Fuel gauge

[Dickinson County Comanche]

So back when I got my truck, I put the (technically) wrong sender in. (I believe it is the later style universal sender for both long and short beds) and, since my truck is a Renix, it runs backwards. My question is, can you take a gauge out of a high output/91-96? Jeep Cherokee and it’ll read correctly? I have the full instrumentation cluster, so it’s the smaller gauge, not the huge one with all the idiot lights.

 

thanks in advance. If this works I’ll have to steal it with the cooling system when I get around to going to the local scrapyard that has one.

[Eagle]

I considered that awhile back, and I think I concluded that it wouldn't work.

 

I was considering tracing the wires for the fuel gauge at the connector that plugs into the cluster, and seeing if it's possible to remove those wires and swap theeir positions in the connector body.

 

[Dickinson County Comanche]

8 hours ago, Eagle said:

I considered that awhile back, and I think I concluded that it wouldn't work.

 

I was considering tracing the wires for the fuel gauge at the connector that plugs into the cluster, and seeing if it's possible to remove those wires and swap theeir positions in the connector body.

 

I guess that makes more sense. I’ll have to try that when I want to swap my interior over.

[AMC86Kid]

If your fuel gauge runs backwards 

then just pop the gauge out and glue over the F with a letter E and glue over the E with a cut out of a letter F

[MJCARENA]

11 minutes ago, AMC86Kid said:

If your fuel gauge runs backwards 

then just pop the gauge out and glue over the F with a letter E and glue over the E with a cut out of a letter F

What a simple fix.

[Eagle]

41 minutes ago, AMC86Kid said:

If your fuel gauge runs backwards 

then just pop the gauge out and glue over the F with a letter E and glue over the E with a cut out of a letter F

 

Since you are the guy who runs his fuel gauge by direct wiring, maybe you know or can tell us if reversing the leads will make the gauge read the other way.

[AMC86Kid]

I only know how to wire the 86 fuel system and on my truck it’s impossible to wire it backwards you either have it wired correct and it works or you have it wired incorrect and it doesn’t work at all 

I am actually amazed that his is even able to work backwards, this leads me to believe that his system is different than what they used on 86 models 

 

i can further explain exactly how the 86 model fuel gauge works if you want me to, but I don’t think it will help him since he has a different unit it probably operates differently  

[Eagle]

4 hours ago, AMC86Kid said:

I only know how to wire the 86 fuel system and on my truck it’s impossible to wire it backwards you either have it wired correct and it works or you have it wired incorrect and it doesn’t work at all 

I am actually amazed that his is even able to work backwards, this leads me to believe that his system is different than what they used on 86 models 

 

i can further explain exactly how the 86 model fuel gauge works if you want me to, but I don’t think it will help him since he has a different unit it probably operates differently  

 

His system is different. Your fuel sender resistance range is 0 ohms empty, 88 ohms full. The HO system is reverse ~~ 105 ohms empty, 5 ohms full.

[Minuit]

Based on a quick look at the schematic for the fuel gauge, I don't think reversing the wires would make it work in the right direction. I might have enough spare parts sitting around that I could throw together an experiment.

[AMC86Kid]

Why don’t you just pop your gauge out and bench test it to see how it’s suppose to be wired, then when you go to connect it back in the truck you know what to look for and what needs to be sent to which prong, this makes modifying it easier 

[Minuit]

This is mainly intended as a response to Eagle, but I think a real fix to this problem would be to swap the fuel gauge to match the sending unit. I haven't tried it, but just comparing the clusters I think it would work out OK. You may or may not be able to keep the low fuel warning light.

 

Torturing a Fuel Gauge for Science

 

Hypothesis: Reversing the connections at the fuel gauge will allow you to use a Renix fuel sending unit in a High Output truck (or vice versa) without the fuel gauge working backwards.

 

Conclusion: NO, the fuel gauge and sending unit must match.

 

Background

All Comanche fuel gauges determine the amount of fuel left in the tank by reading the resistance of the fuel level sensor. The fuel level sensor is a variable resistor, that on all Comanches, has a varying resistance to ground based on the fuel level. As the resistance of the fuel level sensor changes, the current passing through the gauge will also change, causing the gauge to deflect depending on the amount of fuel left in the tank.

 

The fuel gauge has three terminals: ignition power, ground, and fuel level sense. The resistance between "fuel gauge sense" and "ground" varies as follows:

1991-1992: 105 ohms when empty. 5 ohms when full. (Source: 1991 Factory Service Manual)

1986-1990: 0 ohms empty, 88 ohms full. (Source: 1990 Factory Service Manual)

Other years are outside of the scope of this writeup.

 

Due to this difference in fuel level sensors through the years, replacing the fuel sending unit with one from an incorrect year will result in a fuel gauge that reads backwards.

 

Testing Setup

 

Here's the fuel gauge I will be using for this test, out of a High Output Comanche. While I know this gauge worked when I removed it from my truck years ago, we'll go ahead and test it using the correct resistances for a 1991 model year truck.

 

The ground terminal is in the center of the picture, with a black test lead connected to it. The 12V ignition switched terminal is located to the left with a red test lead connected. The fuel level sense terminal is on the right, and a resistor has been placed between it and the ground terminal.

 

For testing purposes, the fuel sending unit can be approximated by connecting resistors between the "fuel level sense" and "ground" terminals when not installed in a vehicle. With no sending unit connected, the gauge reads below empty.

 

With a 100-ohm resistor connected (I don't have a 105-ohm resistor handy), the gauge should read at approximately empty:

 

To simulate a full tank, I have connected two 10-ohm resistors in parallel between the ground and fuel level sense terminals, creating an equivalent resistance of 5 ohms:

Therefore, we can conclude that the gauge is working correctly.

 

Renix Fuel Sending Unit with an HO Gauge

As discussed, the Renix-era fuel level sensor reads from 0-88 ohms empty-full. Not only is the scale the reverse of the HO, but the resistance range is different.

 

Just for demonstration purposes, here is what your fuel gauge will read when connected to a Renix sender. Here's two 47-ohm resistors connected in series, which provides an equivalent resistance of 94 ohms:

Not exactly 88, but close enough for this demonstration.

 

 

As expected, the gauge reads empty:

 

Let's try an empty tank - 0 ohms:

A very predictable "full" on the gauge.

 

But what if we swapped the gauge connections around? There's three leads, so there's a few different combinations you could make. Let's start by switching the fuel level sense and ground terminals:

That's way, way past full, but still a promising start.

 

Let's try an empty tank:

The gauge does not move in this position.

 

Why? Remember how the "fuel level sense" terminal reads the resistance between itself and ground? When you switch the "fuel level sense" and "ground" terminals, you connect the fuel level sense pin straight to ground, which pins the gauge at "full" no matter what!

 

Now, I don't think I'm making a controversial statement when I say that reversing the positive and ground terminals on electronics is generally a bad idea. I'm going to do it anyway:

 

Here's 100 ohms:

 

And 0 ohms:

I guess it works, just upside down? At least you're on the right side of the gauge.

 

Conclusion

In summary, connecting the fuel gauge backwards does not "correct" the gauge in any of the combinations tested here. While this test was done with a High-Output era fuel gauge (with the reasoning that it's much more likely for someone to be installing a Renix sending unit in a High Output truck than the other way around), I believe that repeating this test with a Renix fuel gauge would provide a similar outcome. The gauge and sending unit must match. It looks like the HO and Renix era fuel gauges can be swapped. I do not know about the "low fuel warning module"

 

There are more combinations of leads that could be tried. I tried the combinations that seemed to make the most sense to me. I could test the rest of the possible combinations, but I already have almost two hours of my time into doing this test and writing up the result already.

 

If this is something anyone wants to see more of, let me know.

[AMC86Kid]

So if the gauge is designed to read a certain amount of ohms up and down then by putting a different sending unit can make your gauge work incorrectly because the ohm output levels won’t be where the gauge was designed to read, Makes sense to me

[Minuit]

Yup. No fancy technology in there, but the gauges are designed to work with sending units that work opposite from each other.

[Eagle]

DRAT!

 

Minuit, many thanks for running that test, and documenting it so thoroughly.

 

Now we know -- the only solution is to find a sender that matches the gauge, or find a gauge that matches the sender.

 

[silvertwinkiehobo]

Excellent work, and good on you for not letting the smoke out. Now, here's the $64,000 question: will a Renix sender resistor fit an HO pump hanger? Joe's gauge reads backward, and I'd rather not try installing an HO fuel gauge, just in case it requires slicing and dicing the cluster and cluster wiring to do it.

[Vakarian]

Okay so reading through this I am still a bit confused with solving my fuel gauge issue. The fuel gauge on my Renix JM doesn't move no matter the level of fuel and sits as shown in the picture below. What does it mean? The fuel pump works but could there be a loose wire or did the previous owner put an incorrect sending unit? Thanks for the help.

 

[jdog]

Have you recently changed the cluster out? If not then your wire for the gauge is grounded out somewhere

[derf]

First things I would check is the wires leading up to the gauge.  They might be somehow grounded.  Or they may be broken/separated/etc.

 

My old J-10 had a wire that kept coming off the sender after a couple of weeks and it would do something like that until I plugged it back in.

 

It might also be a wire that's been pinched and grounded somewhere.

 

 

I would try to see if the wires are connected at the tank sender.  If one came off, plug it back in and it should be fixed.

 

If that's fine, I'd pull the cluster and test the wire with an ohm meter.  Find the pin for the wire to the sender and check resistance between it and chassis ground.  If it's 0 or infinite, you know you have a problem in the wire.  0 = pinched wire that needs to be fixed.  Infinite = broken wire that needs to be replaced.  If it's somewhere in the dozens of ohms (depending on fuel level) it's fine.

 

Next, test the gauge following the example above.  If the gauge is broken, that's what you need to fix.  I'd start by testing resistance between the connector on the cluster and the gauge itself.  If that's 0 ohms, it's fine.  If it's infinite, the connection is broken.  Then test the gauge.

[Vakarian]

2 hours ago, jdog said:

Have you recently changed the cluster out? If not then your wire for the gauge is grounded out somewhere

 I have not changed the cluster out. Thank you for the info on the grounded out wire to look for.

 

derf, thank you for the insight and a path to start at.

 

I appreciate the help from you both and thanks for all the information.

[AnotherOldJeepGuy]

My fuel gauge issue is that when full the needle is around 1/4 tank distance beyond Full.  The real problem is that the gauge read somewhere between 1/8 and 1/4 when I run out of gas.  This operation is consistent, but I would still prefer having an accurate fuel gauge!  All parts are original 1990 parts and there have been no modification done that I know of...  The only work I know of was I had the ignition switch swapped out once when I though that was an issue with a start up failure.   At the end of that repair my AUX position no longer works, but I didn't find out the AUX didn't work until months after the work was done since it's not something I use much.  I also don't know when I first noticed the fuel gauge issue so I can't tie the two issues together with confidence.

 

From the post above I can see that I should be able to test the gauge itself pretty easily if I get it out.  I have noticed while surfing some it being said that removing the fuel gauge is a PITA, so before I start, any advice on what I am getting myself into if I want to pull it?  Any thoughts on the potential relationship to the fuel gauge failure and the ignition switch swap?

 

Thanks.

 

 

[Pete M]

5 hours ago, AnotherOldJeepGuy said:

My fuel gauge issue is that when full the needle is around 1/4 tank distance beyond Full.  The real problem is that the gauge read somewhere between 1/8 and 1/4 when I run out of gas.  This operation is consistent, but I would still prefer having an accurate fuel gauge!  All parts are original 1990 parts and there have been no modification done that I know of...  The only work I know of was I had the ignition switch swapped out once when I though that was an issue with a start up failure.   At the end of that repair my AUX position no longer works, but I didn't find out the AUX didn't work until months after the work was done since it's not something I use much.  I also don't know when I first noticed the fuel gauge issue so I can't tie the two issues together with confidence.

 

From the post above I can see that I should be able to test the gauge itself pretty easily if I get it out.  I have noticed while surfing some it being said that removing the fuel gauge is a PITA, so before I start, any advice on what I am getting myself into if I want to pull it?  Any thoughts on the potential relationship to the fuel gauge failure and the ignition switch swap?

 

Thanks.

 

 

 

 

can you pull the needle off and reattach it slightly closer to E? 

[gogmorgo]

6 hours ago, AnotherOldJeepGuy said:

My fuel gauge issue is that when full the needle is around 1/4 tank distance beyond Full.  The real problem is that the gauge read somewhere between 1/8 and 1/4 when I run out of gas.  This operation is consistent, but I would still prefer having an accurate fuel gauge!  All parts are original 1990 parts and there have been no modification done that I know of...  The only work I know of was I had the ignition switch swapped out once when I though that was an issue with a start up failure.   At the end of that repair my AUX position no longer works, but I didn't find out the AUX didn't work until months after the work was done since it's not something I use much.  I also don't know when I first noticed the fuel gauge issue so I can't tie the two issues together with confidence.

 

From the post above I can see that I should be able to test the gauge itself pretty easily if I get it out.  I have noticed while surfing some it being said that removing the fuel gauge is a PITA, so before I start, any advice on what I am getting myself into if I want to pull it?  Any thoughts on the potential relationship to the fuel gauge failure and the ignition switch swap?

 

Thanks.

 

 

I would start by visiting cruiser54.com and do the connector and ground refreshing associated with the fuel pump. But I suspect the sending unit has some sort of issue. There’s a replacement available from MTS.

[AnotherOldJeepGuy]

Thanks Pete - that is a clever fix!  And assuming I can get to the needle easily it would be a quick fix, that depends on if the plastic front can be removed without pulling the full cluster, TBD I guess.

 

Thanks gogmorgo, I am not familiar with cruiser54 or MTS, but I am sure Google is!

[Pete M]

yes, you can pull the clear cover without yanking the entire cluster.  

 

the link in my signature has Cruiser54's website in it near the top.