in volcano, HI. Ironic isnt it. Like 4 miles outside the hawaii volcanos national park on the big island/island of hawaii, i actually kinda volunteer there as an automechanic. Let me tell you working for the government aint half bad.

[HOrnbrod]

Let me tell you working for the government aint half bad.

 

Got that right. :D Spent most of my life in gov DoD service as both military and Fed GS civilian, but never got a chance to visit your AO. Most of my duty was further west (Marshall Islands, Guam, and beyond) but HI was my R&R place. Enjoy, UB in a great location. :cheers:

[702_MJ]

So DoD, you mean department of defense? And AO, i don't understand. Just got the volunteer job cuz my dad is the chief ranger and asked if the auto shop needed help. So kinda new to the whole government thing and abbreviations. But the job is hella tight, and its a foot in the door with the department of interior/national park service. I have a feeling it government work might become a career. But it sounds like you got to see alot.

[HOrnbrod]

So DoD, you mean department of defense? And AO, i don't understand. Just got the volunteer job cuz my dad is the chief ranger and asked if the auto shop needed help. So kinda new to the whole government thing and abbreviations. But the job is hella tight, and its a foot in the door with the department of interior/national park service. I have a feeling it government work might become a career. But it sounds like you got to see alot.

 

Yes, Defense. AO = Area of Operations. :D Gov work has been very good to me, mucho travel, great and not so great assignments (Korea and Nam come to mind), but never a regret. With the economy as it is and seeing no quick relief soon, you for sure have the right idea to get your foot in the door and carry on from there. Sounds like your dad is cool and is looking out for you. Both my daughters are thinking the same way as they are in college now under ROTC scholarships. Enjoy it - you're off to a great start. :cheers:

[jeepdoggydogB]

well thanks hornbrod for correcting me i just thought i would throw my 2 cents in.

 

Thats why i just deal with it. And think if some one steals the truck on the F mark, well buddy you aint got a full tank after all :brows:

 

No, I was not correcting you; you're exactly right. I haven't bench tested how much the difference in the sender pot's resistance would affect the accuracy of the gauge, just theory guessing if polarity was reversed at the sender connection above the tank. You have the ideal platform to do a real world test; maybe the accuracy of the fuel gauge might not as bad as I think? :cheers:

 

BTW, where you at in HI? Hono?

I like Eagle's interpretation (at least that is what I told my Wife E=energy, F=fail). When I get to my balljoint issue I will switch those wires and let folks know the accuracy of the guage. I suspect you are correct and the real HO sending unit is the best solution. :cheers:

[mjeff87]

I did the opposite....put an HO fuel pump assembly into my Renix. I knew going in about the gauge reversal, and looked at the new unit long and hard to try to figger out how to "fix" it before install. The sending unit only has one wire (+) coming out of it, it grounds thrru the assembly, so there's no possiblilty of swapping wires at that end. I then looked at the printed circuit on the back of the cluster, and all the gauges share a common ground as near as I could figure, so I just left it all as is and installed it. Like others have said, you just have to get used to it reading the wrong way :chillin:

 

Jeff

[HOrnbrod]

I just looked at my FSM again and agree with Jeff; there is only one lead coming off the slider of the sender potentiometer that goes to the fuel gauge. The sender potentiometer, like all pots, has two legs and a slider. The slider output goes to the gauge, one leg is tied to ground, and the other leg has no connection. The slider picks off a variable resistance as the float moces up and down and the fuel gauge follows based on the resustance it sees. To reverse polarity, you would have to remove the black ground wire from the sender pot leg and reconnect it to the other empty leg of the pot. This would reverse the resistance readings (high resistance for FULL; low resistance for EMPTY) as the float goes up and down and the gauge would move in the correct direction. Since I don't have a complete sender laying around, can someone look at the sender pot and see if reconnection of the black ground wire to the other end of the pot is even possible? If it is, this would work for sure. But don't know how accurate it would be. :D

[Sir Sam]

I just looked at my FSM again and agree with Jeff; there is only one lead coming off the slider of the sender potentiometer that goes to the fuel gauge. The sender potentiometer, like all pots, has two legs and a slider. The slider output goes to the gauge, one leg is tied to ground, and the other leg has no connection. The slider picks off a variable resistance as the float moces up and down and the fuel gauge follows based on the resustance it sees. To reverse polarity, you would have to remove the black ground wire from the sender pot leg and reconnect it to the other empty leg of the pot. This would reverse the resistance readings (high resistance for FULL; low resistance for EMPTY) as the float goes up and down and the gauge would move in the correct direction. Since I don't have a complete sender laying around, can someone look at the sender pot and see if reconnection of the black ground wire to the other end of the pot is even possible? If it is, this would work for sure. But don't know how accurate it would be. :D

 

In addition to that I would also add that if we could measure the full value and the empty value of both units we could very easily construct a resistor network to correct the resistance.

[Eagle]

As already noted, the Renix sender has a range of 0 to 88 ohms. The HO sender has a range of 105 to 5 ohms. The difference isn't the same on both ends of the scale, but adding a 5-ohm resistor to the Renix-era sender would get it closer to the HO values. The range would then be 5 to 93 ohms. Combined with a bit of bending on the float arm, that might result in a gauge whose readings are at least usable, since factory gas gauges aren't especially accurate anyway.

 

(Side note: My brother used to be service manager at a BMW dealership. One day he got an irate phone call from a woman whose new BMW had run out of gas. Her model had a fuel and mileage computer as well as a gauge. L'il bro asked her if she had checked the computer before she ran out of gas. "Yes I did!" she replied angrily. "It said I had 1.3 miles to go before it was empty.")

 

Keep in mind that the fuel pump is also grounded by the one black wire going into the tank. Switching the black and colored wire for the sensor might result in some odd behavior on the part of the pump. I suppose it might be possible to add a fourth wire as a dedicated ground for the sender - I'd have to pull a fuel pump/sender assembly out of a tank to see if that's really feasible, or if it's just another pie-in-the-sky idea that sounds great in theory but won't work in practice.

[HOrnbrod]

Keep in mind that the fuel pump is also grounded by the one black wire going into the tank. Switching the black and colored wire for the sensor might result in some odd behavior on the part of the pump. I suppose it might be possible to add a fourth wire as a dedicated ground for the sender - I'd have to pull a fuel pump/sender assembly out of a tank to see if that's really feasible, or if it's just another pie-in-the-sky idea that sounds great in theory but won't work in practice.

 

You don't switch the colored wire, that stays on the pot slider and goes to the gauge. You only switch the the black ground wire from one leg of the pot to the other unused leg. The pump will still grounded by the black wire. Wish I had a sender to do a bench test, but I'm certain it will work.

[Eagle]

Keep in mind that the fuel pump is also grounded by the one black wire going into the tank. Switching the black and colored wire for the sensor might result in some odd behavior on the part of the pump. I suppose it might be possible to add a fourth wire as a dedicated ground for the sender - I'd have to pull a fuel pump/sender assembly out of a tank to see if that's really feasible, or if it's just another pie-in-the-sky idea that sounds great in theory but won't work in practice.

 

You don't switch the colored wire, that stays on the pot slider and goes to the gauge. You only switch the the black ground wire from one leg of the pot to the other unused leg. The pump will still grounded by the black wire. Wish I had a sender to do a bench test, but I'm certain it will work.

Ah, so. Muy interesante.

 

I wish you had a sender to bench test, too. I never noticed an unused connector tab.

[Jeff J]

This is a little bit of an old thread that I found a couple years ago. Moving the wire on the pot does make the gauge work in the correct direction. Accuracy is an issue. My gauge would show empty while still having a considerable amount of fuel. I can't remember if the arm is long enough on my sender or not.  Still better than working in the wrong direction since I don't drive my Comanche very often. Now I have to switch the wires back the other way since I'm installing a gauge package with a mechanical speedometer so it will finally work.